Phytotaxa 277 (2): 101–145
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Copyright © 2016 Magnolia Press Article PHYTOTAXA
ISSN 1179-3155 (print edition)
ISSN 1179-3163 (online edition)
Accepted by Li-Bing Zhang: 7 Sept. 2016; published: 27 Sept. 2016
The genus Isoetes (Isoetaceae): a provisional checklist of the accepted and unresolved
ANGELO TROIA1*, JOVANI B. PEREIRA2, CHANGKYUN KIM3 & W. CARL TAYLOR4
1 Dipartimento di Scienze e Tecnologie Biologiche Chimiche e Farmaceutiche (STeBICEF), Sezione di Botanica ed Ecologia Vegetale,
Università degli Studi di Palermo, Via Archirafi 38, I-90123 Palermo, Italy
2 Ruhr-Universität Bochum, Department of Evolution and Biodiversity of Plants, 44780 Bochum, Germany
3 Department of Life Science, Gachon University, Seongnam 13120, South Korea
4 Department of Botany, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-166, USA
* Author for correspondence: firstname.lastname@example.org
Isoetes is a widely distributed lycophyte genus of at least 200 species occurring in diverse habitats. The species can be dif-
ficult to identify because Isoetes, with its apparent simplicity of form and conserved morphology, provides few diagnostic
features to reliably distinguish its species. The last worldwide monograph, published nearly a century ago, listed 77 taxa.
The first step in producing a flora or monograph of all known species of a genus is to compile a list of the acceptable species
names. The list presented here is a compilation of 192 accepted names representing taxa from regions around the world:
chromosome numbers were assigned to 101 of them, with polyploidy settled on 46.7%. Distribution mapping of the accepted
species indicates that South America is the center of diversity for Isoetes and species diversity is the highest in temperate
regions. Many of the species on this list are rare and have limited ranges. The list of taxa can be used to initiate floristic stud-
ies and conservation efforts in keeping with the target goals of the Global Strategy for Plant Conservation.
Isoetes Linnaeus (1753: 1100) is a nearly cosmopolitan genus of heterosporous lycophytes, containing more than 200
species and being the only living genus of the family Isoetaceae (Jermy 1990, Larsén & Rydin 2016). These species
have evolved through an ancient lineage with an extensive fossil record dating from the Devonian Period (Pigg 2001)
and morphology so conserved that members of the genus are recognized in the Triassic Period (Retallack 1997). Today
members of the genus are widely distributed from the tropics to the sub-arctic; they range from submerged, evergreen
aquatics of cold, clear water lakes to upland terrestrial ephemerals of seasonally parched soils over bedrock.
In spite of their long fossil record, widespread distribution, and diverse adaptations, Isoetes species are remarkably
uniform in their morphology. Plants appear simple in form, consisting of a lobed subterranean rootstock producing
a tuft of linear sporophylls above and roots below along the groove between the lobes. This simple, conserved form
provides few characters and character states that can be used to distinguish species. Traditional characters such as
habitat, megaspore size, and megaspore texture are helpful for recognizing species, but they provide few morphological
character states needed to clearly distinguish all the species. More recently, chromosome counts, scanning electron
microscope analysis of spore texture, and DNA sequences have provided critical evidence for diagnosing species, but
these are of little help to botanists in the field.
In addition to the challenge of accurately identifying species with a limited set of morphological characters and
character states, frequent interspecific hybrids produce intermediate character states that further blur distinctions
between species. Isoetes interspecific hybrids are recognized by their production of non-viable, malformed spores of
variable size, shape, and texture (Taylor et al. 1985). DNA cloning and sequencing has revealed that some of these
sterile interspecific hybrids have doubled their chromosome number to reestablish chromosome balance and become
fertile, allopolyploid species (Hoot et al. 2004). Based on chromosome counts it appears that more than half of the
known species of Isoetes are polyploids (Troia 2001).
The latest worldwide taxonomic monograph of Isoetes was published almost a century ago (Pfeiffer 1922).
Pfeiffer’s monograph accounted for 77 taxa (65 as species, 8 as varieties, and 4 as forms). More recent estimates have
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continued to increase the number of species in the genus: ca. 130 species (Jermy 1990), ca. 150 species (Taylor et
al. 1993), ca. 200 (Hoot & Taylor 2001), and ca. 350 species (Hickey et al. 2003). These increases are due to more
botanists interested in the genus, use of new tools to reveal species, and careful searches for Isoetes in the field.
Therefore, it is appropriate to publish a compilation of accepted species names and their known distributions. It is
hoped this information will aid botanists in taxonomic, biogeographic, and phylogenetic studies of the genus and
conservationists with the preservation of the species and their habitats. Understanding plant diversity at the species
level is fundamental as a “basis for both conservation assessments and for management” of species and ecosystems
Target 1 of the Global Strategy for Plant Conservation (GSPC) (Convention on Biological Diversity 2002) is the
production of “An online flora of all known plants.” Before such a flora can be produced, “a widely accessible working
list of known plant species is needed, as a step towards a complete world flora” (Paton et al. 2008). To meet this need,
“The Plant List” was launched online through GSPC at the end of 2010. The Plant List (2013–2016) currently includes
only 93 accepted Isoetes species names out of 230 scientific plant names of species rank. A more complete list of Isoetes
taxa is needed before a flora can be produced. Target 2 of the GSPC states “An assessment of the conservation status
of all known plant species, as far as possible, to guide conservation action”. This cannot be achieved without a more
complete list of the species to be assessed and thus, the achievement of Target 2 is dependent on the accomplishment
of Target 1.
The list presented here is neither a taxonomic revision nor an ultimate checklist, but it addresses a need to have a
more current working checklist available for taxonomic revisions and for the addition of species that will be described
in the future.
Materials & Methods
The list was compiled by regional experts (authors of this paper) on the basis of their experience, i.e. Taylor (North
America), Pereira (Central and South America), Kim (China, Korea, Japan and SE Asia), and Troia (all the remaining
regions). Troia and Taylor served also as general editors, integrating the efforts into one global list.
The nomenclatural data assembled here are based on Index Kewensis and the Gray Herbarium Card Index (both
available at http://www.ipni.org) and Tropicos (http://www.tropicos.org) with later additions of recently described
species. Authors and original publications were checked for each taxon and corrected when necessary, according to
the International Code of Nomenclature (McNeill et al. 2012). Names of authors, dates, and sources were checked
and abbreviated according to The International Plant Names Index (2015), sometimes verified with Stafleu & Cowan
The list includes all accepted species with their main (not all) synonyms. Regional syntheses by Croft (1986) and
Roux (2009) together with several global syntheses available on the Internet (Hassler & Swale 2001–2016, Desfayes
2008–2016, Musselman 2012–2016) are important resources. Hybrids are included only if they were previously
described as species or varieties.
The taxonomy of Isoetes in India is unclear, i.e. most of the last described species “appear to represent no more
than infraspecific variation in spore-surface morphology, of uncertain taxonomic significance” (Chandra et al. 2008).
We agree that “further study is required into separation of species, and their correct nomenclature and synonymy”
(Chandra et al. 2008), but according to the available literature from Indian and not-Indian researchers, we tried to make
a plausible although preliminary assessment of the species occurring in the country, considering as “good species”
more than the two species estimated by Fraser-Jenkins (2008).
For each accepted taxon, information about its distribution has been added, usually on a national scale and
according to the standards proposed by the International Working Group on Taxonomic Databases for Plant Sciences
Finally, we tried to assign to each accepted taxon a chromosome number, on the basis of a critical review of
published data; the reference was made - when possible - to the synthesis published in 2001 (Troia 2001), where the
reader can find details on the reports; only for new species or reports not included in Troia 2001, other references were
GENUS ISOETES Phytotaxa 277 (2) © 2016 Magnolia Press • 139
The resulting checklist (Table 1; Supplemental file) includes 344 names, 192 accepted taxa (183 species, 7
subspecies, 2 varieties), 112 synonyms, 14 hybrids, and 26 unresolved names. Four species, described in 2016
during the processing and revision of this article (I. tamaulipana Mora-Olivo et al., I. vanensis Zare et al., Isoetes
serracarajensis J.B.S. Pereira et al., I. cangae J.B.S. Pereira et al.) have been added as (preliminary) accepted.
Chromosome numbers were assigned to 101 accepted taxa, with a total of 105 counts (because some taxa showed
more than one ploidy level): diploid taxa were 56, whereas polyploid ones were 49 (34 tetraploids, 10 hexaploids, 2
octoploids, 2 decaploids and 1 dodecaploid); note that odd numbers were not reported in this summary because they
usually are typical of hybrid taxa, and also the report of 2n=58 for I. tenuifolia has not been considered. The count
2n=33 reported for I. goebelii (Pereira et al. 2015) has probably to be referred to hybrid specimens, here we confirm
the “accepted” status for that name waiting for the results of additional ongoing research (Pereira in preparation).
Some minor nomenclatural adjustments regarding spelling and dates of publication have been made. For example,
the terminations of some specific epithets (e.g. dixitei) have been corrected (dixitii) according to Art. 60.12 of the
Code (McNeill et al. 2012). Some dates have been corrected (e.g. in I. luetzelburgii U. Weber, I. organensis U. Weber)
according to Stafleu & Cowan (1976–1988): in other cases (I. storkii T.C. Palmer) dates have been verified on the
Although the species list (Table 1) is a provisional checklist, it is useful to generate a synoptic table (Table 2), a
distribution map of these species according to floristic regions “level 2” (Brummit 2001) (Fig. 1), and a synthesis of
the distribution of the different ploidy levels in the genus (Fig. 2).
FIGURE 1. Distribution of Isoetes species richness using the Level 2 regions of Brummitt (2001).
The last global monographic treatment of the Isoetaceae (Pfeiffer 1922) accounted for 77 taxa (65 species, 8 varieties,
4 forms). In more recent global compilations Hassler & Swale (2001–2015) listed 143 species and Desfayes (2008–
2015) accounts for ca. 174 taxa. The checklist presented here lists a total of 188 accepted taxa.
Recent estimates of extant Isoetes species range from ca. 150 (Taylor et al. 1993) to about 350 (Hickey et al.
2003). The authors of the present work estimate the number of species to be about 250. This number takes into account
some yet unresolved species groups and other taxa currently being described: Fig. 3 shows the tendency to describe
new taxa in recent years; it shows that an increasing number of taxa have been described during the last 50 years.
Many Isoetes species are rare, sometimes described and known from only the type locality. They are usually
threatened by pollution and destruction of the lakes, rivers, or wetlands and other places where they live (e.g. Bagella &
Caria 2013, Barni et al. 2013). Their habitats are usually inadequately protected. In addition, climate change may alter
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the hydrological regimes of freshwater inland water bodies (Ruiz 2008). In the online “IUCN Redlist of Threatened
Species” (IUCN 2015), only 28 species of Isoetes have been assessed. Seventeen are listed as threatened or near
threatened, 6 Data Deficient, and 5 Least Concern. Some of those names are synonyms or unresolved taxa, according
to the list presented in Table 1.
TABLE 2. Conspectus showing the number of accepted Isoetes taxa by region, using the Level 2 regions of Brummitt (2001); the number
in front of the region is the code of the region according to Brummitt (2001).
Region Number of taxa
84 Brazil 26
78 Southeastern U.S.A. 21
83 Western South America 19
50 Australia 16
27 Southern Africa 14
12 Southwestern Europe 13
13 Southeastern Europe 12
75 Northeastern U.S.A. 12
85 Southern South America 10
22 West Tropical Africa 9
82 Northern South America 9
34 Western Asia 9
38 Eastern Asia 8
20 Northern Africa 8
79 Mexico 8
71 Western Canada 8
72 Eastern Canada 7
40 Indian Subcontinent 7
76 Southwestern U.S.A. 7
73 Northwestern U.S.A. 7
24 Northeast Tropical Africa 6
25 East Tropical Africa 6
23 West-Central Tropical Africa 5
74 North-Central U.S.A. 5
77 South-Central U.S.A. 5
26 South Tropical Africa 4
36 China 4
43 Papuasia 4
29 Western Indian Ocean 3
31 Russian Far East 3
70 Subarctic America 3
80 Central America 3
81 Caribbean 3
...Continued on next page
GENUS ISOETES Phytotaxa 277 (2) © 2016 Magnolia Press • 141
TABLE 2. (Continued)
Region Number of taxa
10 Northern Europe 2
11 Middle Europe 2
14 Eastern Europe 2
30 Siberia 2
41 Indo-China 2
51 New Zealand 2
21 Macaronesia 1
42 Malesia 1
63 North-Central Pacific 1
28 Middle Atlantic Ocean 0
32 Middle Asia 0
33 Caucasus 0
35 Arabian Peninsula 0
37 Mongolia 0
60 Southwestern Pacific 0
61 South-Central Pacific 0
62 Northwestern Pacific 0
90 Subantarctic Islands 0
91 Antarctic Continent 0
FIGURE 2. Distribution of the different ploidy levels in the genus Isoetes. Odd numbers have not been considered.
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142 • Phytotaxa 277 (2) © 2016 Magnolia Press
The distribution map of Isoetes species presented in Fig. 1, as well as a synoptic Table 2, illustrate global species
distributions. The number of botanists working in these regions, as well as the accuracy with which individual species
can be mapped affects these distribution patterns. However, the resolution is sufficient to identify several distributional
trends. First, South America appears to be the center of taxonomic diversity for the genus, as suggested by Hickey et
al. (2003), Brazil being the richest region with 26 taxa. Second, diversity in the genus seems to be highest in temperate
regions, with the single main exception of Brazil-Western South America (but here many species occur in temperate/
mountainous areas), where we find 26 + 19 = 44 taxa, one being in common: the other four “biodiversity hotspots”
are in fact Eastern USA (21 in SE + 12 in NE = 26 taxa, 7 being in common), Southern Europe (13 in SW + 12 in SE
= 19 taxa, 6 being in common), Australia (16 taxa), and Southern Africa (14 taxa). Third, several regions appear to
be without Isoetes: in the case of the Asiatic regions (Caucasus, Arabian Peninsula, Middle Asia, Mongolia) possibly
because of insufficient field exploration, more than unsuitability of the climates.
FIGURE 3. Number of accepted Isoetes taxa described in the last 50 years.
As regards to incidence of polyploidy in the genus, according to the data here gathered it results 46.7%; it is a
high value, compared to other groups of vascular plants (cf. Wood et al. 2009), but it is significantly lower compared to
previous estimates (e.g. 61.2% in Troia 2001, which considered 67 taxa vs. 101 taxa here considered), probably thanks
to a better definition of the taxa and to an improved amount of data. It has to be stressed that taxa with more ploidy
levels probably hide different cryptic species: it is the case of I. durieui, I. kirkii, I. muelleri.
The present provisional checklist is the more realistic attempt to assess the species richness and geographic
distribution in Isoetes. It is hoped that this compilation of data will provide a stimulus for further work in improving
our knowledge of diversity and conservation status of these interesting plants, as well as to protect them and the
habitats in which they live.
This work has been realized thanks to the decisive stimulus of Richard Lansdown, chair of IUCN-SSC-Freshwater
Plants Specialist Group and through the network of the “Isoetes Research Group” (http://isoetes.myspecies.info/) and
GENUS ISOETES Phytotaxa 277 (2) © 2016 Magnolia Press • 143
the “Isoetes group” within the IUCN-SSC-FPSG (http://iucn.org/ssc/fwpsg), both of which all the authors belong to.
We also thank our colleagues Xing-Liu (Wuhan University, China), Deborah Hofstra and Paul Champion (NIWA, New
Zealand’s Institute of Water and Atmospheric research, New Zealand), and Sarvesh Singh (University of Allahabad,
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