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The role of intraspecific hybridization in the evolution of invasiveness: A case study of the ornamental pear tree Pyrus calleryana


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Hybridization between genetically distinct populations of a single species can serve as an important stimulus for the evolution of invasiveness. Such intraspecific hybridization was examined in Pyrus calleryana, a Chinese tree species commonly planted as an ornamental in residential and commercial areas throughout the United States. This self-incompatible species is now escaping cultivation and appearing in disturbed habitats, where it has the potential to form dense thickets. Using genetic techniques incorporating nine microsatellite markers, we show that abundant fruit set on cultivated trees as well as the subsequent appearance of wild individuals result from crossing between genetically distinct horticultural cultivars of the same species that originated from different areas of China. We conclude that intraspecific hybridization can be a potent but little recognized process impacting the evolution of invasiveness in certain species.
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The role of intraspecific hybridization in the evolution
of invasiveness: a case study of the ornamental pear
tree Pyrus calleryana
Theresa M. Culley ÆNicole A. Hardiman
Received: 20 July 2007 / Accepted: 22 January 2008 / Published online: 28 October 2008
ÓSpringer Science+Business Media B.V. 2008
Abstract Hybridization between genetically dis-
tinct populations of a single species can serve as an
important stimulus for the evolution of invasiveness.
Such intraspecific hybridization was examined in
Pyrus calleryana, a Chinese tree species commonly
planted as an ornamental in residential and commer-
cial areas throughout the United States. This self-
incompatible species is now escaping cultivation and
appearing in disturbed habitats, where it has the
potential to form dense thickets. Using genetic
techniques incorporating nine microsatellite markers,
we show that abundant fruit set on cultivated trees as
well as the subsequent appearance of wild individuals
result from crossing between genetically distinct
horticultural cultivars of the same species that
originated from different areas of China. We con-
clude that intraspecific hybridization can be a potent
but little recognized process impacting the evolution
of invasiveness in certain species.
Keywords Callery Pear Evolution
Intraspecific hybridization Pyrus calleryana
Hybridization is a strong evolutionary force that can
potentially reshape the genetic composition of pop-
ulations and create novel genotypes that facilitate
adaptation to new environments (Stebbins 1950;
Anderson and Stebbins 1954; Arnold 1997). The
importance of hybridization in evolutionary processes
such as speciation has long been acknowledged
(Darlington 1940; Stebbins 1959,1969), but its
application to the field of invasion biology has only
more recently been discussed (Abbott 1992; Ellstrand
and Schierenbeck 2000; Cox 2004; Schierenbeck and
¨nouche 2006), as has the larger role of evolution
itself (Lee 2002; Lavergne and Molofsky 2007;
Novak 2007). Hybridization between genetically
distinct taxa has been proposed as a mechanism for
the evolution of invasiveness in introduced and native
species (Ellstrand and Schierenbeck 2000). Most
well-known examples involve interspecific or inter-
generic processes (Ellstrand and Schierenbeck 2000),
as in Spartina (Aı
¨nouche et al. 2003; Cox 2004)or
Senecio (Abbott 1992).
Less well studied has been intraspecific hybrid-
ization, defined as successful matings between
individuals from well differentiated populations
originally isolated from one another and consequently
with different gene frequencies (Stebbins 1950). As
such, this process does not pertain to crosses between
individuals from the same gene pool that possess
different alleles (Arnold 1997). If resulting F
T. M. Culley (&)N. A. Hardiman
Department of Biological Sciences, University
of Cincinnati, 614 Rieveschl Hall, Cincinnati,
OH 45221-0006, USA
N. A. Hardiman
Biol Invasions (2009) 11:1107–1119
DOI 10.1007/s10530-008-9386-z
individuals or later generation hybrids are fertile,
recombination may lead to novel genetic rearrange-
ments which can allow hybrids to expand their
ecological tolerance and invade new niche environ-
ments (Stebbins 1959; Arnold 1997). Intraspecific
hybridization can also result in increased genetic
variance, altered epistatic interactions, masking or
unloading of deleterious alleles, and/or transfer of
favorable genes (Lee 2002). Alternatively, such
hybridization events can also produce outbreeding
depression by disrupting co-adapted gene complexes
and local adaptation in established species (Arnold
1997). Intraspecific hybridization has been carried
out artificially for centuries to improve agriculturally
or horticulturally important plant species (Khanduri
and Sharma 2002; Johnston et al. 2003), but it has
rarely been examined in natural populations, with few
exceptions (Hufford and Mazer 2003; Erickson and
Fenster 2006; Johansen-Morris and Latta 2006).
Within the context of invasion biology, intraspe-
cific hybridization could potentially explain the
recent spread of certain species, such as those with
extended lag periods (Sakai et al. 2001), during which
time hybridization and selection may act to create
invasive genotypes (Ellstrand and Schierenbeck
2000). Intraspecific hybridization is most likely to
follow multiple introductions of a species (Kolbe
et al. 2004), especially non-native species that are
transplanted from different parts of their native range
into a new locality. Multiple introductions may be
done deliberately, for example, when plant species
are imported for horticulture (Reichard and White
2001; Burt et al. 2007) or accidentally as when seeds
are introduced as contaminants during shipping
(Sakai et al. 2001). Once introduced, genetically
distinct individuals may cross-pollinate to create
novel genotypes through admixture that otherwise
would never have been possible in the native
environment (Arnold 1997; Novak and Mack 2005;
Roman and Darling 2007). In this way, among-
population variation present in the native range is
transformed into within-population variation in the
introduced area (Kolbe et al. 2004). Although most
novel genotypes may be inappropriate in the new
environment, production of individuals combined
with novel selection (e.g. release from native com-
petitors, predators or pathogens) may strongly favor
certain genotypes. Within native species, intraspecific
hybridization can also occur if new genotypes are
introduced from different parts of the native range.
However, not all cases of intraspecific hybridization
will lead to invasiveness (Ellstrand and Schierenbeck
2000; Wolfe et al. 2007), but only when the right
combination of novel genetic rearrangements match
with the appropriate introduced environment in which
invasive traits can be selected.
The purpose of this paper is to examine the role of
intraspecific hybridization in the evolution of inva-
siveness in plant species. To do so, we first define the
process of intraspecific hybridization as it relates to
invasive species, citing specific examples in the
literature. Second, we focus on a case study of the
Callery Pear (Pyrus calleryana; Rosaceae), an
ornamental Asian tree that is increasingly invading
sites throughout the United States. Finally, we
conclude with suggestions for future research. Ulti-
mately, we maintain that intraspecific hybridization is
a potentially important and often overlooked stimulus
for facilitating invasiveness in certain species given
the right conditions.
Evidence of intraspecific hybridization
The process by which intraspecific hybridization can
lead to invasiveness (Wolfe et al. 2007) requires four
steps. If any of these steps do not hold, invasiveness
may not evolve through this mechanism. First,
founders from at least two genetically divergent
populations of a species must be introduced into the
same area. Evidence of such multiple introductions
consist of known introductory history of the species
(e.g., Luken and Thieret 1996) or genetic data tracing
back the origin of introduced populations to different
parts of the native range (e.g., Williams et al. 2005).
Second, individuals from these differentiated popu-
lations must cross and produce fertile offspring. In
some species, this may create novel recombinant
genotypes (i.e. admixture) with a selective advantage
in the new range. The offspring may also exhibit
elevated levels of genetic variation, especially if the
parental populations each experienced separate
genetic bottlenecks or founder events that lowered
levels of variation within their respective populations
(Husband and Barrett 1991; Cox 2004; Novak and
Mack 2005; but see Roman and Darling 2007). Third,
recombinant hybrids must be fit, generate offspring
and able to persist in new environments by possessing
1108 T. M. Culley, N. A. Hardiman
invasive characters that allow them to exploit resources
in current or new habitats. In some F
increased fitness may be evident because of heterosis
due to overdominance (Facon et al. 2005) and not
necessarily to recombination. Finally, natural selection
in the new environment must favor certain gene
combinations in hybrid individuals and consequently
their traits will persist and spread in populations.
There are several cases of invasive species in
which some or all of these steps are present. In the
tree species Schinus terebinthifolius for example,
there is evidence of multiple introductions into North
America from genetically different source popula-
tions along with post-introduction recombination
events (Williams et al. 2005,2007). The same is
also true for the grass Phalaris arundinacea (Laver-
gne and Molofsky 2004,2007), which at one time had
been suggested to be dominated by European culti-
vars that escaped cultivation (see Lavoie and
Dufresne 2005). Although multiple introductions
have been indicated in Alliaria petiolata (Durka
et al. 2005), Ambrosia artemisiifolia (Genton et al.
2005), Bryonia alba (Novak and Mack 1995), and
Hirschfeldia incana (Lee et al. 2004), it remains
unclear whether hybridization has subsequently
occurred because these studies were not designed to
examine recombination events.
Current behavior and past history of an invasive
species can also superficially resemble intraspecific
hybridization. For example, common reed (Phrag-
mites australis) is native to North America where it is
now spreading and forming dense monocultures
(Orson 1999; Wilcox et al. 2003). Recent genetic
evidence suggests that these invasive populations are
composed primarily of a single introduced Eurasian
genotype, which instead of hybridizing with native
genotypes, is now outcompeting and displacing them
(Saltonstall 2002; Wilcox et al. 2003; Lelong et al.
2007). A similar process may also be occurring in
ornamental fountain grass (Pennisetum setaceum), an
introduced Eurasian perennial in which invasive
populations in Hawaii and Arizona contain the same
genotype (Poulin et al. 2005). This genotype was also
found in noninvasive California populations, indicat-
ing phenotypic plasticity in invasiveness within the
species (Poulin et al. 2005). In another case, Silene
latifolia has undergone multiple introductions into
the United States from genetically structured Euro-
pean populations, but artificially crossing plants from
different source populations did not increase hybrid
reproductive output or survival in a common garden
(Wolfe et al. 2007). Consequently detailed examin-
ations of additional species are needed to fully
understand the impact of intraspecific hybridization
in the evolution of invasiveness.
Case study of the Callery Pear
Pyrus calleryana is an ornamental tree species from
Asia that is in the early stages of spread in the United
States (Vincent 2005; Culley and Hardiman 2007;
Hardiman and Culley 2007). The species, commonly
known as the Callery Pear, is a popular cultivated tree
often planted in commercial and residential areas,
where it is prized for its early spring flowers, rapid
growth, and fall color. Until recently, the species was
considered unable to escape from cultivation or to
naturalize because of self-incompatibility, vegetative
propagation, and rare fruit production (Gilman and
Watson 1994). The species is currently recognized as
invasive because volunteer populations have been
reported with increasing frequency over the last
5 years in at least 26 states (Vincent 2005; Culley and
Hardiman 2007), concurrent with recent observations
of abundant fruit set in cultivated and escaped
individuals. Because of its present spread, the species
is now listed by the United States Fish and Wildlife
as a plant invader of Mid-Atlantic natural areas
(Swearingen et al. 2002) and is considered either
invasive or watch-listed in ten states (Culley and
Hardiman 2007).
The reproductive biology of Pyrus calleryana is
conducive to its ability to invade new areas. In early
spring before leaves appear, abundant flowers are
produced with 6–12 flowers per inflorescence (Cuizhi
and Spongberg 2003). Each flower contains 20
stamens and 2–5 fused carpels with two ovules per
locule, giving a maximum seed number of 10 (Jackson
2003). Pollen is dispersed by several generalist
pollinators, including honeybees (Apis mellifera L.),
bumble bees (Bombus terrestris L.) and hover flies
(Farkas et al. 2002). Fruits mature in autumn and are
dispersed by animals such as European starlings,
American Robins, and squirrels (Gilman and Watson
1994). Pyrus calleryana exhibits gametophytic self-
incompatibility (Zielinski 1965) in which compatible
crosses are only possible between haploid pollen and
The role of intraspecific hybridization in the evolution of invasiveness 1109
diploid pistil tissue that do not share a self-incompat-
ibility allele. In this system, crosses can result in full
compatibility, partial compatibility, or complete
incompatibility, depending on the genotypes of the
two individuals being crossed. In invasive popula-
tions, the self-incompatibility (SI) system acts to
maximize outcrossing and hence hybridization events
(Culley and Hardiman 2007) but its effectiveness
depends on the number of SI alleles present within
populations. In new populations, the SI system may
contribute to an Allee effect and slow invasion
(Taylor et al. 2004; Taylor and Hastings 2005)
because low density of individuals may limit the
number of compatible genotypes present and there-
fore their ability to reproduce with one another (i.e.
decrease fitness). However, the Allee effect can be
overcome with time as the number of introductions
increase and gene flow is facilitated by a variety of
pollen and seed dispersers, which in turn increases
diversity of SI alleles.
Here we provide evidence that the invasive nature
of P. calleryana has evolved via intraspecific hybrid-
ization, as outlined by some of the criteria above.
Namely, introduced populations of P. calleryana
consist of cultivars (i.e. cultivated varieties that have
been artificially selected for horticulturally import ant
traits) that represent native genotypes from different
areas of the Asian range. In addition, crossing
between these genetically distinct cultivars has cre-
ated recombinant hybrid genotypes that comprise
invasive populations. Currently we are examining the
fitness of these genotypes relative to the parental
cultivars to determine if they exhibit a fitness
advantage in field conditions. Because several char-
acteristics of P. calleryana described below are also
present in other introduced species, the case of the
Callery Pear can serve as a model for other poten-
tially invasive species.
History of multiple introductions
Pyrus calleryana was originally introduced to breed
fire blight resistance and provide compatible root-
stock for Pyrus communis, the common edible pear
(Culley and Hardiman 2007). The species was
imported into the United States beginning in the
early 1900s, primarily by the USDA plant explorer,
Frank Meyer and plant breeder Frank Reimer, both of
whom collected seed in various regions in China,
Japan and Korea in 1918. According to Meyer’s
(1918) correspondence, the species was found grow-
ing in a wide variety of habitats in China where it had
a thorny phenotype and sparsely occurred in small
populations. Meyer obtained P. calleryana seed from
at least five different geographic locations while
Reimer also collected seed in Korea and Japan
(Cunningham 1984), although Reimer’s seed collec-
tions were not maintained separately. Following
importation to the United States, the species was
primarily maintained and tested at the USDA Intro-
duction Station in Glenn Dale, Maryland and at
Corvallis, Oregon where large numbers of seedlings
were planted and monitored for fireblight testing and
as a source of rootstock for economically important
Pyrus species.
The species was first cultivated as an ornamental
flowering tree several decades later, beginning with
an attractive non-thorny tree found growing at the
Glenn Dale site and first sold commercially in 1962
as the ‘Bradford’ cultivar (Whitehouse et al. 1963).
As a result of its widespread popularity nationwide,
several additional cultivars were subsequently intro-
duced through the latter half of the twentieth century.
Many of these cultivars are derived directly from
different introductions of Asian seed collected in the
earlier part of the century (Table 1). For example,
‘Bradford’ was selected from a seedlot sent to the
USDA in 1919 from Nanking, China while ‘Autumn
Blaze’ originated from Reimer’s 1918/1919 collec-
tion. A limited number of cultivars are also
potentially of hybrid origin; for example, ‘White-
house’ presumably resulted from a cross between
‘Bradford’ and an unknown P. calleryana (Accession
information for PI420995 at the National Clonal
Germplasm Repository; To
maintain the uniform characteristics of each cultivar,
trees are vegetatively propagated by grafting the
desired cultivar (the scion) onto P. calleryana root-
stock; thus, all individuals of a given cultivar should
consist of genetically identical scions, although the
rootstock genotypes often vary (T.M. Culley, N.A.
Hardiman, unpublished data).
Today at least 25 different cultivars are available
(Table 1) with more being developed and the species
remains extremely popular among nurserymen and
horticulturalists. Over 1.5 million Callery Pear trees
were sold in 1998 alone totaling over $30 million
dollars (Li et al. 2004), and the ‘Chanticleer’ cultivar
1110 T. M. Culley, N. A. Hardiman
was named the Urban Street Tree of the Year in 2005
(Phillips 2004). Consequently, the popularity of this
species among the general public combined with its
commercialization has led to a situation where
different Asian genotypes (i.e. cultivars) have pur-
posely been introduced multiple times across the
country and are still planted today.
Genetic differentiation of source populations
Another criterion for intraspecific hybridization lead-
ing to invasiveness is that the introductions must be
from genetically differentiated source populations.
This seems likely in Pyrus calleryana given that
most cultivars originated as seeds collected from
Table 1 List of cultivars of Pyrus calleryana, including the year each became commercially available, the site of origin and the
source and/or parentage of the cultivar, if known
Cultivar Approx. year Site of origin Source
1972 Independence, KY Chinese seed collected by Meyer; selected from
P. calleryana seedlings in 1969
Autumn Blaze 1978 Corvallis, OR Parent originated from Chinese seed from
Reimer’s 1917 or 1919 collection
Avery Park 1970s Corvallis, OR From a population of P. calleryana seedlings
planted in Avery Park, Corvallis, OR
Bradford 1962 Glenn Dale, MD Chinese seed purchased in Nanking, China in
1919; original tree planted at the USDA
station (Santamour and McArdle 1983)
Bursnozam (Burgandy
1990s Perry, OH Unknown
Cambridge Abt 2003 Cambridge City, IN Unknown
Capital 1981 Washington, D.C. ‘Bradford’ 9unknown P. calleryana parent
Select, Stone Hill, Select,
Glenn’s Form)
1965 Olmsted Falls, OH &
Corvallis, OR
Original tree planted in Cleveland, OH was
derived from commercial seed purchased in
1946 (Santamour and McArdle 1983)
Cleprizam (Cleveland
1990 Perry, OH Unknown
Earlyred Unknown Vincennes, IN ‘Bradford’ 9unknown pollen parent
(Edgedell) 1997 DuPage County, IL P. calleryana 9P. betulifolia
Fronzam (Frontier
) 1990s Perry, OH Unknown
Gladzam (Galdiator
) 1993 Perry, OH Unknown
Grant St. Yellow Abt 1980 OR Unknown
Jaczam (Jack
) 1999 Perry, OH Unknown
Jilzam (Jill
) 1990s Perry, OH Unknown
Mepozam (Metropolitan
) 1990s Perry, OH Unknown
New Bradford
(Holmford) 1996 Boring, OR Unknown
Princess 1976 Olmsted Falls, OH Unknown
Rancho 1965 Olmsted Falls, OH Unknown
Redspire 1975 South Brunswick township,
‘Bradford’ 9unknown pollen parent
(XP-005) 1978 Portland, OR Purchased seed
Valzam (Valiant
) 1975 Perry, OH ‘Cleveland Select’ 9unknown pollen parent
Veyna Abt 2004 Visalia, CA ‘Aristocrat’ 9P. kawakammii unknown cultivar
Whitehouse 1977 Glenn Dale, MD ‘Bradford’ 9unknown P. calleryana at USDA
Cultivars analyzed in the genetic study of differentiation (Hardiman and Culley 2007) are italicized
The role of intraspecific hybridization in the evolution of invasiveness 1111
populations in different regions of Asia that appear to
be genetically divergent (N.A. Hardiman, T.M.
Culley, unpublished data). It is thus possible to
confirm the genetic differentiation of cultivars (i.e.
native genotypes). To do so, we used nine microsat-
ellite markers that were originally designed for
closely related P. communis and Malus domestica
(Yamamoto et al. 2002; Gianfranceschi et al. 1998),
and which successfully amplify in P. calleryana
(Hardiman and Culley 2007). To examine genetic
differentiation among cultivars, we acquired samples
from each of eight commercially available cultivars
in Southwestern Ohio (2–22 individuals sampled per
cultivar; Fig. 1) and individual samples from seven
additional cultivars from the National Clonal Germ-
plasm Repository (NCGR; Table 1). Genotypes of
the multiple samples for each of the eight cultivars
were also compared to test whether individuals within
each cultivar were genetically identical.
Most cultivars were genetically differentiated from
one another and identifiable based on their multilocus
genotypes. In a Principle Coordinates Analysis (PCoA)
based on all possible pairwise genetic distances
calculated according to Smouse and Peakall (1999;
Fig. 1), each cultivar generally clustered away from all
others, indicating that it is genetically distinct. ‘Brad-
ford’, ‘Valzam’, ‘Whitehouse’, and ‘Capital’ each
contained a unique private allele and ‘Grant St.
Yellow’ contained two private alleles. However,
‘Chanticleer’, ‘Cleveland Select’ and ‘Stone Hill’
cultivars were all genetically identical, which is
consistent with anecdotal accounts that they are
derived from the same street tree in Cleveland, Ohio
(Hardiman and Culley 2007). Within each cultivar,
individuals were genetically identical, except for
‘Redspire’, ‘Autumn Blaze’ and one ‘Cleveland
Select’ sample. These anomalous individuals differed
by only a single allele at one to four loci, and were
obtained from the same nursery indicating potential
contamination or mutation within the growers stock. In
addition, an AMOVA provided evidence for signifi-
cant genetic differentiation among cultivars, with the
majority of variation explained by genetic structuring
among cultivars (U
=0.961, P\0.001) rather than
within cultivars. These data do not include rootstock
genotypes, which in preliminary analyses are always
genetically different than the scions with which they
are paired (T.M. Culley, N.A. Hardiman, unpublished
data); thus the rootstock has the potential to cross with
the scions if allowed to sprout and flower. Conse-
quently the introduced populations of cultivars of
P. calleryana in the United States represent a mixture
of genetically different Asian genotypes.
Hybridization and genetic recombination
Given that genetically different cultivars are fre-
quently planted in residential and commercial areas
across the United States, there is potential for these
cultivars to naturally outcross-pollinate and produce
fertile hybrids. This is critical because as a self-
incompatible species, fruits cannot be produced in
P. calleryana through selfing or cross-pollination of
individuals of the same cultivar. The ability of
cultivars to successfully hybridize with one another
was examined two different ways.
Axis 1 (44.9%)
Axis 2 (18.9%)
Early R
Grant S
Avery P
t. Yellow
(N = 18)
Fig. 1 Principle
coordinates analysis based
on pair-wise genetic
distance (calculated
according to Smouse and
Peakall 1999) showing
genetic differentiation
among cultivars of P.
calleryana sample sizes are
given in the legend
1112 T. M. Culley, N. A. Hardiman
First, we performed hand-pollinations in a common
garden over 3 years comparing fruit set among
reciprocal crosses of four common cultivars: ‘Brad-
ford’, ‘Chanticleer’, ‘Aristocrat’ and ‘Redspire’.
Hand-pollinations were performed on multiple indi-
viduals of each cultivar, using emasculated flowers on
days 3–4 of anthesis. Each cross combination between
cultivar pairs was replicated at least twice. Self-
pollinations or crosses within each cultivar were also
preformed and occasionally resulted in fruit formation,
but no viable seeds were obtained. Reproductive
success across all cultivar cross combinations was
high, with four of the cross combinations resulting in
100% fruit set and an average percent fruit set of 75%
(Table 2). Overall, few differences were found among
cross combinations, indicating that cultivars are capa-
ble of freely crossing with one another. The single
exception was the ‘Bradford’ 9‘Chanticleer’ cross
with ‘Bradford’ as the maternal parent in which no
fruits were formed, but this was based on a small
sample size. Differences in fruit set across cross
combinations may be primarily driven by the game-
tophytic SI system in P. calleryana, which is currently
being tested at the genetic level. Fruits on average,
yielded approximately 2 seeds (range: 1–4), with
over 87% seed germination expressed in most crosses
(N.A. Hardiman, T.M. Culley, unpublished data);
seedlings are now being monitored in a common
garden to quantify early establishment and photosyn-
thetic performance. Generalist pollinators were also
observed moving frequently between unmanipulated
flowers of different cultivars with fruits developing
soon thereafter, suggesting that hybridization events
are likely under natural conditions. Overall, these
results indicate that with few exceptions, most culti-
vars are cross-compatible and capable of producing
viable hybrid offspring.
The ability of cultivars to cross-fertilize was also
examined a different way by focusing on the parent-
age of existing invasive individuals. If these
individuals result from recent hybridization between
nearby cultivars as proposed (e.g. Vincent 2005), the
genetic contribution of each cultivar should be evident
in the invasive genotypes. Using nine microsatellite
loci (Hardiman and Culley 2007), invasive individuals
were genotyped in three populations in Ohio (OH),
Tennessee (TN), and Maryland (MD) representing
different ages of invasion. The Cincinnati, OH
population recently formed in the last 7 years and is
hypothesized to contain mostly F
hybrids while the
older Nashville, TN population is expected to contain
more advanced generation hybrids. Because the oldest
population occurs in Glenn Dale, MD where the
species was first introduced in the early 1900s, this
population is expected to consist largely of advanced-
generation hybrids with parentage reflecting both
original Asian genotypes and cultivars in the neigh-
boring area. To quantify cultivar composition at each
site (i.e. putative parents), samples were also collected
from cultivars planted in residential and commercial
areas surrounding each invasive population.
Cultivar identification of neighborhood trees in the
residential and commercial areas near each site was fist
assigned with GeneClass2 (Piry et al. 2004)using
Rannala and Mountain’s (1997) Bayesian method.
Results indicated that neighborhood trees always con-
sisted of a mixture of cultivars but the exact combination
differed between sites (Fig. 2). For example, ‘Bradford’
was the most common cultivar in the OH (55.1%), TN
(79.4%), and MD (8.3%), while ‘Aristocrat’ was the
second most popular tree in OH (15.4%) but was absent
from the TN and MDsites where it is more susceptible to
fireblight infection. There was also a higher proportion
of unknown cultivars in the MD neighborhood
(61.1.9%) than in OH (2.6%) or TN (11.9%), most
likely reflecting the greater diversity of Chinese geno-
types historically planted around the USDA station
(Culley and Hardiman 2007).
The relative genetic contribution of these cultivars
as well as admixture within invasive populations was
next examined at the three sites using the Bayesian
genotype clustering program Structure v2.0 (Falush
et al. 2003). This technique determines the most likely
Table 2 Percent fruit set resulting from a 3 year hand-polli-
nation study of Pyrus calleryana cultivars in a common garden
Paternal source
Aristocrat Bradford Chanticleer Redspire
Aristocrat 0 100% (3) 67% (21) 60% (5)
Bradford 75% (12) 0 0% (4) 100% (4)
Chanticleer 58% (24) 81% (16) 0 70% (10)
Redspire 90% (10) 100% (8) 100% (7) 0
The upper diagonal represents fruit set with the given cultivar
as a pollen donor and the lower diagonal represents the given
cultivar as the pollen receiver. Sample sizes indicating the
number of crosses are shown in parentheses next to each
percentage value
The role of intraspecific hybridization in the evolution of invasiveness 1113
number of genetic populations (K) given the observed
data and assigns individuals to those populations
based on their multilocus microsatellite genotypes.
The most likely value of Kis that which maximizes
the log-likelihood of obtaining the observed sample of
multilocus genotypes. Using the admixture model
with correlated allele frequencies, we ran 20,000 steps
with a burn-in of 30,000 for each Ktested. The highest
model log-likelihood was obtained with K=11,
which corresponded to the eight cultivars and popu-
lations at the three sites. The analysis confirmed that
cultivars are genetically distinct (Fig. 3) and that the
invasive populations in Ohio, Tennessee and Mary-
land each consist of a mixture of cultivar genotypes.
Invasive individuals of a single cultivar genotype
were never detected, indicating that cultivars them-
selves are not escaping into natural sites. As expected,
there was a large number of F
hybrids (i.e. possessing
approximately 50% of two cultivar genotypes) in the
youngest Ohio population and greater recombination
and advanced generation hybrids (containing geno-
typic contributions from 3 or more cultivars) in the
older populations. In Ohio, most of the F
resulted from crosses between ‘Bradford’ and ‘Aris-
tocrat’, which were also the most common cultivars
planted in the surrounding residential neighborhood
(Fig. 2). In Tennessee, many hybrids exhibited ‘Brad-
ford’ parentage, consistent with the popularity of that
(a) (b) (c)
N=78 N=59 N=36
10 %79%
Aristocrat Bradf ord Chanticleer Redspire Ca pital Fa ur ie Unknown
Fig. 2 Proportion of cultivars growing in residential and
commercial areas surrounding invasive populations in (a)
Ohio, (b) Tennessee, and (c) Maryland. Sample sizes are
shown below each graph. Unknown individuals could not be
matched to genotypes of 13 reference cultivars and may
represent cultivars yet to be identified
Fig. 3 Graphical output from structure in which each vertical
bar represents an individual tree for (a) multiple individuals of
known cultivars and (b) invasive populations in Ohio
(N=102), Tennessee (N=60) and Maryland (N=97). The
color of the bar indicates the cultivar group to which an
individual has been placed, and the extent of the color is the
percent of the genotype attributable to the corresponding
group. Individuals containing approximately half of each
genotype of two cultivars are considered F
plants. Cultivars
include ‘Aristocrat’ (A), ‘Bradford’ (B), ‘Redspire’ (R),
‘Capital’ (C), ‘Chanticleer’ (Ch), and ‘Autumn Blaze’ (AB)
1114 T. M. Culley, N. A. Hardiman
cultivar in the surrounding neighborhood. Maryland
populations contained a greater number of unknown
genotypes, which may represent additional unknown
cultivars or Asian genotypes (Culley and Hardiman,
2007). Overall, these data indicate that a diverse
combination of cultivar genotypes contribute to the
invasive populations, consistent with intraspecific
Conclusions and future directions
As indicated by the our ongoing study of the Callery
Pear, intraspecific hybridization can be an important
stimulus for the evolution of invasiveness provided
that specific conditions are met. Namely, at least two
introductions of genetically distinct populations must
occur in the same locality with subsequent hybrid-
ization of different individuals, resulting in novel
genotypic combinations with adaptive potential in the
new environment. Just as only a small number of
introduced species become invasive, intraspecific
hybridization will not lead to invasiveness in every
case (e.g. Wolfe et al. 2007). With the increasing
globalization of our world today, however, introduc-
tions of new populations and species continue, thus
increasing the probability of future hybridization
events. Given the high economic and ecological costs
associated with only a few invasive species (Pimentel
et al. 2000,2005), it is crucial that we identify
evolutionary processes that facilitate invasiveness so
as to prevent and control future problem species.
We are still at an early stage in understanding how
frequently intraspecific hybridization leads to inva-
siveness but the cases identified so far (Lavergne and
Molofsky 2004,2007; Williams et al. 2005,2007;
Culley and Hardiman 2007) indicate that it can occur in
several unrelated species. Even studies in which
evidence does not support intraspecific hybridization
(e.g. Wolfe et al. 2007) are still valuable as they
provide the context for establishing the overall
frequency. Additional investigations are now needed
to determine the extent and effect of intraspecific
hybridization in introduced and native plant taxa.
These studies must take into consideration that intra-
specific hybridization may proceed alongside other
processes promoting invasiveness such as polyploidy
(Schierenbeck and Aı
¨nouche 2006), interspecific
hybridization (Ellstrand and Schierenbeck 2000) and
escape from native predators or competitors (Sakai
et al. 2001). In invasive Spartina in California for
example, both intra- and interspecific hybridization
have been documented (Aı
¨nouche et al. 2003; Bando
2005). Some introduced species also may possess
preadaptive traits that are not a product of hybridiza-
tion per se but rather act to enhance invasiveness once
hybridization produces recombinant genotypes. For
example, P. calleryana in its native Asian range is
tolerant of diverse soil moisture conditions, which is
consistent with its ability to invade wet, mesic or dry
sites in the United States (Culley and Hardiman 2007).
In addition, reed canary grass undergoes intraspecific
hybridization (Merigliano and Lesica 1998; Lavergne
and Molofsky 2007) and typically exhibits high
competitive growth especially in nutrient rich habitats
(Lavergne and Molofsky 2004).
The propensity for a species to undergo intraspe-
cific hybridization will depend in part on traits and
processes that promote outcrossing. Breeding systems
such as dicliny, heterostyly or self-incompatibility
maximize fertilization between genetically distinct
individuals and thus increase the potential for hybrid-
ization. Although such systems may induce an Allee
effect in an initial population, the effect may quickly
disappear as outcrossing occurs between populations,
especially with the contribution of multiple introduc-
tions. For example, two plant species with strong
evidence of intraspecific hybridization are dioecious
(Schinus terebinthifolius; Williams et al. 2005,2007)
or self-incompatible (Pyrus calleryana), traits also
noted in several perennial weeds (Price and Jain 1981).
Such obligatory outcrossing is in contrast with the
traditional characterization of invasive species as self-
compatible (Baker 1974; Price and Jain 1981; Roy
1990; but see Novak and Mack 2005). This suggests
that species in which outcrossing is actively promoted
may be more likely to become invasive through
intraspecific hybridization than selfing species. In
P. calleryana, different bee species indiscriminately
visit flowers and often carry pollen between neigh-
boring cultivars, resulting in hybrid fruit set. Finally,
intraspecific hybridization will also be enhanced by
animal-mediated seed dispersal which often results in
seeds being carried long distances (Schiffman 1997),
thus promoting movement of novel recombinant
genotypes across the landscape. In P. calleryana,
most seedlings bear no genetic similarity to nearby
mature trees, presumably because these seedlings
The role of intraspecific hybridization in the evolution of invasiveness 1115
originated from seeds defecated indiscriminately as
birds forage across an area. Overall, traits that promote
outcrossing and gene flow have the potential to
produce intraspecific hybridization, if combined with
multiple introductions of genetically distinct individ-
uals. Consequently, it is important to closely examine
the reproductive biology of introduced species when
determining their potential for spread.
Future investigations of intraspecific hybridization
should especially consider the impactof horticulture and
agriculture on plant invasions, which can facilitate
multiple introductions of cultivated plant species.
Compared to natural processes, commercialization
allows these species to spread more quickly and
extensively because genetically differentiated cultivars
can be mass-produced and sold nationwide to gardeners
and landscapers who plant them in combination within a
variety of locations. In addition, the horticultural
industry is largely driven by consumer demand for
unique and novel plant species, which in turn facilitates
introduction of non-native species. Although the vast
majority of horticulturally important plant species never
become invasive (Reichard and White 2001), there are
cases of invading species that have a horticultural origin,
including honeysuckle (Amur spp.; Luken and Thieret
1996; Schierenbeck 2004), English Ivy (Hedera spp.;
Clarke et al. 2006) and Brazilian peppertree (Schinus
terebinthifolius; Williams et al. 2005,2007). In some
cases, invasive genotypes may originate unintentionally
after crossing occurs between different cultivars planted
in the landscape, as in P. calleryana (described above)
and Lythrum salicaria (Anderson and Ascher 1993).
Cultivar selection prior to introduction itself can also
increase invasiveness, as with selection for showy
appearance and dense foliage of Japanese Ardisia
crenata in the United States (Kitajima et al. 2006).
Consequently, some plant breeders have begun exam-
ining individual cultivars for invasive traits (Anderson
et al. 2006), such as abundant seed set and high seed
germination (e.g. Lehrer et al. 2006;WilsonandKnox
2006). The next stepis to determine if crossing between
cultivars of particular species results in potentially
invasive genotypes before they are released. In some
cases, sterile cultivars of highly popular species are
being developed (Li et al. 2004) and voluntary initia-
tives have been proposed to minimize plant invasions
(Burt et al. 2007).
Finally, the importance of intraspecific hybridiza-
tion in species invasions has several implications for
management. Control plans of invasive species must
first determine if intraspecific hybridization is present
and if so, every effort must be made to prevent
introduction of new genotypes into an area. Because
genotypes may be morphologically indistinguishable
from one another, land managers need to work
closely with scientists to use genetic techniques to
identify problematic genotypes for early removal. In
cases where different genotypes have already been
widely released into the environment, as with the
popular Callery Pear, it is unrealistic that the invasive
species will ever be completely eradicated and
therefore control measures that prevent formation of
new populations will be most effective. These should
involve the following: (1) quick removal of invasive
populations in natural areas after their detection;
(2) replacement of cultivated parental plants with
non-invasive species whenever possible, especially in
locations near natural areas; (3) consideration of
voluntary self-regulation or legislative measures that
minimize introduction of new, compatible genotypes;
(4) education of the general public on the importance
of using suitable alternatives. To this end, the
development of completely sterile cultivars of
highly popular horticultural species may reduce the
number of parental genotypes capable of spawning
invasive populations while still providing a profitable
alternative for the nursery industry. Ultimately,
understanding how invasiveness may evolve in light
of intraspecific hybridization is of paramount impor-
tance to preventing or controlling invasive species
before they exert substantial ecological and economic
impacts on the environment.
Acknowledgments The authors thank D. Ayers, N. Ellstrand
and K. Schierenbeck for organizing the symposium that led to this
special issue, as well as enlightening discussionsand comments on
the manuscript. K. Manbeck provided an invaluable perspective
from the green industry while M. Klooster, S. Rogstad and two
anonymous reviewers provided helpful suggestions that greatly
improved the manuscript. This research was supported by a grant
from the US Department of Agriculture, Cooperative State
Research, Education, and Extension Service, to T.M.C. (USDA
CREES 06-35320-16565).
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The role of intraspecific hybridization in the evolution of invasiveness 1119
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... Intraspecific hybridization is believed to contribute to the invasive spread of species, including Phalaris arundinacea (Lavergne and Molofsky, 2007), Pyrus calleryana (Culley and Hardiman, 2009), and Schinus terebinthifolius (Williams et al., 2005). For example, Pyrus calleryana is self-incompatible but readily sets seeds and forms thickets when crossed with cultivars of the same species (Culley and Hardiman, 2009). Although S. coccinea is beneficial to pollinators and is not currently on a list of invasive species for the United States, its ability to reseed may be undesirable. ...
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Salvia is the largest genus in the Lamiaceae with more than 1000 species. The species S. coccinea used in this study has naturalized in the southeastern United States and is an important plant for pollinators. This project aimed to improve phenotypic characteristics of S. coccinea for use in the landscape by selecting for increased petal size and unique petal color. Two elite accessions were selected for hybridization using the pedigree method. One selection displayed compact habit with bicolored coral and white flowers, while the other was slightly larger with solid red flowers. Selections were made based on improved flower color and larger petal size. The breeding program achieved a 25% increase in petal width and a more vivid petal color for the coral bicolored selections. Additionally, a 60% increase in petal width was achieved for red flowers. These novel selections are attractive plants for the landscape, displaying improved ornamental value and supporting local pollinator populations.
... Generally, population differentiation can be caused by gene flow, genetic drift, or local adaptation (Shah et al. 2020;Wambugu and Henry 2022). Gene flow can, for example, occur via intraspecific hybridization (Stebbins 1950), which is defined as a "successful mating between individuals from well-differentiated populations originally isolated from one another and consequently with different gene frequencies" (Culley and Hardiman 2009). Therefore, hybridization can reshape the genetic composition of populations and create novel combinations of genes that allow adaptation to changing environments (Anderson et al. 2006;Anderson and Stebbins 1954;Arnold 1997;Stebbins 1950). ...
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... However, the invasive potential of some species or their hybrids (interspecific or even intraspecific) introduced into new areas must be monitored through future research. P. calleryana, a self-incompatible Chinese species that is widely planted as an ornamental tree in the United States, is now escaping cultivation and occurring in disturbed environments, where it has the potential to establish dense thickets and migrate into natural and managed lands where these trees could cause complex and varied harmful ecological effects [75,76]. ...
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Pear is one of the most important fruit species grown in the temperate zones of the globe. Besides fruit production, pear species are highly valued in forestry and agroforestry systems; in landscaping, as ornamental features; as fruits of ecological value, and in other areas. The Pyrus species, obtained from a gene bank, were evaluated for the different morphological traits of the trees, leaves, flowers, and fruits, as well as their responses to attacks from principal diseases and pests. Phenotypic data were examined using correlation and multivariate analyses, and a dendrogram of morphological traits was completed via molecular investigations at the DNA level using the RAPD markers. The findings revealed the complexities of the phenotypic and genetic connections among Pyrus species, as well as the difficulty in establishing phylogenetic relationships among pear species. The findings also demonstrated that the wide variability between species with different geographical origins, and their multiple peculiarities of interest, represents a cornerstone as the source of genes of great utility for pear breeding or for utilizing trees for different edible crops and for silvocultural, landscape, or ecological purposes.
... A diversity of genotypes and phenotypes also provides the raw material for an introduced lineage to evolve in the longer term, with admixture among diverse genotypes creating novel trait combinations (Schierenbeck and Ellstrand 2009;Hovick and Whitney 2014). High diversity and cryptic admixture are especially common in escaped ornamental plants (Culley and Hardiman 2009). Cultivars with desirable traits are often sourced from geographically disparate sources in the native range and selected through breeding. ...
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Newly introduced trait diversity can spur rapid evolution and facilitate local adaptation in the introduced plant Lythrum salicaria. The horticultural plant L. virgatum might further introduce meaningful trait variation by escaping into established L. salicaria populations or by hybridizing with L. salicaria. Although many experiments have focused on L. salicaria genotypes, relatively little is known about L. virgatum ecology. We used a greenhouse common garden to compare traits and flood response of L. salicaria and L. virgatum collected from two sources each in their native range. We tested the hypotheses that these two wetland taxa have comparable responses to flooding (inundation), and that flood tolerance correlated to higher fitness. Flooding produced stronger stress responses in L. virgatum. Compared to L. salicaria, L. virgatum shifted more aboveground allocation away from reproduction, decreased inflorescence biomass by 40% more, and produced 7% more stem aerenchymatous phellum, a specialized tissue that maintains aeration. Despite these more pronounced responses to flooding stress, L. virgatum had higher fitness (inflorescence biomass and reproductive allocation) than L. salicaria. Overall, L. virgatum differed from L. salicaria in functionally important ways. Lythrum virgatum persisted under flooding and produced more reproductive biomass than L. salicaria under both flooded and non-flooded conditions. However, inundation stressed L. virgatum more than L. salicaria. Lythrum virgatum is likely able to establish into the wetland habitats in which L. salicaria prevails but may possess broader habitat tolerances.
... Insights gleaned from the fast-growing field of invasion biology, which encompasses studies of rapid evolution, invasibility, and ecoevolutionary dynamics could be potentially translatable to feral models. Although feral crops account for up to 14% of invasive species in the United States, feral ornamental plants account for up to half of invasive species in the United States but are even less wellstudied than feral crops (Culley & Hardiman, 2009;Li et al., 2004;Reichard & Campbell, 1996). Understanding pathways to domestication and feralization in ornamentals could provide a useful parallel study system to crops. ...
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Societal Impact Statement: Given the rapidly increasing drought and temperature stresses associated with climate change, innovative approaches for food security are imperative. One understudied opportunity is using feral crops—plants that have escaped and persisted without cultivation—as a source of genetic diversity, which could build resilience in domesticated conspecifics. In some cases, however, feral plants vigorously compete with crops as weeds, challenging food security. By bridging historically siloed ecological, agronomic, and evolutionary lines of inquiry into feral crops, there is the opportunity to improve food security and understand this relatively understudied anthropogenic phenomenon. Summary: The phenomenon of feral crops, that is, free-living populations that have established outside cultivation, is understudied. Some researchers focus on the negative consequences of domestication, whereas others assert that feral populations may serve as useful pools of genetic diversity for future crop improvement. Although research on feral crops and the process of feralization has advanced rapidly in the last two decades, generalizable insights have been limited by a lack of comparative research across crop species and other factors. To improve international coordination of research on this topic, we summarize the current state of feralization research and chart a course for future study by consolidating outstanding questions in the field. These questions, which emerged from the colloquium “Darwins' reversals: What we now know about Feralization and Crop Wild Relatives” at the BOTANY 2021 conference, fall into seven categories that span both basic and applied research: (1) definitions and drivers of ferality, (2) genetic architecture and pathway, (3) evolutionary history and biogeography, (4) agronomy and breeding, (5) fundamental and applied ecology, (6) collecting and conservation, and (7) taxonomy and best practices. These questions serve as a basis for ferality researchers to coordinate research in these areas, potentially resulting in major contributions to food security in the face of climate change.
... Potentially, this discordance may be explained by intraspecific gene flow by hybridisation between the morphotypes in Lake Albert, or incomplete lineage sorting (Després, 2019). The former, intraspecific hybridisation, was earlier linked with the evolution of invasiveness (Culley and Hardiman, 2008). ...
The Nile perch (Lates niloticus) is a notorious invasive species. The introductions of Nile perch into several lakes and rivers in the Lake Victoria, Uganda, region led to the impoverishment of the trophic food webs, particularly well documented in Lake Victoria. Additionally, its parasites were co-introduced, including Dolicirroplectanum lacustre (Monogenea, Diplectanidae). Dolicirroplectanum lacustre is the single monogenean gill parasite of latid fishes (Lates spp.) inhabiting several major African freshwater systems. We examined the intra-specific diversification of D. lacustre from Lates niloticus in Lake Albert, Uganda (native range) and Lake Victoria (introduced range) by assessing morphological and genetic differentiation, and microhabitat preference. We expected reduced morphological and genetic diversity for D. lacustre in Lake Victoria compared with Lake Albert, as a result of the historical introductions. We found that D. lacustre displayed high morphological variability within and between African freshwaters, with two morphotypes identified, as in former studies. The single shared morphotype between Lake Albert and Lake Victoria displayed similar levels of haplotype and nucleotide diversity between the lakes. Mitonuclear discordance within the morphotypes of D. lacustre indicates an incomplete reproductive barrier between the morphotypes. The diversification in the mitochondrial gene portion is directly linked with the morphotypes, while the nuclear gene portions indicate conspecificity. Based on our results, we reported reduced genetic and morphological diversity, potentially being a result of a founder effect in Lake Victoria.
... A number of mechanistic drivers and pathways can lead to de-domestication and ultimately independent reproducing feral populations that have greater invasive potential. Wu et al. (2021) outline how the reacquisition of wild-like characteristics that enable establishment beyond cultivation can occur via one of three pathways: from an endoferal origin where there is spontaneous mutations in genes underlying key traits, [e.g., Weedy rice (Qiu et al., 2017), Tibetan semiwild wheat (Guo et al., 2020), feral apple (Cronin et al., 2020) and feral olive (Mekuria et al., 2002)]; an exo-endoferal origin from natural hybridization between domesticate-derived forms with divergent genotypes [e.g., feral Callery pear (Culley and Hardiman, 2009)], or lastly, an exoferal origin from introgression between weedy or wild relatives [e.g., California wild radish (Hegde et al., 2006), johnsongrass (Paterson et al., 1995), weed beet (Fénart et al., 2008)]; (Ellstrand et al., 2010;Kanapeckas et al., 2016;Wu et al., 2021). The classification of de-domesticated populations depends on their impacts and where they colonize (Wu et al., 2021). ...
Cultivated plants provide food, fiber, and energy but they can escape, de-domesticate, colonize agroecosystems as weeds, and disrupt natural ecosystems as invasive species. Escape and invasion depend on traits of the species, type and rate of domestication, and cultivation context. Understanding this “de-domestication invasion process” is critical for managing conservation efforts to reduce unintended consequences of cultivated species in novel areas. Cannabis (Cannabis sativa L.) is an ideal case study to explore this process because it was one of the earliest plants to co-evolve with humans, has a crop to weed history, and has been introduced and cultivated globally. Moreover, recent liberalization of cannabis cultivation and use policies have raised concerns about invasion risk. Here, we synthesize knowledge on cannabis breeding, cultivation, and processing relevant to invasion risk and outline research and management priorities to help overcome the research deficit on the invasion ecology of the species. Understanding the transition of cannabis through the de-domestication-invasion process will inform policy and minimize agricultural and environmental risks associated with cultivation of domesticated species.
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Biological invasions cause multi-trillion-dollar impacts worldwide. However, the development of approaches to predict drivers and magnitudes of economic costs remain limited. The use of fitness-relevant traits offers a promising, yet neglected, avenue to close this gap. Certain traits acquired during evolutionary history predispose species to succeed in non-native regions and determine variation in impact within and among invasive alien species. Invader’s performance can also rapidly be optimized via natural selection and phenotypic plasticity once exposed to the newly invaded environmental conditions. Given that invader impacts are increasingly viewed through an economic lens, this generates a trait-mediated component of economic impacts that can be quantified through individual traits and the synergistic effects across multiple traits. We discuss these new concepts and highlight emerging transdisciplinary avenues to quantify invasion costs from species traits, and the key roles that big data, museum collections, and machine learning approaches are expected to play.
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March 2023 ECOLOGICAL RESTORATION 41:1 • 25 This open access article is distributed under the terms of the CC-BY-NC-ND license ( and is freely available online at: Supplementary materials are freely available online at: ABSTRACT Pyrus calleryana (Callery pear) is an invasive plant that threatens ecosystems in the eastern United States. We investigated the efficacy of various control techniques on P. calleryana invasion in grasslands. Treatments were applied to (a) P. calleryana stems that had experienced mowing annually for several years and were sprouting (n = 100 stems; "trees-sprouting") and (b) stems that had established ca. 10 years earlier, had never been cut, and were single-stemmed trees (n = 40 stems; "trees-intact"). In both experiments, existing stems were cut and randomly assigned one of the following treatments: cut only (control), burning, freezing, or herbicide, and in the trees-sprouting experiment there was also a negative control of monitoring existing sprouts. All trees in which the cut stumps were treated with herbicide were effectively killed, whereas stems in all other treatments, in both experiments, generated a vigorous sprout response. In the trees-sprouting experiment , there was a strong overall effect of treatments (RMANOVA; p < 0.001) and prescribed fire created a statistically significant increase in sprout number in relationship to the negative control (post-hoc test; p = 0.036). In the trees-intact experiment, there was vigorous sprouting in response to all treatments other than herbicide. Stump freezing resulted in a delay in sprout response; however, all frozen stems eventually sprouted. The ability of this species to sprout vigorously, even after experiencing frequent and intense ecological disturbance, creates the potential for a fundamental alteration of old-field succession in habitats where this species is present.
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Generally, urbanization is a major threat to biodiversity; however, urban areas also provide habitats that some species can exploit. Flying-foxes (Pteropus spp.) are becoming increasingly urbanized; which is thought to be a result of increased availability and temporal stability of urban food resources, diminished natural food resources, or both. Previous research has shown that urban-roosting grey-headed flying-foxes (Pteropus poliocephalus) preferentially forage in human-modified landscapes. However, which land-use areas and food plants support its presence in urban areas is unknown. We tracked nine P. poliocephalus roosting in Adelaide, South Australia, between December 2019 and May 2020, using global positioning systems (GPS), to investigate how individuals used the urban landscape mosaic for feeding. The most frequently visited land-use category was “residential” (40% of fixes) followed by “road-side,” “reserves” and “primary production” (13–14% each). However, “reserves” were visited four times more frequently than expected from their areal availability, followed by the “residential” and “road-side” categories that were visited approximately twice more than expected each; in contrast, the “primary production” category was visited approximately five times less than expected. These results suggest that while residential areas provide most foraging resources supporting Adelaide’s flying-fox population, reserves contain foraging resources that are particularly attractive to P. poliocephalus. Primary production land was relatively less utilized, presumably because it contains few food resources. Throughout, flying-foxes visited an eclectic mixture of diet plants (49 unique species), with a majority of feeding fixes (63%) to locally indigenous Australian native species; however, in residential areas 53% of feeding visits were to non-locally indigenous species, vs only 13% in reserves. Flowering and fruiting phenology records of the food plants visited further indicated that non-locally indigenous species increase the temporal availability of foraging resources for P. poliocephalus in urban Adelaide. Our findings demonstrate the importance of residential areas for urban-roosting P. poliocephalus, and suggest that the anthropogenic mixture of food resources available in the urban landscape mosaic supports the species’ year-round presence in urban areas. Our results further highlight the importance of conserving natural habitats within the urban landscape mosaic, and stress the need for accounting for wildlife responses to urban greening initiatives.
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Abstract Genetic incompatibilities and low offspring fitness are characteristic outcomes of hybridization between species. Yet, the creative potential of recombination following hybridization continues to be debated. Here we quantify the outcome of hybridization and recombination between adaptively divergent populations of the North American legume Chamaecrista fasciculata in a large-scale field experiment. Previously, hybrids between these populations demonstrated hybrid breakdown, suggesting the expression of adaptive epistatic interactions underlying population genetic differentiation. However, the outcome of hybridization ultimately rests on the performance of even later generation recombinants. In experiments that compared the performance of recombinant F6 and F2 generations with nonrecombinant F1 and parental genotypes, we observed that increasing recombination had contrasting effects on different life-history components. Lifetime fitness, defined as the product of survivorship and reproduction, showed a strong recovery of fitness in the F6. The overall gain in fitness with increased recombination suggests that hybridization and recombination may provide the necessary genetic variation for adaptive evolution within species. We discuss the mechanisms that may account for the gain in fitness with recombination, and explore the implications for hybrid speciation and phenotypic evolution
We describe extensions to the method of Pritchard et al. for inferring population structure from multilocus genotype data. Most importantly, we develop methods that allow for linkage between loci. The new model accounts for the correlations between linked loci that arise in admixed populations (“admixture linkage disequilibium”). This modification has several advantages, allowing (1) detection of admixture events farther back into the past, (2) inference of the population of origin of chromosomal regions, and (3) more accurate estimates of statistical uncertainty when linked loci are used. It is also of potential use for admixture mapping. In addition, we describe a new prior model for the allele frequencies within each population, which allows identification of subtle population subdivisions that were not detectable using the existing method. We present results applying the new methods to study admixture in African-Americans, recombination in Helicobacter pylori, and drift in populations of Drosophila melanogaster. The methods are implemented in a program, structure, version 2.0, which is available at
Biology of Apples and Pears is a comprehensive reference book on all aspects of pomology at the organ, tree and orchard level. It provides detailed information on propagation, root and shoot growth, root stock effects, canopy development in relation to orchard design, flowering, pollination, fruit set, fruit growth, fruit quality factors and quality retention in store. It also deals with mineral nutrition, water-relations and irrigation, diseases and pests and biotechnology. The book emphasises the scientific basis of modern tree and orchard management and fruit storage. It describes key cultivar differences and their physiology and genetics and environmental effects and cultivar x environment interactions in tropical and sub-tropical as well as temperate zone conditions. It is written for fruit growers, extension workers, plant breeders, biotechnologists and storage and crop protection specialists as well as for researchers and students of pomology and horticulture.
Bryonia alba (Cucurbitaceae) is a Eurasian herbaceous vine that spreads vegetatively through the production of many stems from a large, tuberous root. The only known U.S. populations of this aggressive apomict are in Idaho, Montana, Utah, and Washington and likely stem from deliberate (and subsequent accidental) introductions. To assess levels and patterns of genetic diversity of B. alba across its introduced range, 23 populations were analyzed for allozyme variation using 12 enzyme systems. On average, 14.9% of loci are polymorphic per population (1.19 alleles per locus)—low values compared to other vascular plant taxa. Mean percent polymorphic loci differed among regions, with the highest values in the Washington and northern Idaho region (20.2%) and Montana (19.0%) and lower values in populations from the Utah and southern Idaho region (7.4%). Observed heterozygosity exceeded that expected at Hardy-Weinberg equilibrium in six of 23 populations, and a statistically significant excess of heterozygosity was detected at Pgm-1 in 18 of 23 populations. The level of population differentiation is high (GST = 0.544); however, the level of differentiation among populations within regions is much lower. These results are consistent with the genetic variation and structure expected for an apomict. Based on the level of genetic differentiation among populations, the current disjunct distribution of B. alba in its new range results from two, and possibly three, separate introductions in the western United States. These introductions may stem, in part, from the vine's 19th century use as a medicinal and ornamental plant.
Inter-racial hybridization was performed successfully in Pinus roxburghii taking three different provenances, i.e. Pauri, Badiyargarh and Srinagar (locality-specific) at lower (900 m a msl) and higher (1900 m a msl) altitudes. The results revealed that cone and seed setting percentages in the selected provenances varied from 38.57 to 60.00% and 76.00 to 88.00% at the lower, and 36.00 to 58.33% and 68.00 to 84.67% at the higher altitudes, respectively. Controlled pollination resulted in enormous fertilization success, with no signs of incompatibility. Ovulate strobili remained receptive up to 5 days.
A unique attribute of invaders is that they thrive in a country in which they did not evolve. In this chapter, I review the physiological, demographic and genetic attributes of invaders sensu stricto (excluding native weeds or colonists). When compared to similar native species, invaders often have features likely to endow them with higher relative fitness. However, the few available comparisons may constitute a biased sample. Attempts to generalize show that the invasive flora of a country is composed of a large array of plant types and that there are no attributes with which to characterize invaders in general. More specific approaches of invasions, centered on the invaders or the recipient habitats, are reviewed. The need for an approach combining ecolological and evolutionary features of habitats and introduced species is emphasized.