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THE SEPARATION OF GENERATIONS: BIOLOGY AND BIOGEOGRAPHY
OF LONG-LIVED SPOROPHYTELESS FERN GAMETOPHYTES
Jerald B. Pinson,
1,
* Sally M. Chambers,* Joel H. Nitta,†Li-Yaung Kuo,‡and Emily B. Sessa*
*Department of Biology, University of Florida, Box 118525, Gainesville, Florida 32611, USA; †Department of Organismic and Evolutionary
Biology, Harvard University, 22 Divinity Avenue, Cambridge, Massachusetts 02138, USA; and ‡Institute of Ecology
and Evolutionary Biology, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
Editor: Erika Edwards
Premise of research. Ferns (monilophytes) and lycophytes are unique among land plants in having two in-
dependent life stages: the gametophyte generation, which is generally small, cordiform, and short-lived, se-
nescing after fertilization, and the sporophyte generation, which is considered the dominant, long-lived portion
of the life cycle produced following fertilization. In many species of epiphytic ferns, however, the gametophyte
generation is capable of sustained vegetative growth, and some are able to reproduce asexually via gemmae.
These two characteristics have increased the independence of these gametophytes, so much so that some species
never produce sporophytes at all, while other species produce sporophytes only in parts of their geographic
range, a trend we term here the “separation of generations.”
Pivotal results. Long-lived fern gametophytes have evolved independently in several families and can be
found around the world. We present a comprehensive review of the long-lived fern gametophytes that are able
to forgo the production of a sporophyte, including accounts of their discovery, taxonomy, biology, ecology,
and biogeography. We also present several hypotheses concerning why these species do not produce sporo-
phytes, identify gaps in our knowledge about these organisms, and suggest areas of future study.
Conclusions. While several populations of independent gametophytes have been identified and character-
ized in temperate regions, it is likely that the bulk of species with spatially separated generations occur in the
tropics, where little work has been done. Additionally, virtually no studies have been undertaken that attempt
to determine the underlying factors inhibiting sporophyte production in ferns. As 2017 marks the fiftieth an-
niversary of the first comprehensive study published on independent fern gametophytes, we can think of no
better time for a review on their biology and an assessment of the work that still needs to be done.
Keywords: alternation of generations, life cycle, gametophyte, sporophyte, fern, gemmae.
We are accustomed to see and to marvel at the
great varied form and adaptation of the spo-
rophytes, which are the ferns as we know them,
but indeed there must be nearly as much vari-
ety of adaptation among the gametophytes.
(Holttum 1938, pp. 421–422)
Introduction
By definition, all land plants (embryophytes) cycle between
diploid sporophyte and haploid gametophyte stages, known
as the “alternation of generations.”In the two largest groups
of land plants, bryophytes and spermatophytes (seed plants),
one stage is nutritionally dependent on the other; however, in
ferns (monilophytes) and lycophytes, the two life stages are in-
dependent and can live freely from one another. In ferns and
lycophytes, as in seed plants, the diploid sporophyte is tradi-
tionally defined as the “dominant”generation. The fern sporo-
phyte produces haploid spores via meiosis that are dispersed
into the surrounding environment once they mature. If these
spores land in a suitable environment, they will germinate into
haploid gametophytes. The gametophyte is the sexual stage of
the life cycle, producing antheridia and archegonia via mitosis,
which produce sperm and eggs, respectively. Once successful
fertilization has occurred, a diploid sporophyte grows directly
from the fertilized egg cell within the archegonium on the hap-
loid gametophyte, and eventually the gametophyte tissue senes-
ces after the new sporophyte is established.
Although most ferns follow the standard alternation of gen-
erations cycle closely, the independence of these two life stages
allows for both the gametophyte and the sporophyte to ex-
plore novel ecological niches (Sato and Sakai 1981) and life-
history strategies (Dassler and Farrar 2001). In some species,
the gametophyte can be long-lived, a pattern that can be seen
multiple times among epiphytic ferns, which have stretched the
limits of physiological tolerance by taking to the canopy, a
1
Author for correspondence; e-mail: jbp4166@ufl.edu.
Manuscript received April 2016; revised manuscript received July 2016;
electronically published November 23, 2016.
Int. J. Plant Sci. 178(1):1–18. 2017.
q2016 by The University of Chicago. All rights reserved.
1058-5893/2017/17801-0001$15.00 DOI: 10.1086/688773
1
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stressful environment that imposes various constraints on plant
growth and establishment (Watkins and Cardelús 2012). At the
extreme, some ferns have done away with the sporophyte gen-
eration altogether, surviving exclusively in the gametophyte
stage. Globally, three known species of ferns fall into this cat-
egory, and 24 others have gametophytic ranges that extend the
species’overall geographic ranges into regions where sporo-
phytes are produced infrequently or not at all (table 1). We
term this spatial disjunction between gametophytic and sporo-
phytic ranges the “separation of generations.”
Most epiphytic ferns occur in tropical habitats (Gentry and
Dodson 1987; Dubuisson et al. 2009), where competition can
be high. In response to the challenges of these environments,
epiphytic fern gametophytes have evolved a three-dimensional
morphology that is much more complex than the typical two-
dimensional cordiform (heart-shaped) shape seen in terrestrial
fern gametophytes (Dassler and Farrar 2001; Pitterman et al.
2013). Epiphytic gametophytes are often branching and dis-
sected and are capable of sustained vegetative or clonal growth,
allowing them to prolong their life spans (fig. 1). This branch-
ing morphology is thought to increase the likelihood of the
thalli of two individuals coming into close enough proximity
for outcrossing to occur (Farrar and Dassler 2001), and the
longevity of these gametophytes also means that one individ-
ual can persist for an extended period of time before another
spore lands nearby and produces another gametophyte (Wat-
kins and Cardelús 2012), thus increasing the chances for out-
crossing. Additionally, this morphology may promote desicca-
tion tolerance, the crevices created by the three-dimensional
morphology helping to retain water for a longer period of time,
thus slowing the drying rate and allowing for tolerance of the
stressful canopy conditions. A study conducted to examine the
degree to which gametophytes of tropical ferns are desiccation
tolerant indicated that gametophytes with this morphology were
capable of withstanding greater levels of desiccation than their
terrestrial, cordate counterparts (Watkins et al. 2007b).
This complex morphology has evolved independently in
members of at least six separate fern families (Hymenophyl-
laceae, Polypodiaceae, Pteridaceae, Lomariopsidaceae, Dryop-
teridaceae, and Schizaeaceae; fig. 2). With a few exceptions,
the majority of species in the first five families that have this
elongated and branched morphology are epiphytic, indicating
the importance of this morphology in tropical canopies. The
genera Schizaea and Bolbitis are notable outliers, which are,
for the most part, terrestrial ferns. In the first three families,
several species have further evolved the ability to reproduce
asexually via small vegetative propagules called gemmae that
are produced mitotically from gametophyte thallus tissue (figs.
1H,1J,1M, 3). Gemmae are thought to enhance the likelihood
of establishment of fern populations in the canopy (Ebihara
et al. 2008), as only one gemma needs be dispersed to give rise
to an entirely new, albeit clonal, population. These gemmae
also have the ability to produce antheridia, thus promoting out-
crossing (Emigh and Farrar 1977).
Table 1
Species Discussed in This Review
No. Family Species Supporting publication(s)
1 Hymenophyllaceae Crepidomanes intricatum Ebihara et al. 2008
2 Hymenophyllaceae Didymoglossom petersii Farrar 1990
3 Hymenophyllaceae Hymenophyllum tayloriae Raine et al. 1991
4 Lomariopsidaceae Lomariopsis kunzeana Possley et al. 2013
5 Polypodiaceae Moranopteris nimbata Farrar 1967
6 Pteridaceae Vittaria graminifolia Farrar and Landry 1987
7 Pteridaceae Vittaria appalachiana Farrar and Mickel 1991
8 Hymenophyllaceae Hymenophyllum wrightii Duffy et al. 2015
9 Hymenophyllaceae Callistopteris baldwinii Dassler and Farrar 1997
10 Hymenophyllaceae Callistopteris apiifolia Ebihara et al. 2013; Nitta et al., forthcoming
11 Hymenophyllaceae Hymenophyllum recurvum NA
12 Hymenophyllaceae Vandenboschia cyrtotheca NA
13 Pteridaceae Vaginularia paradoxa Nitta et al., forthcoming
14 Hymenophyllaceae Hymenophyllum badium Ebihara et al. 2013
15 Hymenophyllaceae Unknown Ebihara et al. 2013
16 Hymenophyllaceae Unknown Ebihara et al. 2013
17 Hymenophyllaceae Vandenboschia kalamocarpa Ebihara et al. 2009
18 Pteridaceae Antrophyum henryi Chen et al. 2013b
19 Pteridaceae Antrophyum parvulum Chen et al. 2013b
20 Pteridaceae Haplopteris heterophylla Chen et al. 2013a, 2013b
21 Pteridaceae Haplopteris sp. Kuo et al., forthcoming
22 Pteridaceae Haplopteris sp. Kuo et al., forthcoming
23 Lomariopsidaceae Lomariopsis lineata Li et al. 2009
24 Lomariopsidaceae Lomariopsis sp. Ebihara et al. 2013
25 Hymenophyllaceae Vandenboschia speciosa Rumsey et al. 1990
Note. A complete list of the 25 species discussed in this review, along with the publication in which a given species was first mentioned (or
named) in a scientific study. For those taxa reported here through personal communication, the publication cell is listed as nonapplicable (NA).
2INTERNATIONAL JOURNAL OF PLANT SCIENCES
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Fig. 1 Photographs illustrating the morphological diversity of fern gametophytes. A, B, Typical cordiform gametophytes (species unknown).
The left-hand gametophyte has an emergent young sporophyte slightly out of focus in the foreground. C–E,Crepidomanes intricatum
(Hymenophyllaceae). F,Hymenophyllum tayloriae (Hymenophyllaceae). G,Vittaria appalachiana (Pteridaceae). H–J,Hymenophyllum wrightii
(Hymenophyllaceae). K,Callistopteris apiifolia (Hymenophyllaceae). L,Vaginularia paradoxa (Pteridaceae). M,Haplopteris heterophylla
(Pteridaceae). N,Danaea nodosa (Marattiaceae). O,Anetium citrifolium (Pteridaceae). A,N, and Oby J. E. Watkins Jr. Band Fby E. Sessa.
C–Eand H–Jby A. Duffy. Gby S. Chambers. Kand Lby J. Nitta. Mby C.-W. Chen.
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Fig. 2 Phylogeny of extant ferns with representative drawings of gametophyte morphology in four families that include members displaying
the separation of generations pattern. Note that there is no illustration for Schizaeaceae or Dryopteridaceae. The topology is based primarily on
Smith et al. (2006) but is also an amalgamation of several other studies. The position of Equiesetaceae follows that of Knie et al. (2015) and
Rothfels et al. (2015). Families within Eupolypods II are based on Rothfels et al. (2012). The recognition of Cystodiaceae and Lonchitidaceae
as separate from Lindseaceae is based on Christenhusz et al. (2011). The families Didymochlaenaceae and Desmophlebiaceae were established by
Zhang and Zhang (2015) and Mynssen et al. (2016), respectively. The systematic position of Dennstaedtiaceae was resolved by Lu et al. (2015)
and Rothfels et al. (2015), and the position of Lomariopsidaceae was established by Kuo et al. (2011).
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The evolution of independent, morphologically complex,
perennial gametophytes capable of asexual reproduction has
several corollaries. Nayar and Kaur (1971) noted that fern
gametophytes and sporophytes share nearly the same niche
space requirements, because a sporophyte will form only if
a gametophyte is able to persist long enough for fertilization
to occur. However, it has been shown since that gametophytes
of some species are able to withstand a wider range of envi-
ronmental conditions than the sporophytes of their own spe-
cies, and there are several well-documented cases where gameto-
phytes extend the species’geographic range beyond that of their
respective sporophytes (Nauman 1986; Farrar and Landry
1987; Farrar 1990; Rumsey et al. 1998a;DasslerandFarrar
1997; Ebihara et al. 2013; Duffy et al. 2015). In terrestrial ferns
with cordiform gametophytes, any individuals growing beyond
the range limits of conspecific sporophytes would likely not last
more than a few growing seasons (Watkins et al. 2007a). Pop-
ulations of perennial gametophytes capable of asexual reproduc-
tion, however, can persist for virtually indefinite amounts of
time. In the three most extreme examples, Crepidomanes intri-
catum (Farrar) Ebihara and Weakley, Hymenophyllum tayloriae
Farrar and Raine, and Vittaria appalachiana Farrar and Mickel,
a viable sporophyte has never been observed (Farrar and Mickel
1991).
The goal of this article is to provide an overview of the bi-
ology and natural history of the various fern species exhib-
iting the separation of generations, which we define as species
that have gametophyte populations spatially separated from
conspecific sporophyte populations or that have no sporophytic
counterpart. Although ferns with these unique life histories can
be found around the world, they have been documented and
studied most thoroughly in eastern North America and west-
ern Europe; thus, the information presented here focuses most
heavily on species from these areas. We review the morphology,
ecology, reproductive strategies, biogeographic patterns, and
habitat specificity of these organisms, and we discuss future
directions for research along with probable causes for the spa-
tial separation of the two generations.
Morphology
Fern gametophytes are often depicted in textbooks (e.g.,
Evert and Eichorn 2013; Reece et al. 2014) as the small,
short-lived, cordiform portion of the life cycle. The cordiform
thallus is common among terrestrial taxa of the Polypodiales,
but a variety of gametophyte morphologies exist (fig. 1). The
quote above serves to illustrate just how varied fern gameto-
phyte morphology can be. If we consider only leptosporan-
giate ferns, which make up the majority of the approximately
9000 taxa (Smith et al. 2006), gametophyte morphology can
be broadly categorized into four types sensu Farrar et al. (2008):
cordiform, strap shaped (sometimes referred to as “elongate-
cordate”;fig. 4), ribbon shaped (fig. 3), and, recognized here
as a distinct morphology, filamentous (fig. 6).
Cordiform, or “heart-shaped,”gametophytes (fig. 1A,1B)
grow from a single apical meristem located in a notch at the
apex of the thallus. These gametophytes grow quickly but re-
main small and are generally short lived, lasting a year or less
(Farrar et al. 2008). It has been demonstrated that establish-
ment of many terrestrial fern gametophytes requires distur-
bance, which creates an environment relatively free of compe-
tition and may expose spore banks in the soil (Watkins et al.
2007a). But this proclivity toward disturbed habitats means
that terrestrial gametophytes must grow and produce sporo-
phytes quickly (rselected) before another disturbance destroys
their populations. Experimentally, however, it has been shown
that, when gametophytes of this type are grown separately to
prevent fertilization in a laboratory setting, some can live for
indefinite periods of time. For example, gametophytes of Os-
munda claytonia L. and Pteris nodulosa Nieuw. were kept alive
for over 3 yr (Mottier 1927). Walp and Proctor (1946) grew
cordate gametophytes of several species together as a demon-
stration for freshman biology students, but when they failed
Fig. 3 Vittaria graminifolia (Pteridaceae) with a magnified illus-
tration of gemmae. Illustrated by Simon Parsons.
Fig. 4 Prosaptia contigua (G. Forst.) C. Presl (Polypodiaceae).
Illustrated by Simon Parsons.
PINSON ET AL.—THE SEPARATION OF GENERATIONS 5
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to produce sporophytes, possibly due to overcrowding or the
addition of potassium manganite, they kept the culture growing
for at least 8 yr.
Epiphytic gametophytes generally live in a more stable en-
vironment with high levels of competition. They invest more
time and resources into growth (Kselected), and sporophyte
production can be delayed if the gametophyte is growing in less
than optimal conditions. Strap-shaped gametophytes (fig. 4)
retain a defined apical meristem and develop a notch at the
tip of the growing region, similar to cordiform gametophytes
(Farrar et al. 2008); however, this morphology differs in hav-
ing multiple meristems and proliferation points that grow more
in length than they do in width. These gametophytes grow
slowly and are long-lived, capable of sustaining indefinite mer-
istematic activity. As new thallus projections proliferate, each
with the ability to produce gametangia and sporophytes, older
portions of the thallus tend to die back. Thus, multiple sporo-
phytes may be produced from a complex, clonal collection of
thalli that were initially one gametophyte (Farrar et al. 2008).
Strap-shaped gametophytes are characteristic of epiphytic taxa
belonging to Polypodiaceae and Dryopteridaceae (e.g., Elapho-
glossum;fig. 2), as well as less proliferate types in some euspo-
rangiate ferns (e.g., Osmundaceae).
Ribbon-shaped gametophytes (figs. 3, 5) have disrupted
marginal meristems along the length of the thallus rather than
the typical apical meristem (Farrar et al. 2008). The meristem
increases the length of the gametophyte, but at certain points
growth ceases, leading to the formation of several distinct
ribbon-like sections that continue to grow and branch on their
own. Like the strap-shaped type, these gametophytes are also
perennial and can grow indefinitely. The ribbon-shaped mor-
phology commonly occurs in epiphytic or epipetric ferns be-
longing to the families Hymenophyllaceae, Polypodiaceae, Lo-
mariopsidaceae, and Pteridaceae (fig. 2).
Filamentous gametophytes (fig. 6) occur in the families Hy-
menophyllaceae and Schizaeaceae. Gametophytes of this type
are highly reduced, bearing a strong resemblance to the vege-
tative cells of algae. These gametophytes produce uniseriate fil-
aments that are capable of repeated branching and indefinite
growth, and some are capable of producing gemmae (Farrar
1992).
Asexual Reproduction: Gemmae
Sporophytes in a number of fern species are capable of veg-
etative reproduction, usually via the process of budding (Mc-
Veigh 1937; Johns and Edwards 1991). Gametophytes, as
well, have a number of mechanisms that allow them to repro-
duce asexually. Multiple species in several unrelated families
produce proliferations that bud from the main portion of the
thallus, which, either upon physical detachment in a laboratory
setting or as older portions of the thallus begin to senesce in na-
ture, can grow into clonal gametophyte individuals. This type of
growth can be observed in species of the Polypodiaceae (Chiou
and Farrar 1997), Dryopteridaceae (Chiou et al. 1998), Pla-
giogyriaceae, and Cyatheaceae (Atkinson and Stokey 1964). An-
other type of proliferative growth is the production of fila-
mentous strands from otherwise cordate thalli that eventually
broaden to form new adult gametophytes, which can be ob-
served in the genus Asplenium (Testo and Watkins 2011) and
the species Stegnogramma burksiorum (J.E. Watkins & Farrar)
Weakley (Watkins and Farrar 2002). Distinct fromboth of these
types, and perhaps the most well-studied form of asexual re-
production in fern gametophytes, are the vegetative propagules
termed gemmae.
The production of gemmae by fern gametophytes is rela-
tively rare and is predominantly restricted to gametophytes
with strap- and ribbon-shaped morphologies. In groups with
these morphologies, some species have the ability to produce
gemmae along the margins of the thallus, which occurs most
commonly in the grammitid (Polypodiaceae; Stokey and At-
kinson 1958) and vittarioid (Pteridaceae; Goebel 1888) ferns,
as well as in some filmy ferns (Hymenophyllaceae; Bower,
1888). Gemmae add a new dimension to the independence of
long-lived fern gametophytes, allowing them to produce ex-
tensive colonies of clones and aiding in short-distance dispersal.
Outside of these groups, the fern genera Ophioglossum and
Psilotum, both eusporangiate ferns (fig. 2), have been found to
Fig. 5 Lomariopsis lineata (Lomariopsidaceae). Illustrated by Si-
mon Parsons.
Fig. 6 Didymoglossum petersii (Hymenophyllaceae) with a mag-
nified illustration of the uniseriate geometry of the cells. Illustrated by
Simon Parsons.
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produce gemmae in subterranean portions of sporophytes,
gametophytes, or both (Bierhorst 1971; Farrar and Johnson-
Groh 1990). The in vitro production of gemmae has also been
observed in cultured gametophytes of Osmunda regalis L.
(Fernández 1997; Magrini and Scoppola 2012).
Developmentally, gemmae are often borne on short stalks
called gemmifers (fig. 3), which grow directly from the margin
and the surface of the gametophyte (Chen et al. 2013b). The
size of the gemmifer varies among families and species, from
as little as one cell in the vittarioids (Farrar 1974) to four to
six cells in Trichomanes alatum Sw. (Bower 1888). In Hymeno-
phyllum eximium Kunze, gemmifers do not appear to be pres-
ent, and gemmae are instead produced from a protruding cell
(Goebel 1888). Additionally, the number of gemmae produced
per gametophyte varies highly among species, as does their ar-
rangement and the number of cells per individual gemma.
Gemmae grow linearly from the gemmifer and may occur in
pairs (one attached to the other), as in Vittaria graminifolia
Kaulf. (fig. 3) and Vittaria lineata (L.) Sm., or as solitary prop-
agules, as in Radiovittaria stipitata (Kunze) E.H. Crane (Farrar
1974). Gemmifers can hold from one to multiple sets of gemmae
or gemma pairs. The gemmae can be composed of anywhere
from two to 16 cells in different species, and they generally have
two smaller terminal cells that act as the rhizoidal primordia,
which can contain chlorophyll or starch (Farrar 1974). Once
the gemmae are mature, an abscission layer forms and the prop-
agules naturally detach. From there, they can either develop into
new genetically identical gametophytes, or, in the presence of a
gibberellin-like hormone that promotes the development of an-
theridia, called antheridiogen, they can begin to directly grow
antheridia, theoretically increasing the chances for outcrossing
(Farrar 1974; Emigh and Farrar 1977).
Gemmae range in size from about 0.2 to 1.0 mm in length
(Farrar 1990) and are therefore relatively large compared with
spores, which in homosporous ferns range from 15 to 150 mm
(Tryon and Lugardon 1991). This makes long-distance dis-
persal of gemmae unlikely. Field studies of the gametophyte-
only fern Vittaria appalachiana have shown that, when trans-
planted beyond its northern range boundary, gametophytes of
this species are capable of surviving, suggesting that dispersal
ability defines the northern range limit of this species (Stevens
and Emery 2015). Vittaria appalachiana is also absent from re-
cently anthropogenically disturbed sites, such as rockfaces that
have been cut for roads and tunnels, suggesting slow rates of
dispersal and establishment (Farrar 1990).
Despite evidence suggesting that the long-distance dispersal
capabilities of gemmae are limited, these propagules are quite
effective dispersers over short distances. For example, V. appa-
lachiana frequently occurs in dense, clonal populations within
the rockhouses it inhabits. Abiotic factors, such as water, may
act as dispersal agents to some extent, and Farrar (1985) postu-
lated that animals might aid in dispersal as well. This has been
demonstrated to be true for bryophytes, with slugs (Kimmerer
and Young 1995) and potentially ants (Rudolphi 2009) dis-
persing various vegetative propagules, but these types of stud-
ies, even for bryophytes, are few and far between. To gain a
better understanding of how gametophyte-only ferns obtained
their current distributions, further investigations into the dis-
persal ability of fern gemmae will be needed. Currently, the as-
sumption is that the distributions of widespread gametophyte-
only taxa are the result of ancient spore dispersal that occurred
before these ferns lost their sporophyte counterparts (Farrar
2006; see “Drivers of Geographic Separation”below).
Biogeographic Patterns: The Separation of Generations
We are aware of 25 fern species in which gametophytes per-
sist indefinitely in the absence of conspecific sporophytes (ta-
ble 1), and this number is increasing rapidly. In some cases,
gametophytes and conspecific sporophytes may cover the same
geographic range, but independent gametophyte populations
can be observed in microhabitats that appear to be unsuitable
for sporophyte production, likely due to fine-scale environmen-
tal differences or the inability of a gametophyte to self fertilize.
Alternatively, gametophyte populations may exist allopatrically,
apart from conspecific sporophytes and as extensions of the
species’geographic range. At the extreme, sporophytes may
not exist at all.
In the sections below, the 25 species are organized broadly
into areas of occurrence, beginning in eastern North America
and proceeding westward to western North America, the Pa-
cific Islands, Asia, and ending in Europe. The greater number
of species in temperate zones likely reflects sampling bias—
most fern species capable of producing long-lived gametophytes
occur in families typically found in the tropics, and recent stud-
ies suggest that many additional independent gametophytes
await discovery in tropical areas.
Eastern North America
Temperate regions have by far the most thoroughly studied
populations of long-lived gametophytes, due largely to the ef-
forts of Donald Farrar, Frederick Rumsey, and Elizabeth Shef-
field. To date, we are aware of eight fern species, belonging to
five families (table 1), showing the spatial separation of game-
tophyte and sporophyte generations in eastern North America:
Hymenophyllaceae: Crepidomanes intricatum (Farrar) Ebihara
and Weakley, Didymoglossum petersii (A. Gray) Copel., and
Hymenophyllum tayloriae Farrar and Raine; Lomariopsidaceae:
Lomariopsis kunzeana (Underw.) Holttum; Polypodiaceae: Mo-
ranopteris nimbata (Jenman) Proctor; and Pteridaceae: Vittaria
graminifolia Kaulfuss and Vittaria appalachiana Farrar and
Mickel.
Several of the species listed above have geographic ranges
that extend, at least in part, into the Appalachian Mountains
and Plateau, where they can often be found growing in scat-
tered rock outcrops, which are the eroded remains of the Cre-
taceous uplift of the Plateau (Miller and Duddy 1989). Known
colloquially as “rockhouses”or “rock shelters,”these outcrops
generate environmental conditions characterized by extremely
low light levels (0–5.99 mmol m
22
s
21
; S. Chambers, unpublished
data) and high relative humidity (85%–95%; Chambers and
Emery 2016). One unique characteristic of this habitat is its abil-
ity to buffer seasonal and daily temperature variation, creating
warmer conditions in the winter, cooler conditions in the sum-
mer, and generally a more stable thermal environment relative
to surrounding conditions (Farrar 1998; Chambers and Emery
2016).
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Although the species in eastern North America occupy sim-
ilar habitats, there is disparity in the size of their ranges. For
example, gametophytes of V. graminifolia are currently known
from two locations: St. Helena Parish in Louisiana (Farrar and
Landry 1987) and Broxton Rocks Preserve in southern Georgia
(J. B. Pinson and S. M. Chambers, unpublished data), while
Moranopteris nimbata is found in only a single site in Macon
County, North Carolina (Farrar 1990). In contrast, the two spe-
cies with the most extensive ranges in North America, C. intri-
catum and V. appalachiana, both grow throughout the Appala-
chian Mountains, with similar habitat specificity and almost
identical geographic ranges. Below, we summarize what is cur-
rently known about each of these species.
Hymenophyllaceae
1. Crepidomanes intricatum (fig. 1C–1E) completely lacks a
sporophyte and is by far the most common of the Hymeno-
phyllaceae in the continental United States. This species can
be found throughout much of the Appalachian Mountains
and Plateau, from Georgia and Alabama, through parts of Il-
linois and Indiana, and as far north as New England and Ver-
mont. The gametophytes are filamentous, and given that their
appearance is similar to that of algae, the first formal descrip-
tion of this species was actually included in an 1887 book on
the freshwater algae of North America (Wolle 1887). The au-
thor correctly theorized, however, that it might be a fern on
the basis of a few scant but defining features that separate it
from filamentous algae (Farrar 1992). Originally described
as Trichomanes intricatum Farrar (Farrar 1992), it was un-
known whether sporophytes of this species existed at all. In
2008, however, a team of investigators discovered that T. in-
tricatum had the exact same rbcL sequence as an accession of
the Asian fern Crepidomanes schmidtianum and differed in
only one base pair for a second accession, indicating a rel-
atively recent formation of this species (Ebihara et al. 2008).
Accordingly, the species has been transferred to Crepidomanes
(Weakley et al. 2011).
There are a number of plausible hypotheses for how this
species formed and came to occupy its current distribution.
In one scenario, long-distance dispersal of C. schmidtianum
from Asia to eastern North America initially gave rise to pop-
ulations with the normal alternation of generations (Ebihara
et al. 2008). Then, in response to glacial expansion, this spe-
cies lost its sporophytic generation and retreated into rock
shelters, resulting in the current distribution and allowing for
reproductive isolation that promoted speciation. Crepidoma-
nes intricatum occupies habitat that extends well north of the
Pleistocene glaciation boundary, so if a fertile sporophyte was
responsible for its current distribution, that sporophyte must
only recently have gone extinct as well (!12,000 yr ago), as-
suming poor dispersal ability of gametophyte-only popula-
tions. It is also possible that C. intricatum and C. schmidtianum
are the product of separate hybridization events involving the
same maternal progenitor or that one gave rise to the other
via hybridization, which would explain the matching rbcL se-
quences. Efforts to determine the population structure of C.
intricatum are currently under way at Utah State University
(A. Duffy, personal communication), which may help to deter-
mine its origins.
2. Didymoglossum petersii (fig. 6) grows mostly in the Neo-
tropics, including Guatemala, Honduras, Costa Rica, and Nic-
aragua (Mickel and Smith 2004), but it has several populations
in eastern North America as well in the southern Appalachians,
with additional small populations in Florida and Louisiana
(Farrar 1993a). Gametophytes are filamentous and can be found
growing independently of sporophytes in certain parts of its
range in North America, such as at Broxton Rocks Preserve
in Georgia (J. B. Pinson and S. M. Chambers, unpublished data),
as well as at several sites in Arkansas, where gametophytes were
documented growing up to 50 km from the nearest sporophyte
population (Farrar 1992; Bray 1996). This species is both epi-
phytic and epipetric; in Louisiana, gametophytes and sporo-
phytes were observed growing on the trunks of beech (Fagus
grandifolia Ehrh) and magnolia (Magnolia grandiflora L.) trees
(Allen 1975; Farrar and Landry 1987). Despite its wide range,
very little work has been done on this species.
3. Hymenophyllum tayloriae (fig. 1F) is endemic to the
United States and is extremely rare, occurring in only a few
counties in North Carolina and Alabama (Farrar 1998). This
species was first reported in 1936, when Mary Taylor collected
a small juvenile fern sporophyte in Pickens County, South
Carolina. The identity of the collection remained somewhat in-
tractable, as it was originally identified as Hymenophyllum
hirsutum (L.) Sw., but on closer examination, it was determined
to be a new species. It would be another 55 yr, however, until it
was given the specific epithet of tayloriae, commemorating its
initial collector (Raine et al. 1991). The gametophytes are rib-
bon shaped and produce copious amounts of gemmae, but ju-
venile sporophytes are rare, and, to date, a mature sporophyte
for this species has never been observed. The single juvenile
sporophyte collected by Taylor in 1936 was the only observed
occurrence of a sporophyte until 1993, when several were dis-
covered at a single site in Alabama by the bryologist Paul
Davison. About 50 sporophytes were found at this location, all
in microhabitats of small crevices in sandstone rock outcrop-
pings (Farrar and Davison 1994). All of these sporophytes were
juvenile, however, and it is unknown whether they ever matured
or produced spores.
Lomariopsidaceae
4. Lomariopsis kunzeana grows on the tropical islands of
Cuba and Hispaniola but is also present in small numbers
in solution holes throughout southern Florida, where it is con-
sidered endangered (Gann et al. 2006). In some of these solu-
tion holes, L. kunzeana has been observed growing as both a
sporophyte and ribbon-shaped gametophytes, but in others, it
has been observed growing only as independent gametophytes
(Possley et al. 2013). Solution holes are formed when water
collects at the surface of carbonate rock and subsequently
percolates down to subterranean aquifers, dissolving the rock
as it goes (Ford and Williams 2007). These depressions can
vary in size and depth, some spanning several meters in width
and/or reaching below the water table (Kobza et al. 2004).
The differences in depth among sinkholes and solution holes
likely produce differing fine-scale environmental conditions,
which may drive the lack of sporophyte production in certain
depressions. More study of this phenomenon and of its effects
on fern distributions in these microhabitats is needed.
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Polypodiaceae
5. Moranopteris nimbata has strap-shaped gametophytes
(Farrar 1967). The species is generally found in the tropics of
the Caribbean, including the islands of Cuba, Hispaniola,
and Jamaica (Smith 1993), but a single population at a single
site in North Carolina was discovered in 1966 (Farrar 1967,
1971, 1985). This population was growing epipetrically and
intermixed with bryophytes about 800 mi from the nearest
documented occurrence of the species in the tropics. Unlike
other long-lived gametophytes endemic to the Appalachians,
M. nimbata was growing where it received continuous spray
from a waterfall. Although sporophytes have been observed
growing in this North Carolina population, the species is
considered to be an independent gametophyte, because the
dozen or so sporophytes observed at the site over the course
of 4 yr were all juvenile and infertile, whereas the gameto-
phytes were present in copious numbers. Long-distance dis-
persal, possibly of even just a single spore, is the likeliest ex-
planation to account for the occurrence of this species in the
United States (Farrar 1967). A recent attempt to observe the
species at the same location was unsuccessful (F. W. Li, per-
sonal communication); thus, it is unclear whether the popu-
lation still exists.
Pteridaceae
6. Vittaria graminifolia (fig. 3) is common in the Neotrop-
ics, growing in several Central and South American countries
(Mickel and Smith 2004). In the United States, it can be found
as both independent, ribbon-shaped gametophytes and sporo-
phytes. Edgar Wherry (1964) first reported V. graminifolia as
occurring in the United States, but the occurrence was based
on a misidentified collection of sporophyte tissue (Gastony
1980; Farrar 1993b). Farrar and Landry (1987) would later
find independent gametophytes of V. graminifolia, as deter-
mined by enzyme electrophoresis, growing in a single county
in Louisiana on beech and magnolia trees. More recently,
Frankie Snow and Carl Taylor found specimens of V. grami-
nifolia in Broxton Rocks Preserve, Georgia (J. B. Pinson and
S. M. Chambers, unpublished data). The specimens here were
locally abundant but restricted to small, moist, shaded sand-
stone outcrops of the Altamaha Grit formation (Edwards et al.
2013). A few sporophytes were found at this site, but large
gametophyte populations without sporophytes could also be
seen. These are the only two confirmed recordings of V.
graminifolia in the United States. It is unknown how long the
population has existed at Broxton Rocks, but both instances
in the United States are likely the result of long-distance dis-
persal. Thespecies has reported chromosome counts of 2np120
and 2np240,and both appear to be fertile (Gastony 1977; Smith
and Mickel 1977).
7. Vittaria appalachiana (fig. 1G) is the independent game-
tophyte with the second-largest distribution after C. intri-
catum in the eastern United States, occurring in 12 states,
from Alabama to New York (Farrar 1993b). Gametophytes
are ribbon shaped (Farrar and Mickel 1991), and mature
sporophytes have never been observed for this species, although
Farrar (1978) found three juvenile sporophytes (!1 cm tall) in
Jackson County, Ohio. These seemed to have been produced
apogamously due to the presence of budding protrusions on
other gametophytes at the same location that lacked gametan-
gia. Similarly, Alma G. Stokey grew gametophytes in culture
that produced apogamous growth, but most of these develop-
ing sporophytes died before even producing vascular tissue.
The largest of these, according to her notes, was less than a
centimeter tall (Farrar 1978). Because the sporophytes died,
she never published this work. Caponetti et al. (1982) had sim-
ilar results, and their cultures produced six sporophytes (all
!1 cm) before an air conditioner failure killed all of their cul-
tured specimens. Although gametangia and mobile sperm
have been observed (Farrar 1978), it appears that the species
has lost the ability to produce sporophytes via fertilization or
apogamy.
Recent work has focused on elucidating the origin of this
species. Farrar (1990) found fixed heterozygosity at several
allozyme loci, which could be interpreted as evidence of hy-
bridization in the ancestry of V. appalachiana. Pinson and
Schuettpelz (2016) subsequently tested for hybridization using
a suite of plastid markers and the nuclear marker det1. They
found that, rather than grouping with two putative parental
species, as would be expected in the case of hybridization,
the alleles for V. appalachiana all nested within a clade con-
taining two genetically distinct groups of V. graminifolia,making
the later paraphyletic. These results suggest that either the two
species are conspecificorV. graminifolia sensu lato is actually
composed of two separate species. Vittaria appalachiana is a
polyploid (np120; Gastony 1977), and the molecular results
suggest that it is likely of autopolyploid origin.
Farrar (1990) also used allozymes to uncover population
structure within V. appalachiana. For the gene PGI, six geno-
types were recovered from a total of 92 populations located in
seven states. Of those, only three states (Alabama, Ohio, and
North Carolina) had multiple genotypes, and the remaining
four states each contained only a single genotype among all
sampled populations. When comparing multilocus genotypes
in the same study, the same relative pattern of diversity was
recovered in Ohio and Alabama, and homogeneity was found
elsewhere. This suggests that Ohio and Alabama may have
been the center of diversity from which gametophyte popula-
tions were established in outlying areas. Alternatively, V. ap-
palachiana may have been equally diverse throughout the Ap-
palachian Mountains, but certain genotypes and alleles became
extirpated due to bottlenecking events (Farrar 1990). Since ge-
netic drift and bottlenecking can have an inordinate effect on
asexual populations, this seems to be a plausible hypothesis for
many of the species with independent fern gametophytes.
The northern range limit of V. appalachiana is in the south-
western portion of New York (Farrar 1978). This range cor-
responds with the southern limit of the Illinoian glacial bound-
ary, the last of the major glacial incursions of the Pleistocene.
Stevens and Emery (2015) recently determined that transplanted
individuals from populations throughout the geographic range
of the species could successfully survive in latitudes to the north
of contemporary range limits. This suggests that dispersal limi-
tation drives the geographic range boundary of the species and
that the contemporary boundary reflects historical limitations
established during thePleistocene glaciation (Stevens and Emery
2015). Additionally, the previously discussed C. intricatum has
very similar habitat requirements to that of V. appalachiana,
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and its distribution extends well beyond the glacial boundary,
into Vermont (Farrar 1992). This indicates that neither tem-
perature limitation nor lack of suitable available habitat is the
cause of V. appalachiana’s truncated range; rather, the species
is physically unable to migrate into more northern areas where
suitable habitat exists. By proxy, this further suggests that V.
appalachiana lost the ability to produce mature sporophytes
before the last glaciation event, whereas C. intricatum lost this
ability sometime after the glaciers receded and after it had al-
ready extended its range northward (Farrar 1978).
Western North America
8. Hymenophyllum wrightii Bosch (Hymenophyllaceae) has
ribbon-shaped gametophytes and is the only known fern spe-
cies in western North America that displays the pattern of
spatially separated generations (fig. 1H–1J). First collected as
a sporophyte by Herman Persson on the Queen Charlotte Is-
lands (who published the record “almost as a footnote”in
1958), H. wrightii had previously been known only from Asia
(Persson 1958; Taylor 1967). Although Persson had obtained
a sporophyte specimen, it became apparent through later col-
lections that sporophyte production was rare. Duffy et al.
(2015) recently collected specimens from several populations
and found that the gametophytes have a wide distribution,
from Washington State up through parts of British Colombia
and Alaska; however, documented occurrences of the sporo-
phyte have only ever been reported from the Queen Charlotte
Islands. Genetic analysis of the gametophyte samples collected
by Duffy and colleagues showed no variation between popula-
tions at two plastid loci (rbcL and rps4-trnS) but showed two to
three nucleotide differences from Asian H. wrightii accessions.
Sporophytes from the QueenCharlotte Islands were not included
in that analysis.
Besides growing much farther north than the independent
gametophytes in eastern North America, H. wrightii also has
slightly different habitat requirements. It can be found grow-
ing in shady areas of moist cliff faces, but gametophytes are
also commonly found growing epiphytically on living conifers,
around the tree base or on exposed roots. It can also be found
growing on decaying plant matter, especially in the dark re-
cesses of fallen trees. Given the exposed nature of these hab-
itats and lack of consistent water availability, H. wrightii is
found only within about a 1.5-km distance from the Pacific,
where maritime weather keeps humidity high and temperatures
moderated (Duffy et al. 2015). The gametophytes are ribbon
shaped and produce copious gemmae. This, combined with
the rare occurrence of sporophytes, indicates that individual
populations of independent gametophytes are likely sustained
by vegetative reproduction. Since they can be found growing
on isolated logs and trees, this suggests that the gemmae may
be relatively successful short-distance dispersers, and H. wrightii
perhaps represents an ideal system for the study of gemma dis-
persal.
Pacific Islands
The Pacific Islands, along with the Paleo- and Neotropics,
likely harbor large numbers of independent fern gametophytes,
but researchers are just now beginning to survey in these areas.
In this section, we report on five species that have independent
populations of gametophytes. The only currently available data
come from the Hawaiian Islands and French Polynesia.
Hymenophyllaceae
9. Callistopteris baldwinii Copel. is a terrestrial filmy fern
with gemmiferous, ribbon-shaped gametophytes that displays
a trend of sporophyte reduction and loss along elevational
gradients. On tropical oceanic islands, rainfall generally in-
creases with increasing elevation, a trend caused by the oro-
graphic uplift of trade winds (Loope and Giambelluca 1998).
As a result, cloud forest ecosystems are common at higher
elevations. The sporophytes of this species are highly sensitive
to desiccation and appear to be restricted to these cloud forests
(Dassler and Farrar 1997). Sporophytes are rarely observed at
lower elevations, and dwarfed sporophytes observed at mid-
elevation in Hawai‘i were not mature (Dassler and Farrar
1997). In contrast, gametophytes are present at all elevations.
At high elevation, where fog cover and rainfall are consistently
high, gametophytes were observed growing abundantly on a
number of substrates, including rocks, fallen logs, living tree
trunks, and soil banks (Dassler and Farrar 1997). Near sea
level, the gametophytes appeared to be restricted to vertical
rocks and soil banks, forming large, thick mats, presumably
maintained primarily by dispersal of gemmae, as sporophytes
were absent. Dassler and Farrar (1997) determined that gam-
etangia were present in sufficient numbers to sustain sexual re-
production, and embryos were found at all elevations in Ha-
wai‘i. It would therefore seem that sporophyte production
and development are limited by the death of either embryos
or juvenile sporophytes in drier conditions.
10. Callistopteris apiifolia (C. Presl) Copel. (fig. 1K; see also
“Asia”), a species currently considered to be closely related to
C. baldwinii, has also been observed by Nitta and colleagues
(Nitta et al., forthcoming) to show a pattern of reduced spo-
rophyte production along an altitudinal gradient on the islands
of Mo‘orea and Tahiti, French Polynesia, with ribbon-shaped
gametophytes distributed over a wide range of elevations but
sporophytes confined to moist cloud forest habitats at high el-
evation. Furthermore, Nitta and colleagues ( J. H. Nitta, J. E.
Watkins, N. M. Holbrook, R. Taputuarai, T. Wang, C. C.
Davis, unpublished data) tested the ability of C. apiifolia game-
tophytes to withstand desiccation and found that they were no
more tolerant than sporophytes, suggesting that these game-
tophytes are exploiting protected microhabitats rather than rely-
ing on desiccation tolerance to survive beyond the range of spo-
rophytes.
11. Hymenophyllum recurvum Gaudich. is a species endemic
to the Hawaiian Islands that has ribbon-shaped gametophytes,
which can be found growing independently in honeycomb-like
indentations on the surfaces of boulders deposited by basaltic
lava flows. Within these hexagonal pockets, populations are
often found without any associated sporophytes (D. R. Farrar,
personal communication). Yet both gametophytes and sporo-
phytes of this species can be found growing sympatrically
throughout the islands in epiphytic communities. Although
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the two generations overall are geographically sympatric, the
occurrence of independent gametophyte populations in a rad-
ically different habitat than that of sporophytes underscores
the potential for ecological differences between the generations
and the sometimes-narrow environmental conditions required
for sporophyte production. It is also possible that the game-
tophytes growing in the lava flow crevices rarely receive the
free water necessary for fertilization, which may explain the
lack of sporophytes in that environment.
12. Vandenboschia cyrtotheca Copel. grows as both game-
tophytes and sporophytes at high elevations on the Ko‘olau
Mountains on the eastern side of the island of O‘ahu. This
species appears to exhibit a similar elevation-dependent distri-
bution as C. baldwinii. Farrar conducted an isozyme analysis
on gametophyte populations collected from the Waianae Moun-
tains in eastern O‘ahu and determined that independent game-
tophytes found at lower elevations also belonged to V. cyrto-
theca (D. R. Farrar, personal communication). No further study
of this species has been conducted.
Pteridaceae
13. Vaginularia paradoxa (Fée) Mett. (pMonogramma
paradoxa (Fée.) Bedd.) is a small vittarioid fern distributed
primarily in the South Pacific with ribbon-shaped gameto-
phytes (fig. 1L). Nitta and colleagues (Nitta et al., forthcom-
ing) used DNA bar codes (short diagnostic DNA sequences
that can discriminate between species; Hebert et al. 2003) to
compare ranges of sporophytes and gametophytes along an
elevational gradient from 200 to 2000 m on the islands of
Mo‘orea and Tahiti, French Polynesia. They found over 20 pop-
ulations of V. paradoxa gametophytes growing on trees or rocks
from ca. 200 to 600 m but never observed any sporophytes of
this species, despite intense effort. Sporophytes of V. paradoxa
are apparently extremely rare in Tahiti and Mo‘orea, having
been collected only a handful of times (only three specimens
from Mo‘orea, all from the 1850s, and nine specimens from
Tahiti, with the most recent collection in the 1980s at P). Ad-
ditional gametophyte populations have also been discovered
on other islands in the Society Islands from which sporophytes
of V. paradoxa have never been recorded ( J. H. Nitta, unpub-
lished data). As in other independent vittarioid gametophytes,
V. paradoxa produces copious gemmae and forms clonal mats
up to several square centimeters. It is unknown how it main-
tains these large, frequent populations despite sporophytes be-
ing extremely rare or lacking sympatrically, although it is pos-
sible that long-distance dispersal from other islands plays a key
role.
Asia
Asia has been the focus of recent efforts to find and identify
populations of independent gametophytes, and several have
been discovered in the last few years. Similar to the study on
V. paradoxa conducted on the islands of Mo‘orea and Tahiti
described above, recent studies in Japan and Taiwan have also
employed DNA bar coding to conduct surveys of local game-
tophyte populations and identify them to species without hav-
ing to rely on morphological characters (Ebihara et al. 2010,
2013; Chen et al. 2013b; Kuo et al., forthcoming). There are
currently 12 known instances of independent gametophyte pop-
ulations that grow at least 20 km (and more often 1100 km)
away from any known populations of sporophytes. Unfortu-
nately, some of these cannot be identified to species or even ge-
nus due to a lack of genetic studies. In addition, although many
populations have been identified, little additional research on
their ecology or natural history has been conducted; thus,
we know relatively little about these ferns’habitat require-
ments and whether they are able to produce even juvenile
sporophytes. Below, we describe what is known about these
species’ranges and occurrence.
Hymenophyllaceae
14. Hymenophyllum badium (Hook. and Grev.) is an epi-
phytic fern with ribbon-shaped gametophytes distributed
throughout southeast Asia, southern China, Vietnam, Taiwan,
and Japan. In Japan, it occurs primarily in the southeastern
half of the country. Ebihara et al. (2013) conducted a DNA
bar code survey of gametophytes at eight sites throughout Ja-
pan and compared these with sporophyte records. Gameto-
phytes of H. badium were found growing in Saitama Prefec-
ture, Japan, 100 km from the closest-known sporophyte. These
gametophytes were growing beyond the northern limit of the
sporophyte in Japan, which is itself the northern edge of the
geographic distribution of this species.
(10). Callistopteris apiifolia (fig. 1K) was observed as game-
tophyte populations without nearby sporophytes on the is-
land of Iriomote by Ebihara et al. (2013). The sporophyte
of this species is extremely rare in Japan and is considered en-
dangered. The species is primarily distributed on islands in the
tropical Pacific and in Southeast Asia (see Pacific Islands), and
it is possible that subtropical Iriomote represents the northern
limit of the sporophyte. That the gametophytes can still thrive
there may be another example of fern gametophytes able to
live at or beyond the northern range boundaries of their related
sporophytes.
15, 16. In addition to C. apiifolia, gametophytes of two
more filmy fern species were observed growing on Iriomote
by Ebihara et al. (2013) that did not match any known fern
species from Japan for the DNA bar code marker rbcL. Ef-
forts are ongoing to locate sporophyte matches for these taxa,
should they exist (A. Ebihara, personal communication).
17. Vandenboschia kalamocarpa (Hayata) Ebihara is a mem-
ber of the Vandenboschia radicans species complex in Japan.
Ebihara et al. (2009) investigated a single site in Shizuoka Pre-
fecture where three hybrid sporophytes in this complex oc-
curred: diploid Vandenboschia #stenosiphon (H. Christ)
Copel. (genotype ab), triploid Vandenboschia #quelpaertensis
(Nakai) Ebihara (genotype aag or agg), and tetraploid Van-
denboschia orientalis (C. Chr.) Ching (genotype aagg). Of
these, only V. orientalis is fertile, so it was unclear how the hy-
brid taxa were being produced at the site. By carefully sam-
pling and sequencing filamentous gametophytes found at the
site, Ebihara et al. (2009) discovered haploid V. kalamocarpa
(genotype a) gametophytes, which they inferred were contrib-
uting the atype genome to the hybrids but were themselves
incapable of producing nonhybrid sporophytes. The closest
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known location of V. kalamocarpa sporophytes is the Izu
Islands, located 50 km off the coast of Japan. Ebihara and col-
leagues speculated that this species was able to contribute to
hybrid formation but was unable to produce nonhybrid sporo-
phytes due to differences in environmental conditions on the
Izu Islands compared with Honshu. To our knowledge, this
is the only documented example of an independent gameto-
phyte contributing to the formation of hybrid species outside
the range of conspecific, nonhybrid sporophytes.
Pteridaceae
18, 19, 20. Antrophyum henryi Hieron., Antrophym par-
vulum Blume, and Haplopteris heterophylla C. W. Chen,
Y. H. Chang and Y. C. Liu (fig. 1M). Chen et al. (2013a,
2013b) also used DNA bar coding to identify field-collected
gametophytes to species, but they focused on vittarioid ferns
in Taiwan. They found gametophytes of these three species
growing at least 20 km away from the nearest known conspe-
cific sporophytes and in different elevations or habitats than
typically observed for the sporophytes.
21, 22. Haplopteris species. In another study in Taiwan us-
ing DNA bar codes, Kuo and colleagues (L.-Y. Kuo, C.-W.
Chen, W. Shinohara, A. Ebihara, H. Kudoh, H. Sato, Y.-M.
Huang, W.-L. Chiou, unpublished data) comprehensively sam-
pled gametophytes every 2 mo for a year at a single site in the
Fushan area (northern Taiwan). They found that species rich-
ness of gametophyte populations changed over the course of
the year and exceeded that of sympatric mature sporophytes
by a factor of 2–3 overall. Furthermore, they documented two
species of Haplopteris with independent gametophyte popula-
tions at this site. The first lacked gametangia and is likely con-
specific with Monogramma (Haplopteris)capillaris Copel., a
species with a distribution in Southeast Asia, owing to their
near-identical (199%) chloroplast DNA (cpDNA) sequences
(chlL 1matK 1ndhF). Two additional populations were dis-
covered of the second Haplopteris species, one in northern
Taiwan and one in Japan (Yakushima). At one of the sites
in northern Taiwan, the authors observed both male and female
gametangia along with juvenile sporophytes. In the other two
populations, however, gametangia and juveniles were absent.
Interestingly, both the gametophytes and juvenile sporophytes
in the former population were epiphytic but were observed to
be epipetric or terrestrial at the latter two populations, implying
that microhabitat conditions may have an effect on the produc-
tion of gametangia and/or sporophytes. It is currently unknown
whether this second species has a mature sporophyte counter-
part, although the occurrence of distant populations in Taiwan
and Japan (and divergence times of less than 0.5 mya) suggests
recent spore dispersal. Further population genetic studies are
needed to shed light on their geographic origin and phylogenetic
relationships.
Lomariopsidaceae
23. Lomariopsis lineata (C. Presl) Holttum (fig. 5) is a spe-
cies that was originally thought to be a liverwort, and its iden-
tification as a fern gametophyte is atypical, since independent
gametophyte populations of this species are known only from
the aquarium trade. In 2001, Christel Kasselmann, an avid
aquarium designer, propagated the plants and shared the prop-
agules with other aquarium enthusiasts, after which it be-
came popular in the aquarium market under the name Süß-
wassertang, German for “freshwater seaweed”(Kasselmann
2010). Li et al. (2009) sequenced several plastid loci for the
species, resolving it as being most closely related to L. lineata
(Holttum), an epiphytic fern that grows in several countries in
Asia (Barcelona et al. 2006; Ke et al. 2013). Little is known
about this species, and to date, we are unaware of any record
in which the gametophytes of L. lineata have been observed
naturally growing in aquatic environments. Kasselmann (2010),
however, reported that specimens have been found growing
on rocks in seasonally dry riverbeds. During the time that it
has been in the aquatic trade, there are no reports of sporo-
phyte production, even though archegonia and antheridia have
been observed. It is currently unknown whether the ornamen-
tal represents an independent occurrence of L. lineata game-
tophytes or whether it represents a unique species (Li et al.
2009). The gametophytes lack gemmae, which is likely why
they have not been observed independently in nature. Because
there was only one known introduction by Kasselmann into the
aquatic trade, it can be reasonably assumed that all of the
plants currently being sold are exact clones of each other and
have reproduced entirely by continuous meristematic growth.
24. Gametophytes belonging to the genus Lomariopsis sp.
were discovered by Ebihara et al. (2013) on Iriomote Island.
Genetic analysis revealed that the species failed to match rbcL
sequences from any Lomariopsis known from Japan, indicat-
ing its potential as an independent gametophyte. Current ef-
forts are focusing on sampling a broader range of Lomariop-
sis from Asia to try to find a match for this species. Thus, a
total of four apparently gametophyte-only species (C. apii-
folia, the two unidentified filmy ferns, and Lomariopsis sp.)
have been documented on Iriomote Island, again suggesting that
its location as the northern limit for many tropical taxa and its
isolated nature could promote occurrence of gametophyte-only
populations.
Europe
Hymenophyllaceae
25. Vandenboschia speciosa (Willd.) Kunkel (Hymenophyl-
laceae), known colloquially as the Killarney fern, has filamen-
tous gametophytes and is the only fern in Europe known to
display the pattern of spatially separated generations. First
collected in 1724, populations of V. speciosa may have suf-
fered severe depletion as avid collectors scoured the country-
side for ferns during the Victorian fern craze (∼1850–1890;
Allen 1969; Rumsey et al. 1998a, 1998b; Whittingham 2009).
The last reported observation of sporophytes at the site of its
initial collection was in 1785, the area having undergone ex-
tensive disturbance. Farrar, however, returned to this location
in 1989 and found a gametophyte population still thriving. He
had predicted that gametophytes of V. speciosa might persist
in the absence of sporophytes and was the first to discover
such independent colonies (Rumsey et al. 1998b). The species
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is primarily located in eastern Europe (e.g., Poland) and several
Mediterranean islands (Rumsey et al. 1998b), but populations
also occur in central and northern Europe (Rasbach et al.
1993; Vogel et al. 1993; Krukowski and Swierkosz 2004).
The sporophytes, however, appear to be mainly restricted to
the wetter and warmer climates in the south and are sparse
to effectively nonexistent in the drier portions of central Eu-
rope (Rumsey et al. 1998b; Krippel 2001). Additionally, many
of the sporophytes that are produced in northern Europe, par-
ticularly in Great Britain and Ireland, never reach maturity or
produce spores (Ratcliffe et al. 1993).
Two cpDNA (trnL) haplotypes can be found among V. spe-
ciosa populations, which correspond to different geographical
regions, the first occurring in the Azores and the second prev-
alent in other Mediterranean islands, from Madeira to the Ca-
naries (Rumsey et al. 1996). These islands are the only places
where populations of V. speciosa are abundant and regularly
complete their life cycle, and therefore, presumably, the pop-
ulations on these islands serve as source populations for those
found in Europe. The haplotype present in the Azores can be
found in central Europe, while the haplotype from Madeira
and the Canary Islands corresponds to populations in Spain
and Italy. Both haplotypes, however, can be found in Great
Britain and Ireland (Rumsey et al. 1996).
Further genetic analyses of allozymes conducted by Rumsey
et al. (1999) found a total of seven multilocus polymorphisms
(MLPs) among populations in southwestern Scotland, whereas
complete homogeneity was uncovered in individual popula-
tions, thus suggesting little dispersal among populations. In
Great Britain, where most populations are gametophyte only,
the few observed sporophytes appear to be produced apog-
amously based on identical sporophyte MLPs with surrounding
gametophytes (Rumsey et al. 1999). In Spain, there is evidence
for some dispersal as MLPs are shared among populations.
Additionally, intergametophytic reproduction may occur in this
region as population-specific MLPs also exist (Rumsey et al.
2005). Conversely, in central Europe, the presence of multiple
MLPs among distant populations suggests that sporophytes
likely existed in the region at one time. Long-distance dispersal
to various locations may have established populations after the
Pleistocene glaciations, followed by secondary dispersal to
nearby areas (Rumsey et al. 1998b). Finally, in Italy, a unique
allele of the PGM enzyme locus has also been found for V. spe-
ciosa, which has yet to be detected in any other population,
suggesting that these may be relict Pleistocene populations
(Rumsey et al. 2005).
Drivers of Geographic Separation
Field observations and common-garden studies have shown
that some gametophytes can tolerate a wider range of envi-
ronmental conditions, particularly colder temperatures, com-
pared with their respective sporophyte counterparts (Farrar
1978; Sato and Sakai 1981). Plant responses to desiccation
and cold temperatures invoke a similar metabolic pathway;
thus, plants that can physiologically tolerate cold tempera-
tures are likely tolerant of desiccating environments as well
(Knight and Knight 2001; Sinclair et al. 2013). Tolerance of
these cold temperatures may be one reason that ferns with
temperate distributions can exist without a sporophyte. Spe-
cifically, reductions in temperature associated with Pleistocene
glaciation expansions may have created cold, dry conditions
that the sporophyte could not survive. Farrar (1978) hypoth-
esized that warmer temperatures during the early Tertiary al-
lowed some species of tropical epiphytic ferns to occupy hab-
itats in North America and that subsequent climatic conditions
associated with the Pleistocene glaciations contributed to the
loss of sporophytes in these species. The remaining sporo-
phyteless ferns would therefore be considered ancient relicts of
once flourishing populations. This seems particularly plausible
for Vittaria appalachiana and Crepidomanes intricatum, both
of which have ranges throughout the Appalachian Mountains
and Plateau.
Earth’s climate over the past 50 Myr has, for the most part,
been much warmer than that of the ice-capped planet we are
familiar with today. Even parts of the Pliocene (∼3 Myr ago)
had average global temperatures between 27and 37C higher
than preindustrial temperatures (Jansen et al. 2007), and while
the generic vegetation schemes in eastern North America closely
resembled those seen at present, temperatures were still slightly
elevated compared with those seen today at midlatitudes (Cro-
nin et al. 1994) and along the Atlantic Coastal Plain during
parts of the Pliocene and into the Quaternary (Groot 1991).
With the onset of the glaciers in the Pleistocene, any species
that could not adapt would have relocated to suitable climates
or protected refugia or gone extinct (Davis and Shaw 2001;
Beatty and Provan 2010). For many bryophyte and monilo-
phyte species in the Appalachians, these colder temperatures
may have led to aberrations in the alternation of generations,
where sporophytes and specific sexes are limited to certain
portions of the geographic range or are entirely nonexistent.
In bryophytes, the Appalachians harbor several taxa that ap-
pear to lack one sex or the other; dioicous liverwort genus
Plagiochila has ca. 22 species in the Appalachians, many of
which exist only as a single sex (Longton and Schuster 1983).
In the tropics, however, these species grow as both sexes and un-
dergo the normal alternation of generations (Farrar 1978;
Schuster 1983, 1992). Yet again, this pattern may be driven
by stress tolerance, specifically in response to desiccating envi-
ronments. In the dioicous desert moss, Syntrichia caninervis,
studies have show that sexes respond differently to stressful en-
vironmental conditions generated by low moisture availability
and high light levels,with female plants being significantly more
tolerant than their male counterparts (Stark and McLetchie
2006; Stark et al. 2005). This pattern is also apparent in dioicous
bryophyte taxa in the Appalachians, because there is evidence to
suggest that, in suboptimal conditions, functional archegonia
are produced, but males fail to produce antheridia (Longton
and Schuster 1983; Schuster 1992), a pattern the bryologist
R. M. Schuster termed “sexual regression.”To date, no studies
have been conducted to determine whether this pattern, termed
“spatial segregation of sexes”(Bierzychudek and Eckhart 1988),
occurs in fern gametophytes or whether this may be one reason
for the lack of sporophyte production in disjunct populations.
Although rare in some gametophytes with filamentous mor-
phology (i.e., C. intricatum), long-lived ribbon-shaped game-
tophytes in the Appalachians often retain the ability to produce
gametangia (i.e., V. appalachiana), but sporophytes either are
PINSON ET AL.—THE SEPARATION OF GENERATIONS 13
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found in small numbers or are not produced at all. Following
the hypothesis that these ferns are relictual populations from
the Pleistocene, their inability to produce sporophytes seems
to indicate the presence of selective pressures on the sporo-
phyte generation. For example, it is possible that individuals
that did not allocate resources to the production of a sporo-
phyte that was likely to die had higher survival rates. Thus,
only those individuals with deleterious alleles associated with
sporophyte production survived historical environmental pres-
sures and subsequently generated large populations via asexual
gemmae. To date, there have been no genetic studies attempt-
ing to test this hypothesis.
There are varying degrees of sporophyte loss in some species.
As seen in the Pacific Island species Callistopteris baldwinii,it
is likely that the first stage in sporophyte loss is a reduction in
size. Lomariopsis kunzeana in southern Florida rarely pro-
duces mature sporophytes, but when it does, they are generally
smaller than those produced in Cuba and Hispaniola. Steg-
nogramma burksiorum, endemic to Winston County in Ala-
bama, produces dwarfed sporophytes compared with its sister
species Stegnogramma pilosa (M. Martens & Galeotti) K.
Iwats., which occurs throughout central Mexico (Watkins and
Farrar 2005). In culture studies, S. burksiorum has been noted
above for the ability of its cordate gametophytes to grow thin
filaments that later develop into widened prothalli, a form of
asexual reproduction. If the cold temperatures of the Pleisto-
cene had continued, it is possible that this species may have
lost the ability to produce sporophytes as well.
However, although the Pleistocene relict hypothesis offers a
plausible explanation for the existence of gametophyte-only
species in eastern North America and appears to work well
in cases of dioicous bryophytes, it cannot suitably account
for the majority of fern species with spatially separated gener-
ations. In Europe, for example, the distribution of populations
of Vandenboschia speciosa with and without sporophytes does
not appear to reflect glacial patterns. In the United Kingdom,
there are gametophyte-only populations not far removed from
populations with sporophytes, suggesting that there may be
genetic restrictions underlying the inability of some popula-
tions to produce sporophytes or fine-scale microclimatic con-
ditions that inhibit the production of sporophytes. In west-
ern North America, gametophytes of Hymenophyllum wrightii
can be found in the United States, Canada, and Alaska, but
sporophytes are produced only on the Queen Charlotte Is-
lands, a distribution pattern that cannot be explained by range
expansions/contractions concomitant with glacial movement.
Additionally, many of the independent gametophytes discussed
in this review were discovered in tropical regions that were de-
void of glaciers in the Pleistocene.
It seems that environmental effects may thus be extremely
important in explaining the separation of generations pattern.
To test this hypothesis, transplant and common-garden stud-
ies, similar to those employed by Chambers and Emery (2016),
may be utilized. Specifically, gametophyte explants may be
transplanted into populations in which sporophyte production
is known to occur or placed in simulated environmental con-
ditions (i.e., growth chambers) that reflect natural conditions
associated with sporophyte-bearing populations. Furthermore,
specific environmental factors believed to be driving the lack of
sporophyte production may be isolated in manipulative studies
conducted in growth chambers, thus allowing for the examina-
tion of environmental thresholds to sporophyte production.
It should be noted that there are limits to the establishment
of sporophytes after the dispersal of a single spore to a previ-
ously uncolonized environment or the migration of clonal game-
tophyte propagules, whether by gemmae or portions of the thal-
lus. Not all fern gametophytes are hermaphroditic, and despite
recent evidence that the gametophytes of a large number of fern
species are capable of selfing, many appear to have genetic bar-
riers to the production of a completely homozygous sporophyte
(Sessa et al. 2016). So while gametophytes with strap, ribbon, or
filamentous morphologies are able to produce extensive vege-
tative and clonal growth from a single spore, that clonal game-
tophyte population may not have the capacity to produce
sporophytes, due to an inability to self-fertilize. Currently,
we are aware of a few instances in which gametophytes
translocated from the tropics to greenhouse environments in
North America have failed to produce sporophytes, despite be-
ing established for many years. J. B. Pinson and S. M. Cham-
bers (unpublished data) determined that independent gameto-
phytes that have thrived in the greenhouse of orchid biologist
Mark Whitten (personal communication) for 20 yr without
any known instances of sporophyte production are closely re-
lated to Polytaenium lineatum (Sw.) J. Sm. (96% GenBank
match for rbcL). Additionally, Charles Alford, a fern horticul-
turalist in south Florida, has dense populations of gameto-
phytes closely related to Hecistopteris pumila (Spreng.) J. Sm.
(97% GenBank match for rbcL) growing in his greenhouse as
mats of gametophyte thalli without sporophytes (C. Alford,
personal communication). Both species grow as sporophytes
in Central and South America. Although it has yet to be exper-
imentally tested for these species (or any long-lived gameto-
phyte), it seems likely that the gametophytes in the examples
above are simply unable to produce sporophytes because they
each comprise only a single genotype.
Conclusions and Future Directions
Until recently, fern gametophytes were often thought of as
the Achilles’heel of the fern life cycle (e.g., Page 2002). Game-
tophytes lack stomata and roots, have no vascular tissue, are
mostly one cell layer thick, and require free water for fertiliza-
tion, making them seem ill-equipped to survive in fluctuating en-
vironments. But “natural selection does not tolerate mistakes”
(Farrar, as quoted in Watkins and Cardelús 2012, p. 695), and
researchers have recently begun to discover the unique inno-
vations and evolutionary adaptations that fern gametophytes
possess. Nowhere is this more evident than in the long-lived epi-
phytic gametophytes discussed here. With the capacity for in-
creased longevity and asexual reproduction, some species have
even jettisoned the sporophyte generation entirely and yet still
manage to maintain large populations and distributions. Al-
though these species have been relatively well studied and charac-
terized in temperate regions, where they are the most conspicuous,
we are only just now beginning to uncover patterns of spatially
separated fern generations in the tropics. There are several
obstacles that will need to be overcome to thoroughly document
and understand this pattern in tropical regions, not the least of
which is successful identification of gametophytes.
14 INTERNATIONAL JOURNAL OF PLANT SCIENCES
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With technological advances in DNA sequencing, such as
the DNA bar-coding approaches used in Japan, Taiwan, and
Tahiti described above, we can now identify fern gameto-
phytes to the species level without having to rely on morphol-
ogy. These molecular tools may be especially useful in iden-
tifying novel species with independent gametophytes in the
tropics. For example, in the tropical forests of Central and
South America, one can easily find dense patches of fern game-
tophytes, but it is unclear how these relate to nearby species
and populations that readily produce sporophytes. DNA bar
coding can be successful, however, only with a well-sampled
reference library, as highlighted by the inability to identify
several of the putative independent gametophytes in Asia de-
scribed above. Next-generation sequencing technologies, such
as sequence capture technology that can recover genetic data
from degraded DNA sources, including herbarium specimens
(Straub et al. 2012), would greatly increase the coverage of
the reference library and may enhance our ability to confidently
identify gametophytes to species. Results from studies such as
these will likely uncover novel species and identify even more
disjunct populations, increasing the number of ferns that dis-
play the separation of generations. These genetic resources
can also be used to examine phylogenetic relationships, biogeo-
graphic patterns, patterns of gene flow, and genetic structure
among populations.
Beyond fundamental issues of identification, our understand-
ing of the basic ecology and demography of many of these fern
species is sorely lacking. Given the possibilities of microcli-
matic variation to restrict sporophyte production, further re-
search should be conducted to determine what environmental
factors may be driving this pattern and at what threshold spo-
rophyte production is inhibited. Classic ecological approaches,
including common-garden and manipulative experiments, can
be used to directly test the role of microclimate. These are crit-
ical questions to address, because changes in the Earth’s climate
may make many areas unsuitable for these gametophyte-only
populations and may also limit sporophyte production, which
would compromise species’ability to colonize new habitats if
they have small populations and/or gametophytes that lack
gemmae. For those fern species that appear to have lost the
ability to produce sporophytes entirely, quantitative trait loci
or association genetic studies may be of use in determining
what genes code for the production of sporophytes and which
genes have been lost in these species.
The first account in which independent fern gametophytes
were formally recognized came with the description of Vitta-
ria appalachiana by Wagner and Sharp (1963), which graced
the cover of Science. This was followed soon after by Farrar’s
(1967) first publication of four independent gametophytes in
eastern North America, making 2017 the fiftieth anniversary
of this fundamental work. Since then, it is clear that, although
we have made great strides in cataloging and understanding
these species around the world, there is still much to learn. It
is our hope that this review will serve as a primer for future stud-
ies to both identify new species with independent gametophytes
and quantify the conditions that prohibit their sporophytes
from forming.
Acknowledgments
We thank Simon Parsons for contributing the illustrations
of fern gametophytes (figs. 3–6), as well as James Watkins Jr.
and Aaron Duffy for contributing photographs in figure 1.
We would also like to thank all of those who supplied infor-
mation via personal communication, including Dr. Donald
Farrar, who pointed us in the direction of unpublished occur-
rences and has contributed significantly to the study of inde-
pendent fern gametophytes. We are also thankful for the help-
ful comments of the two anonymous reviewers, as well as
Dr. Walter Judd, who supplied comments on the earliest ver-
sion of the draft.
Literature Cited
Allen CM 1975 Trichomanes petersii in Louisiana. Am J Bot 65:97–
98.
Allen DE 1969 The Victorian fern craze: a history of pteridomania.
Huthinson, London.
Atkinson LR, AG Stokey 1964 Comparative morphology of the
gametophyte of homosporous ferns. Phytomorphology 14:51–70.
Barcelona JF, NE Dolotina, GS Madroñero, WG Granert, DD Sopot
2006 The ferns and fern allies of the karst forests of Bohol Island,
Phillipines. Am Fern J 96:1–20.
Beatty GE, J Provan 2010 Refugial persistence and postglacial recol-
onization of North America by the cold tolerant herbaceous plant
Orthilia secunda. Mol Ecol 19:5009–5021.
Bierhorst DW 1971 Morphology of vascular plants. Macmillan, New
York.
Bierzychudek P, V Eckhart 1988 Spatial segregation of the sexes of
dioecious plants. Am Nat 132:34–43.
Bower FO 1888 On some normal and abnormal developments of
the oophyte in Trichomanes. Ann Bot 1:269–305.
Bray J 1996 Microhabitat diversity of Trichomanes petersii in Arkan-
sas. Am Fern J 86:125–126.
Caponetti J, M Whitten, M Beck 1982 Axenic culture and induction
of callus and sporophytes of the Appalachian Vittaria gametophyte.
Am Fern J 72:36–40.
Chambers SM, NC Emery 2016 Population differentiation and coun-
tergradient variation throughout the geographic range in the fern
gametophyte Vittaria appalachiana. Am J Bot 103:86–98.
Chen CW, YM Huang, LY Kuo, YM Chang, YC Liu, WL Chiou
2013aA new vittarioid fern species, Haplopteris heterophylla (Pteri-
daceae). Syst Bot 38:901–909.
Chen CW, YM Huang, LY Kuo, QD Nguyen, HT Luu, JR Callado,
DR Farrar, WL Chiou 2013b trnL-F is a powerful marker for
DNA identification of field vittarioid gametophytes (Pteridaceae).
Ann Bot 111:663–673.
Chiou WL, DR Farrar 1997 Comparative gametophyte morphology
of selected species of the family Polypodiaceae. Am Fern J 87:77–86.
Chiou WL, DR Farrar, TA Ranker 1998 Gametophyte morphology
and reprodu ctive biology in Elaphoglossum.Can J Bot 76:1967–1977.
Christenhusz M, XC Zhang, H Schneider 2011 A linear sequence of
extant families and genera of lycophytes and ferns. Phytotaxa
19:7–54.
PINSON ET AL.—THE SEPARATION OF GENERATIONS 15
This content downloaded from 223.140.246.146 on January 14, 2017 07:38:05 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).
Cronin TM, DA Willard, HJ Dowsett, SE Ishman, TR Holtz Jr 1994
The Yorktown Formation of Virginia: implications for late Pliocene
climate and sea level history. Geol Soc Am Abstr Program 25:
A334.
Dassler C, D Farrar 1997 Significance of form in fern gametophytes:
clonal, gemmiferous gametophytes of Callistopteris baueriana (Hy-
menophyllaceae). Int J Plant Sci 158:622–639.
——— 2001 Significance of gametophyte form in long-distance col-
onization by tropical, epiphytic ferns. Brittonia 53:352–369.
Davis M, R Shaw 2001 Range shifts and adaptive responses to qua-
ternary climate change. Science 292:673–679.
Dubuisson JY, H Schneider, S Hennequin 2009 Epiphytism in ferns:
diversity and history. C R Biol 332:120–128.
Duffy A, M Stensvold, D Farrar 2015 Independent gametophytes of
Hymenophyllum wrightii in North America: not as rare as we
thought. Am Fern J 105:45–55.
Ebihara A, D Farrar, M Ito 2008 The sporophyte-less filmy fern of
eastern North America Trichomanes intricatum (Hymenophyllaceae)
has the chloroplast genome of an Asian species. Am J Bot 95:1645–
1651.
Ebihara A, S Matsumoto, M Ito 2009 Hybridization involving inde-
pendent gametophytes in the Vandenboschia radicans complex (Hy-
menophyllaceae): a new perspective on the distribution of fern hy-
brids. Mol Ecol 18:4904–4911.
Ebihara A, J Nitta, M Ito 2010 Molecular species identification with
rich floristic sampling: DNA barcoding the pteridophyte flora of Ja-
pan. PLoS ONE 5:e15136.
Ebihara A, A Yamaoka, N Mizukami, A Sakoda, J Nitta, R Imaichi
2013 A survey of the fern gametophyte flora of Japan: frequent in-
dependent occurrences of noncordiform gametophytes. Am J Bot
100:735–743.
Edwards L, J Ambrose, LK Kirkman, eds 2013 Coastal Plain
ecoregion. Pages 347–510 in The natural communities of Georgia.
University of Georgia Press, Athens.
Emigh VD, DR Farrar 1977 Gemmae: a role in sexual reproduction
in the fern genus Vittaria. Science 198:297–298.
Evert RF, SE Eichorn 2013 Raven biology of plants. 8th ed. WH
Freeman, New York.
Farrar DR 1967 Gametophytes of four tropical fern genera repro-
ducing independently of their sporophytes in the southern Appala-
chians. Science 155:1266–1267.
——— 1971 The biology of ferns with asexually reproducing game-
tophytes in the eastern United States. PhD diss. University of Mich-
igan, Ann Arbor.
——— 1974 Gemmiferous fern gametophytes—Vittareaceae. Am J
Bot 61:146–155.
——— 1978 Problems in the identity and origin of the Appalachian
Vittaria gametophyte, a sporophyteless fern of the eastern United
States. Am J Bot 65:1–12.
——— 1985 Independent fern gametophytes in the wild. Proc R Soc
Edinb B 86:361–369.
——— 1990 Species and evolution in asexually reproducing inde-
pendent fern gametophytes. Syst Bot 15:98–111.
——— 1992 Trichomanes intricatum: the independent Trichomanes
gametophyte in the eastern United States. Am Fern J 82:68–74.
——— 1993aHymenophyllaceae. Pages 190–197 in Flora of North
American Editorial Committee, eds. Flora of North America: north
of Mexico. Oxford University Press, New York.
——— 1993bVittariaceae. Pages 187–189 in Flora of North Amer-
ican Editorial Committee, eds. Flora of North America: north of
Mexico. Oxford University Press, New York.
——— 1998 The tropical flora of rockhouse cliff formations in the
eastern United States. J Torrey Bot Soc 125:91–108.
——— 2006 Conservation assessment for Appalachian vittaria (Vit-
taria appalachiana Farrar & Mickel). US Department of Agricul-
ture Forest Service, Eastern Region, Milwaukee, WI.
Farrar DR, C Dassler, JE Watkins Jr, C Skelton 2008 Gametophyte
ecology. Pages 222–256 in TA Ranker, CH Haufler, eds. Biology
and evolution of ferns and lycophytes. Cambridge University Press,
Cambridge.
Farrar DR, PG Davison 1994 Hymenophyllum tayloriae: sporophyte
and gametophyte together again after 58 years. Am J Bot 81(suppl.):
128 (abstract).
Farrar DR, CL Johnson-Groh 1990 Subterranean sporophytic gem-
mae in moonwort ferns, Botrychium subgenus Botrychium.AmJ
Bot 77:1168–1175.
Farrar DR, GP Landry 1987 Vittaria graminifolia in the United States,
again. Am J Bot 74:709–710.
Farrar DR, JT Mickel 1991 Vittaria appalachiana—a name for the
Appalachian gametophyte. Am Fern J 81:69–75.
Fernández H, AM Bertrand, R Sánchez-Tamés 1997 Gemmation in
cultured gametophytes of Osmunda regalis. Plant Cell Rep 16:358–
362.
Ford D, PW Williams 2007 Karst hydrogeology and geomorphology.
Wiley, Chichester.
Gann GD, KN Hines, EV Grahl, SW Woodmansee, CS Smith, J Bell-
Willcox 2006 Rare plant monitoring and restoration on Long Pine
Key, Everglades National Park: year end report, year 3. Institute
for Regional Conservation, Miami, FL.
Gastony GJ 1977 Chromosomes of the independently reproducing
Appalachian gametophyte: a new source of taxonomic evidence.
Syst Bot 2:43–48.
——— 1980 The deletion of Vittaria graminifolia from the flora of
Florida. Am Fern J 70:12–14.
Gentry AH, CH Dodson 1987 Diversity and biogeography of neo-
tropical vascular epiphytes. Ann Mo Bot Gard 74:205–233.
Goebel K 1888 Morphologische und biologische studien. II. Zur
keimungsgeschichte einiger farne. Ann Jard Bot Buitenzorg 7:74–
119.
Groot JJ 1991 Palynological evidence for late Miocene, Pliocene and
early Pleistocene climate changes in the middle U.S. Atlantic coastal
plain. Quat Sci Rev 10:147–162.
Hebert PDN, A Cywinska, SL Ball, JR DeWaard 2003 Biological
identifications through DNA barcodes. Proc R Soc B 270:313–321.
Holttum RE 1938 The ecology of tropical pteridophytes. Pages 421–
422 in F Verdoorn, ed. Manual of pteridology. M Nijhoff, The
Hague.
Jansen E, J Overpeck, KR Briffa, J-C Duplessy, F Joos, V Masson-
Delmotte, D Olago, et al 2007 Palaeoclimate. Pages 435–497 in
S Solomon, D Qin, M Manning, Z Chen, M Marquis, KB Averyt,
M Tignor, HL Miller, eds. Climate change 2007: the physical sci-
ence basis. Contribution of Working Group I to the Fourth Assess-
ment Report of the Intergovernmental Panel on Climate Change.
Cambridge University Press, Cambridge.
Johns RJ, PJ Edwards 1991 Vegetative reproduction in pteridophytes.
Curtis’s Bot Mag 8:109–112.
Kasselmann C 2010 Aquarienpflanzen: 450 Arten im Porträt. Ulmer,
Stuttgart.
Ke TJ, X Fuwu, W Faguo, Z Xianchung, M Kato, D Barrington 2013
Lomariopsidaceae. Pages 725–726 in ZY Wu, PH Raven, DY Hong,
eds. Flora of China. Science, Beijing.
Kimmerer RW, CC Young 1995 The role of slugs in dispersal of the
asexual propagules of Dicranum flagellare. Bryologist 98:149–153.
Knie N, S Fischer, F Grewe, M Polsakiewicz, V Knoop 2015 Horsetails
are the sister group to all other monilophytes and Marattiales are
sister to leptosporangiate ferns. Mol Phylogenet Evol 90:140–149.
Knight H, MR Knight 2001 Abiotic stress signalling pathways: spec-
ificity and cross-talk. Trends Plant Sci 6:262–267.
Kobza RM, JC Trexler, WF Loftus, SA Perry 2004 Community
structure of fishes inhabiting aquatic refuges in a threatened Karst
wetland and its implications for ecosystem management. Biol Con-
serv 116:153–165.
16 INTERNATIONAL JOURNAL OF PLANT SCIENCES
This content downloaded from 223.140.246.146 on January 14, 2017 07:38:05 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).
Krippel Y 2001 Aire de répartition et statut de Trichomanes spe-
ciosum Willd. (Hymenophyllaceae) au Luxembourg. Bull Soc Nat
Luxem 102:3–13.
Krukowski M, K Swierkosz 2004 Discovery of the gametophytes of
Trichomanes speciosum (Hymenophyllaceae: Pteridophyta) in Po-
land and its biogeographical importance. Fern Gaz 17:79–84.
Kuo LY, CW Chen, W Shinohara, A Ebihara, H Kudoh, H Sato, YM
Huang, WL Chiou Forthcoming Not only in the temperate zone:
independent gametophytes of two vittarioid ferns (Pteridaceae, Po-
lypodiales) in East Asian subtropics. J Plant Res.
Kuo LY, FW Li, WL Chiou, CN Wang 2011 First insights into fern
matK phylogeny. Mol Phylogenet Evol 59:556–566.
Li FW, BC Tan, V Buchbender, RC Moran, G Rouhan, CN Wang,
D Quandt 2009 Identifying a mysterious aquatic fern gameto-
phyte. Plant Syst Evol 281:77–86.
Longton R, RM Schuster 1983 Reproductive biology. Pages 386–
462 in RM Schuster, ed. New manual of bryology. Hattori Botan-
ical Laboratory, Nichinan, Japan.
Loope LL, TW Giambelluca 1998 Vulnerability of island tropical
montane cloud forests to climate change, with special reference to
East Maui, Hawaii. Clim Change 39:503–517.
Lu JM, N Zhang, XY Du, J Wen, DZ Li 2015 Chloroplast phy-
logenomics resolves key relationships in ferns. J Syst Evol 53:448–
457.
Magrini S, A Scoppola 2012 First results from conservation studies
of chlorophyllous spores of the Royal fern (Osmunda regalis, Os-
mundaceae). Cryobiology 64:65–69.
McVeigh I 1937 Vegetative reproduction of the fern sporophyte. Bot
Rev 3:457–497.
Mickel J, AR Smith 2004 The pteridophytes of Mexico. New York
Botanical Garden, Bronx.
Miller DS, IR Duddy 1989 Early cretaceous uplift and erosion of the
northern Appalachian basin, New York, based on apatite fission
track analysis. Earth Planet Sci Lett 93:35–49.
Mottier DM 1927 Behavior of certain fern prothallia under pro-
longed cultivation. Bot Gaz 83:244–266.
Mynssen CM, A Vasco, RC Moran, LS Sylvestre, G Rouhan 2016
Desmophlebiaceae and Desmophlebium: a new family and genus of
Eupolypod II ferns. Taxon 65:19–34.
Nauman CE 1986 Trichomanes in Florida. Am Fern J 76:179–183.
Nayar BK, S Kaur 1971 Gametophytes of homosporous ferns. Bot
Rev 37:295–396.
Nitta JH, J-Y Meyer, R Taputuarai, CC David Forthcoming Life cy-
cle matters: DNA barcoding reveals contrasting community structure
between fern sporophytes and gametophytes. Ecol Monographs.
Page CN 2002 Ecological strategies in fern evolution: a neopterido-
logical overview. Rev Palaeobot Palynol 19:1–33.
Persson H 1958 The genus Takakia found in North America. Bryol-
ogist 61:359–361.
Pinson JB, E Schuettpelz 2016 Unraveling the origin of the Appala-
chian gametophyte, Vittaria appalachiana. Am J Bot 103:668–676.
Pittermann J, CB Brodersen, J Watkins Jr 2013 The physiological re-
silience of fern sporophytes and gametophytes: advances in water re-
lations offer new insights into an old lineage. Front Plant Sci 4:285.
Possley J, J Maschinski, S Hodges, E Magnaghi, D Powell, K
Weclawska, S Wright, V Pence 2013 Year 11 report: biological
monitoring for plant conservation in Miami-Dade County natural
areas. Miami-Dade County Resolution R-808-07. Fairchild Tropi-
cal Botanic Garden, Miami, FL.
Raine CA, DR Farrar, E Sheffield 1991 A new Hymenophyllum spe-
cies in the Appalachians represented by independent gametophyte
colonies. Am Fern J 81:109–118.
Rasbach H, K Rasbach, C Jéro
̂me 1993 Über das Vorkommen des
Hautfarns Trichomanes speciosum (Hymenophyllaceae) in den Vo-
gesen (Frankreich) und dem benachbarten Deutschland. Carolinea
51:51–52.
Ratcliffe DA, HJB Birks, HH Birks 1993 The ecology and conserva-
tion of the Killarney fern Trichomanes speciosum Willd. in Britain
and Ireland. Biol Conserv 66:231–347.
Reece JB, LA Urry, MI Cain, SA Wasserman, PV Minorsky, RB
Jackson 2014 Campbell biology. 10th ed. Benjamin Cummings,
Menlo Park, CA.
Rothfels CJ, FW Li, EM Sigel, L Huiet, A Larsson, DO Burge, M
Ruhsam, et al 2015 The evolutionary history of ferns inferred
from 25 low-copy nuclear genes. Am J Bot 102:1089–1107.
Rothfels CJ, MA Sundue, LY Kuo, A Larsson, M Kato, E Schuett-
pelz, KM Pryer 2012 A revised family-level classification for eu-
polypod II ferns (Polypodiidae: Polypodiales). Taxon 61:515–533.
Rudolphi J 2009 Ant-mediated dispersal of asexual moss propagules.
Bryologist 112:73–79.
Rumsey FJ, JA Barrett, M Gibby, SJ Russell, JC Vogel 2005 Repro-
ductive strategies and population structure in the endangered pteri-
dophyte Trichomanes speciosum (Hymenophyllaceae: Pteridophyta).
Fern Gazette 17:205–215.
Rumsey FJ, AC Jermy, E Sheffield 1998aThe independent gameto-
phytic stage of Trichomanes speciosum Willd. (Hymenophyllaceae),
the Killarney fern and its distribution in the British Isles. Watsonia
22:1–19.
Rumsey FJ, SJ Russell, J Ji, JA Barrett, M Gibby 1996 Genetic var-
iation in the endangered filmy fern Trichomanes speciosum Willd.
Pages 161–165 in JM Camus, M Gibby, RJ Johns, eds. Pteridology
in perspective. Royal Botanic Gardens, Kew.
Rumsey FJ, E Sheffield, DR Farrar 1990 British filmy-fern gameto-
phytes. Pteridologist 2:40–42.
Rumsey FJ, JC Vogel, SJ Russell, JA Barrett, M Gibby 1998bClimate,
colonization and celibacy: population structure in central European
Trichomanes speciosum (Pteridophyta). Plant Biol 111:481–489.
——— 1999 Population structure and conservation biology of the
endangered fern Trichomanes speciosum Willd. (Hymenophyllaceae)
at its northern distributional limit. Biol J Linn Soc 66:333–344.
Sato T, A Sakai 1981 Cold tolerance of gametophytes and sporophytes of
some cool temperate ferns native to Hokkaido. Can J Bot 59:604–608.
Schuster RM 1983 Phytogeography of the Bryophyta. Pages 463–
626 in RM Schuster, ed. New manual of bryology. Hattori Botan-
ical Laboratory, Nichinan.
——— 1992 On Megaceros aenigmaticus Schust. Bryologist 95:305–
315.
Sessa EB, WL Testo, JE Watkins 2016 On the widespread capacity
for, and functional significance of, extreme inbreeding in ferns.
New Phytol 211:1108–1119.
Sinclair BJ, LV Ferguson, G Salehipour-Shirazi, HA MacMillan 2013
Cross-tolerance and cross-talk in the cold: relating low temperatures
to desiccation and immune stress in insects. Integr Comp Biol 53:
545–556.
Smith AR 1993 Grammitidaceae. Pages 309–311 in Flora of North
American Editorial Committee, eds. Flora of North America: north
of Mexico. Oxford University Press, New York.
Smith AR, JT Mickel 1977 Chromosome counts for Mexican ferns.
Brittonia 29:391–398.
Smith AR, KM Pryer, E Schuettpelz, P Korall, H Schneider, PG Wolf
2006 A classification of extant ferns. Taxon 55:705–301.
Stark LR, DN McLetchie 2006 Gender-specific heat-shock tolerance
of hydrated leaves in the desert moss Syntrichia caninervis. Physiol
Plant 126:187–195.
Stark LR, DN McLetchie, BD Mishler 2005 Sex expression, plant
size, and spatial segregation of the sexes across a stress gradient
in the desert moss Syntrichia caninervis. Bryologist 108:183–193.
Stevens SM, NC Emery 2015 Dispersal limitation and population
differentiation in performance beyond a northern range limit in
an asexually reproducing fern. Divers Distrib 21:1242–1253.
Stokey AG, LR Atkinson 1958 The gametophyte of the Grammiti-
daceae. Phytomorphology 8:391–403.
PINSON ET AL.—THE SEPARATION OF GENERATIONS 17
This content downloaded from 223.140.246.146 on January 14, 2017 07:38:05 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).
Straub SC, M Parks, K Weitemier, M Fishbein, RC Cronn, A Liston
2012 Navigating the tip of the genomic iceberg: next-generation
sequencing for plant systematics. Am J Bot 99:349–364.
Taylor TM 1967 Mecodium wrightii in British Columbia and Alaska.
Am Fern J 57:1–6.
Testo WL, JE Watkins Jr 2011 Comparative development and game-
tophyte morphology of the hart’s-tongue fern, Asplenium scolopen-
drium L. J Torrey Bot Soc 138:400–408.
Tryon AF, B Lugardon 1991 Spores of the Pteridophyta: surface,
wall structure, and diversity based on electron microscope studies.
Springer, New York.
Vogel JC, S Jeßen, M Gibby, AC Jerymy, L Ellis 1993 Gametophytes
of Trichomanes speciosum (Hymenophyllaceae: Pteridophyta) in
central Europe. Fern Gaz 14:227–232.
Wagner WH, AJ Sharp 1963 A remarkably reduced vascular plant
in the United States. Science 142:1483–1484.
Walp RL, GR Proctor 1946 Long-lived fern prothallia. Am Fern J
36:109–112.
Watkins JE, CL Cardelús 2012 Ferns in an angiosperm world: Cre-
taceous radiation into the epiphytic niche and diversification on the
forest floor. Int J Plant Sci 173:695–710.
Watkins JE, DR Farrar 2002 A new name for an old fern from north
Alabama. Am Fern J 92:171–178.
——— 2005 Origin and taxonomic affinities of Thelypteris (subgen.
Stegnogramma)burksiorum (Thelypteridaceae). Brittonia 57:183–201.
Watkins JE, MK Mack, SS Mulkey 2007aGametophyte ecology and
demography of epiphytic and terrestrial tropical ferns. Am J Bot
94:701–708.
Watkins JE, MC Mack, TR Sinclair, SS Mulkey 2007bEcological
and evolutionary consequences of desiccation tolerance in tropical
fern gametophytes. New Phytol 176:708–717.
Weakley AS, LD Estes, KG Mathews, CT Witsell, K Gandhi, A
Ebihara 2011 New combinations, rank changes, and nomencla-
tural and taxonomic comments in the vascular flora of the south-
eastern United States. J Bot Res Inst Tex 5:437–455.
Wherry E 1964 The southern fern guide, southeastern and south-
midland United States. Doubleday, Garden City, NY.
Whittingham S 2009 The Victorian fern craze. Shire, Oxford.
Wolle F 1887 Fresh water algae of North America. Vol 1. Comenius,
Bethleham, PA.
Zhang LB, L Zhang 2015 Didymochlaenaceae: a new fern family of
eupolypods I (Polypodiales). Taxon 64:27–38.
18 INTERNATIONAL JOURNAL OF PLANT SCIENCES
This content downloaded from 223.140.246.146 on January 14, 2017 07:38:05 AM
All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).
