Int. J. Plant Sci. 164(5):703–710. 2003.
䉷 2003 by The University of Chicago. All rights reserved.
1058-5893/2003/16405-0004$15.00
HABITAT DIFFERENTIATION BETWEEN DIPLOID AND TETRAPLOID
GALAX URCEOLATA (DIAPENSIACEAE)
Marc T. J. Johnson,1 Brian C. Husband, and Tracy L. Burton2
Department of Botany, University of Guelph, Guelph, Ontario N1G 2W1, Canada
Polyploid plants are typically geographically separated from their diploid progenitors, but it is rarely known
whether this segregation is maintained by habitat differentiation or by selection against triploid hybrids. We
tested for habitat differentiation between diploid and tetraploid populations of Galax urceolata in the southeastern United States by comparing the abundance and richness of vegetation associated with their respective
habitats and testing for spatial autocorrelation in the distributions of diploid and tetraploid plants. Diploid
and tetraploid habitats did not differ with respect to plant species richness. Herbaceous plants were 500%
more common in tetraploid than diploid populations, while shrubs and trees were more common in diploid
populations. Differences were found for the relative abundances of the most common species, with three species
being significantly more prevalent in tetraploid than diploid populations. Spatial patchiness was detected for
diploids and tetraploids, on a scale of 25 km. These results provide evidence for habitat differentiation, which
may contribute to the maintenance of spatial separation between diploid and tetraploid G. urceolata.
Keywords: adaptation, Appalachian Mountains, ecotype, geographic differentiation, isolation by distance,
polyploidy, reproductive isolation, speciation.
Introduction
dependent processes may affect diploids and polyploids where
the taxa involved must be differentiated with respect to their
ecophysiological requirements. Alternatively, spatial segregation between diploids and polyploids may be maintained by
environmentally independent selection. Such selection results
from the inherently low fitness of triploid hybrids formed from
diploid-tetraploid matings (Levin 1975; Ramsey and Schemske
1998; Burton and Husband 2000), as well as the transmission
disadvantage experienced by the minority cytotype (Levin
1975; Husband 2000). Selection against triploids, as with
homoploid hybrids, will restrict or prevent the invasion of a
population of one cytotype by another and thereby maintain
any geographical structure in cytotype distribution that has
arisen from past dispersal or colonization events (Barton and
Hewitt 1985).
Previous attempts to test the mechanisms of geographic differentiation between diploids and polyploids have often involved comparisons of performance in a common environment
(Bretagnolle and Thompson 1996). Any observed differences
in growth or morphology are taken as evidence for genetic
differentiation, but it is unclear how such differences affect
cytotype distributions. To this end, some researchers have compared the habitats of polyploids and diploids (Rothera and
Davy 1986; Lumaret et al. 1987; Felber-Girard et al. 1996;
Husband 2000), but many of these studies have been based
on casual observation or qualitative measures of habitat (Nesom 1983; Rothera and Davy 1986; Lumaret et al. 1987; Husband and Schemske 1998). Such an approach may be sufficient
when the cytotypes’ habitats are distinct, but qualitative measures give little indication of the magnitude of differentiation
and often preclude the use of statistical inference. In contrast,
Felber-Girard (1996) employed a rigorous statistical approach
and demonstrated that habitat differentiation based on simi-
Polyploidy is the state of having more than two chromosome
sets per nucleus (Thompson and Lumaret 1992) and is a widespread phenomenon in angiosperms (Masterson 1994). Plant
biologists have long been interested in the consequences of
polyploidy for the geographical distribution of plants (Stebbins
1950) because both allo- and autopolyploids are typically separated from their diploid ancestors (Lewis 1980; Levin 1983,
2002). Although such geographic separation is common, range
size and degree of overlap between polyploids and their progenitors vary widely among species (Nesom 1983; Stebbins
and Dawe 1987; Keeler 1990; van Dijk et al. 1992; Masumori
et al. 1995; Husband and Schemske 1998). The magnitude of
such geographic differentiation is critical to understanding the
maintenance of diploid and polyploid cytotypes within species
and the role of polyploidization in the evolution of ecological
tolerances. Despite their importance, little is known about the
mechanisms underlying the maintenance of geographic separation between cytotypes within species.
Geographic separation between polyploids and diploids can
be maintained by either environmentally dependent or environmentally independent selection. The geographical separation of taxa is often interpreted as the result of ecological
sorting along an abiotic or biotic environmental gradient (Endler 1977; Fowler and Levin 1984). Similar environmentally
1
Author for correspondence; current address: Department of Botany, University of Toronto, Toronto, Ontario M5S 3B2, Canada; email [email protected].
2
Current address: Canadian Food Inspection Agency, Nepean, Ontario K1A 0Y9, Canada.
Manuscript received February 2003; revised manuscript received May 2003.
703
704
INTERNATIONAL JOURNAL OF PLANT SCIENCES
larity of species was occurring between diploid and tetraploid
plants of Anthoxanthum alpinum. Such an approach along
with quantitative data may be particularly important for detecting habitat differentiation that is less obvious, as is often
the case when ploidy varies within species (Lewis 1980).
This study builds on recent research on the distribution of
diploid (2np2xp12) and autopolyploid (2np3xp18;
2np4xp24) cytotypes of Galax urceolata (Poiret) Brummitt
(Diapensiaceae). Galax urceolata is a clonal herbaceous perennial plant that is endemic to the Blue Ridge Mountains of
southeastern United States (Nesom 1983; Burton and Husband
1999). Populations occur in forested habitats and range in size
from fewer than 10 to several thousand plants (Baldwin 1941;
Nesom 1983; Burton and Husband 1999). In a sample of 1570
individuals from 42 locations, Burton and Husband (1999)
showed that G. urceolata comprises discrete populations that
form a mosaic of cytotype frequencies across the species’ range.
Forty-two percent of populations contained a single cytotype,
with diploids more common in the northeast and tetraploids
more common in the southwest. Although Nesom (1983) suggested that diploids and tetraploids prefer different moisture
regimes, the mechanisms underlying their spatial differences
have never been tested quantitatively.
Using two approaches, we tested the hypothesis that diploid
and tetraploid populations of G. urceolata are differentiated
with respect to their habitats: (1) We compared the vegetation
associated with diploid and tetraploid populations. The habitats may differ with respect to any number of abiotic and
biotic factors, but the composition and abundance of the surrounding vegetation should reveal ecologically meaningful differences (Tuomisto et al. 2003). We predicted that if cytotypes
are differentiated, their habitats should differ with respect to
plant species richness, relative abundance, or total abundance.
(2) We used spatial autocorrelation analysis to determine
whether diploid and tetraploid populations were distributed
heterogeneously. Because the abiotic environment is likely to
be spatially autocorrelated (Engen et al. 2002; Tuomisto et al.
2003), evidence for spatial heterogeneity in cytotype frequency
would be consistent with habitat differentiation between
cytotypes.
Material and Methods
Study Sites
In total, 42 populations of Galax urceolata, primarily from
the Blue Ridge Mountains, were used in this study. Cytotype
frequencies, geographic locations, and code numbers for all
populations were reported in Burton and Husband (1999, fig.
1). Eight of these populations were used in the vegetation comparison, while all 42 populations were used in the spatial autocorrelation analysis. The cytotype frequencies for populations were derived from estimates of DNA content for four to
166 plants per population (mean p 37.3/population). Sample
sizes for estimating DNA content ranged from 14 to 95 for
the eight populations in the vegetation comparison (Burton
and Husband 1999).
Associated Vegetation
In June 1998, we tested for habitat differences by comparing
the associated vegetation of four diploid G. urceolata populations (population codes 8, 21, 31, 36 in Burton and Husband
1999) with four tetraploid G. urceolata populations (codes 9,
10, 17, 32). Populations were chosen haphazardly from
throughout the range with the conditions that diploid and
tetraploid populations be roughly paired in space and that the
frequency of the most common cytotype should exceed 90%.
In each population, we delineated the area in which G. urceolata was found. This area was divided into four (populations 31, 21, 9, 32, 17) or five (populations 8, 36, 10) equalsized strata; within each stratum a sampling point was
randomly located. A nested quadrat was placed at each sampling point, and species richness, species abundance, and relative abundance were estimated for three growth-form categories: herbaceous plants (herbs), shrubs, and trees. Herbs
included all nonwoody species; shrubs were defined as any
woody plants between 0 and 3 m height (including species that
can exceed 3 m in height) and woody vines; and trees were
defined as any woody plant (excluding vines) greater than 3
m in height (Mueller-Dombrois and Ellenberg 1974).
Sampling procedures and quadrat sizes for each plant
growth form were adjusted to capture comparable amounts
of variation (Goldsmith et al. 1986). Herbs and shrubs were
sampled in square quadrats with an area of 1 m2 and 4 m2,
respectively, while quadrats for censusing trees were circular
with an area of 314 m2 (10-m radius). Quadrats for herbs and
shrubs were subdivided into 100 and 400 equal-sized cells
(0.1 # 0.1 m), respectively. Species abundance was measured
as the number of cells that contained at least one basal stem.
For trees, abundance was measured as the absolute number
of tree trunks of a given species that were entirely within the
circumference of the circular quadrat. For each species, we
measured relative abundance as its abundance divided by the
sum of abundances for all species within the same plant type,
that is, herb, shrub, or tree.
Plants were identified to species in the field or collected for
later identification using standard floras and the local herbarium at Highlands Biological Station, North Carolina. Nine rare
taxa were not identified to species but were included in the
abundance and species richness analyses as morphospecies. All
nomenclature follows Gleason and Cronquist (1991).
Analyses. Three categories of response variables were analyzed to characterize the vegetative community: total abundance, relative abundance, and species richness. Total abundance was calculated separately for each growth form by
summing the abundance measures of each species within a
nested quadrat. Relative abundance was compared for the nine
most common species, which were chosen based on their presence in at least 15% of quadrats (table 1). To improve homogeneity of variance and normality, total and relative abundance data were transformed using natural log and arcsine–
square root transformations, respectively. Species richness was
summed across herbs, shrubs, and trees, at each sampling
point; no transformations were needed.
Using separate analyses, we tested for differences between
diploid and tetraploid habitats in total abundance of each
growth form and relative abundance of common species using
JOHNSON ET AL.—HABITAT DIFFERENTIATION AND POLYPLOIDY IN GALAX
MANOVA in the PROC GLM program (SAS Institute,
Cary, N.C.). Significance tests in MANOVA were based on P
values of the F-test, using the Wilks’s l criterion. The statistical
model for both analyses was dependent variables p
cytotype + population(cytotype) + error. For the MANOVA
on total abundance, we stipulated population(cytotype) as the
denominator error to test for the cytotype effect because there
was significant variation between populations of a particular
cytotype (P ! 0.001). For the MANOVA on relative abundance, it was not possible to use the population(cytotype) effect
as the denominator because there were insufficient degrees of
freedom. Instead, we used the residual error matrix as the
denominator, which is appropriate in this case since the effect
of population(cytotype) was not significant (P p 0.39).
We also performed univariate analyses on all dependent variables inputted into the MANOVAs using the PROC MIXED
program (SAS), which uses restricted maximum likelihood
(REML) estimation and the Satterthwaite correction to adjust
the degrees of freedom (Littell et al. 1996). Cytotype was designated as a fixed effect and population was nested within
cytotype (as with the MANOVAs). Significance of random effects was determined using a one-tailed x 2 test calculated as
the log likelihood ratio (Littell et al. 1996, p. 44). Species
richness was analyzed using the same univariate model.
Spatial Autocorrelation of Cytotypes
We generated spatial correlograms and chloropleth maps to
test for spatial autocorrelation in cytotype frequency (Sokal
and Jacquez 1991). Correlograms are based on the value of
Moran’s I, which ranges from 1 to ⫺1, indicating positive and
negative associations in cytotype frequency, respectively. No
spatial dependency is inferred when I p 0. Correlograms give
the value I for a range of distance intervals, and the statistical
significance of the overall correlogram and individual I values
indicate whether spatial patterns deviate from random (Sokal
and Oden 1978). Chloropleth maps show locations of populations reproduced to scale, where populations are shaded
for one of 10 cytotype frequency intervals. Maps are analyzed
visually to detect coarse patterns.
Spatial correlograms were generated for both diploid and
tetraploid cytotypes in SAAP 4.3 (Wartenberg 1989). Distance
Table 1
Common Vascular Plant Species Used in the Multivariate
and Univariate Comparisons of Relative Abundance
between Diploid and Tetraploid Populations
of Galax urceolata
Herb (nonwoody plants):
G. urceolata (Poiret) Brummitt
Shrub (woody plants !3 m and all wood vines):
Acer rubrum L.
Kalmia latifolia L.
Smilax rotundifolia L.
Tree (woody plants 13 m):
A. rubrum L.
Nyssa sylvatica Marshall.
Quercus prinus L.
Quercus rubra L.
Tsuga canadensis (L.) Carrière.
705
Fig. 1 Mean abundance of vascular plants for herbs, shrubs and
trees, compared between single-cytotype diploid (solid bars) and tetraploid populations (open bars). The mean abundance is the least
squares mean (LSM) of the nontransformed data from the nested
ANOVA. One standard error of the LSM is displayed. No differences
are statistically significant at 0.05.
classes were chosen a priori to enable detection of evolutionary
processes acting at small and large scales while simultaneously
maximizing the number of populations in each class. The upper
distance class boundaries for the first eight intervals were 25,
50, 85, 119, 157, 205, 263, and 332 km, respectively. We
followed the standard convention by excluding the last two
distance classes (i.e., 9 and 10) because values at these classes
are generally not interpretable (Slatkin and Arter 1991; Sokal
and Jacquez 1991). SAAP 4.3 requires latitude and longitude
coordinates to calculate distance between populations. These
coordinates were taken from United States Geological Survey
topographical maps (scale p 124,000) and detailed road
maps. The software also estimates the overall significance of
the correlograms, using the Bonferonni technique (Oden 1984)
and the significance of the I values at each distance class.
Results
Associated Vegetation
Eighty-nine species of vascular plants were recorded in the
quadrats, including 33 herbaceous, 42 shrub, and 32 tree species (appendix); 18 species occurred as both a shrub and a
tree. Total abundance of herbs, shrubs, and tree species did
not differ statistically between diploid and tetraploid populations (MANOVA: Wilks’s l p 0.24, F3, 4 p 4.14, P p 0.10)
(fig. 1). In univariate analyses, herbaceous plants were 500%
more abundant in tetraploid than diploid populations, an effect that was weakly significant (table 2; fig. 1). Shrubs and
trees were 57% and 29% more abundant in diploid than in
tetraploid populations, respectively, but these differences were
not significant (table 2; fig. 1). Populations of a particular
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706
Table 2
Summary of the Effects of Cytotype on Three Measures of Total
Abundance and Relative Species Abundance for the Nine
Common Species in Galax urceolata Habitats,
Based on Nested ANOVA
Variable
Total abundance:
Herb
Shrub
Tree
Relative abundance:
G. urceolata
Acer rubra (shrub)
Kalmia latifolia
Smilax rotundifolia
A. rubra (tree)
Nyssa sylvatica
Quercus prinus
Quercus coccinea
Tsuga canadensis
df
F
1, 6
1, 6
1, 6
4.88
0.83
0.69
1, 33
1, 33
1, 6
1, 6
1, 6
1, 6
1, 6
1, 6
1, 6
26.31
1.01
5.71
2.38
0.11
0.03
0.05
2.01
6
P
0.07
0.4
0.44
0.0001
0.35
0.023
0.18
0.75
0.88
0.83
0.21
0.053
Note. The sources of variation in each analysis were cytotype
(fixed effect) and population nested within cyotype (random). The
analyses were conducted in PROC MIXED (SAS); we stipulated the
Sattherthwaite correction method (Littell et al. 1996), which adjusted
the df for G. urceolata and A. rubra.
cytotype varied significantly in total abundance (MANOVA:
Wilks’s l p 0.2, F18, 71 p 3.07, P ! 0.001).
Relative abundance of the common species differed between
diploid and tetraploid populations when Galax urceolata was
either included (MANOVA: Wilks’s l p 0.27, F9, 19 p 5.59,
P p 0.001) or removed (Wilks’s l p 0.52, F8, 20 p 2.3, P p
0.063) (fig. 2), but the effect was marginally significant in the
latter case. In univariate analyses, the relative abundance of
G. urceolata, Kalmia latifolia, and Tsuga canadensis, was six,
four, and 12 times greater in tetraploid than diploid populations, respectively (table 1; fig. 2). Populations within a cytotypic level did not vary significantly (MANOVA: Wilks’s
l p 0.10, F54, 101 p 1.06, P p 0.39).
Mean species richness did not differ between tetraploid
(mean p 11.94, SE p 1.52) and diploid populations
(mean p 10.56, SE p 1.45) (F1, 6 p 0.45, P p 0.52). Populations within a cytotype varied by as much as 94% in mean
species richness (x12 p 10, P p 0.001).
the range, with high frequency in the north and low frequency
in the south, or patches of unequal size. Diploids were predominant north of the Virginia–North Carolina border, but
additional clusters of high diploid frequency occurred in central and southern North Carolina. Tetraploids showed several
clear patches of populations with high frequency from northwestern North Carolina to northeastern Georgia.
Discussion
This study provides evidence for habitat differentiation between diploid and tetraploid populations of Galax urceolata.
Two results support this conclusion. First, there was relative
abundance of the nine common species (in MANOVA), and
three of the nine species (in ANOVA) were significantly different between diploid and tetraploid populations (table 2; fig.
2). The multivariate pattern remained unchanged when G.
urceolata was removed from the analysis, although the
strength of the result decreased. Second, diploid and tetraploid
cytotype frequencies were both positively spatially autocorrelated at small distance classes (fig. 3). This result combined
with population genetic data is congruent with habitat differentiation. Additionally, results on the total abundance of herbs,
shrubs, and tree species are suggestive of differences between
diploid and tetraploid populations (fig. 1).
Associated Vegetation
Previous research on habitat differentiation between diploid
and tetraploid G. urceolata has been inconclusive. Baldwin
(1941) reported that diploid and tetraploid plants are separated by no obvious ecological barriers. However, Nesom
Spatial Autocorrelation
Correlograms revealed spatial autocorrelation for both diploids (overall correlogram, P ! 0.001) and tetraploids (overall
correlogram, P ! 0.002) (fig. 3). Moran’s I value varied among
distance classes from ⫺0.21 to 0.51 for diploids and ⫺0.22
to 0.46 for tetraploids. Diploid frequency was positively autocorrelated at distance classes 1 (I p 0.51, P ! 0.001) and 5
(I p 0.162, P ! 0.05) (fig. 3). Tetraploid frequency was also
positively autocorrelated at the first distance class (I p 0.46,
P ! 0.002) but negatively autocorrelated at distance class 7
(I p ⫺0.224, P ! 0.05) (fig. 3).
Spatial patterns discerned in chloropleth maps were congruent with the results from correlograms. In chloropleth
maps, diploid frequency appeared as either a weak cline across
Fig. 2 Mean relative abundance (as a percentage) of the nine species
used in the MANOVA, compared between single-cytotype diploid
(solid bars) and tetraploid (open bars) populations. The mean relative
abundance is the least squares mean (LSM) from the nested ANOVA
using the nontransformed data. One SE of the LSM is displayed. The
abundance of species is relative to the total abundance of plants within
the appropriate plant type. The nine species used in the MANOVA
were the herb Galax urceolata (Gu); the shrubs Acer rubrum (Ar1)
and Kalmia latifolia (Kl); the vine Smilax rotundifolia (Sr); and the
trees Acer rubrum (Ar2), Nyssa sylvatica (Ns), Quercus prinus (Qp),
Quercus rubra (Qr), and Tsuga canadensis (Tc). Statistically significant
differences (using transformed data) in mean relative abundance between diploid and tetraploid populations: ∗ p P ! 0.05; ∗∗∗ p P !
0.001.
JOHNSON ET AL.—HABITAT DIFFERENTIATION AND POLYPLOIDY IN GALAX
707
erance, our results indicate that G. urceolata thrives in relatively open, tetraploid-like habitats where more light can
penetrate the forest floor than in diploid locations. By extension, diploid plants may perform best in habitats that are more
tetraploid-like but have been competitively excluded from
them. Alternatively, differences in abundance may reflect a difference in life history between diploids and tetraploids. Specifically, tetraploids may be more robust and produce more
ramets per clonal patch, resulting in higher estimates of abundance. These hypotheses could be tested with growth comparisons in common environments or reciprocal transplant
experiments.
Spatial Autocorrelation
Fig. 3 Spatial correlograms to test if the frequency of diploid and
tetraploid plants were spatially autocorrelated among populations.
The overall correlograms were significant for the occurrence of diploids
(P ! 0.001) and tetraploids (P p 0.002). Open points indicate significant I values (P ! 0.05). Abscissa shows distance class for calculations
of Moran’s I value. The upper bounds of distance classes 1 through
8 are 25, 50, 85, 119, 157, 205, 263, and 332 km, respectively. Ordinate: Moran’s I value.
(1983) stated that diploids occupied more xeric locations than
tetraploids, but clear differences between cytotypes were often
lacking. Our study demonstrates that diploid and tetraploid
habitats of G. urceolata differ but by a matter of degree. Habitats for diploid and tetraploid G. urceolata did not differ in
total species richness, and no common species were unique to
either habitat, although some less common ones were. Rather,
differences were related to changes in the relative abundances
of the species present. Therefore, it is likely that much overlap
exists in the habitat preferences of diploid and tetraploid
plants. Such overlap may explain why mixed populations
(diploid-tetraploid) occur in some portions of the range and
why triploid hybrids are found in these same environments
(Burton and Husband 1999). Additional work is required,
however, to determine whether hybrids are maintained by a
fitness advantage in intermediate environments or by recurrent
formation through diploid-tetraploid matings.
In general, habitats of tetraploid G. urceolata had more
herbaceous vegetation but less shrub and tree cover than diploid environments. This pattern may reflect habitat differences
in soil moisture and soil nutrient levels, which are generally
positively associated with abundance of herbaceous plants and
sometimes negatively related to shrub and tree abundance
(Drewa et al. 2002; Turkington et al. 2002). The pattern is
also consistent with Nesom’s (1983) observation that diploid
G. urceolata seem to be associated with more xeric microenvironments. However, disturbance from fire and other anthropogenic agents can also lead to reductions in woody plants
and increases in abundance of herbaceous species (Arthur et
al. 1998; Elliott et al. 1998; Briggs et al. 2002). All of these
factors are of potential importance in our system.
Galax urceolata was more abundant in tetraploid than diploid environments. If cover is any reflection of ecological tol-
The spatial patchiness of diploids and tetraploids, as indicated by the autocorrelation analysis, is consistent with the
hypothesis that single cytotype populations are maintained by
habitat differentiation. This interpretation is based on the
premise that G. urceolata cytotypes are sorting along an environmental gradient that in itself is autocorrelated (Engen et
al. 2002; Tuomisto et al. 2003). By extension, the scale of
autocorrelation in the plants should mirror that of the biotic
and abiotic environment. Both diploid and tetraploid G. urceolata exhibited a significant positive autocorrelation at the
first distance class (i.e., 0–25 km) but not in the subsequent
three distance classes. There was variation in patch size, however, evident from the distribution of diploid populations. A
cluster of diploid-dominated populations was large in the
north compared to smaller patches in the south (fig. 1 in Burton
and Husband 1999). These results show that, on average,
patchiness occurs on the relatively small scale of 25 km in
diameter, which is not surprising given the topographical variation of the Blue Ridge Mountains.
Spatial autocorrelation may also reflect the effects of drift
or founder effects, coupled with isolation by distance among
different regions where G. urceolata occurs. While we cannot
exclude this possibility, recent population genetic studies on
this species indicate isolation by distance processes are not a
dominant force. Burton (2000) examined the genetic similarities among seven populations (four diploid and three tetraploid) of G. urceolata, which were separated by distances ranging from 35 km to 353 km. Using random amplified
polymorphic DNA markers, she found no spatial structure nor
any relationship between geographic and genetic distance.
Neighboring populations were in fact genetically less similar
than more distant populations (Burton 2000). Taken together,
the spatial autocorrelations and the genetic analyses indicate
that habitat differentiation is more important than isolation
by distance processes in maintaining single-cytotype populations.
Habitat Differentiation between Polyploids
and Their Progenitors
Other studies have sought to determine the role of habitat
differentiation as a mechanism behind geographic separation
of cytotypes within a single taxon (Lumaret et al. 1987; FelberGirard et al. 1996; van Dijk and Bakx-Schotman 1997).
Lumaret et al. (1987) examined the fine-scale distribution of
diploid and tetraploid Dactylis glomerata in sympatric pop-
INTERNATIONAL JOURNAL OF PLANT SCIENCES
708
ploid G. urceolata populations. We did not test whether
environment-independent selection is also operating to maintain geographic separation between cytotypes; future studies
could provide insight into the relative importance of environment-dependent versus independent selection by performing
reciprocal transplants of cytotypes between populations (sensu
Fritsche and Kaltz 2000) and measuring plant fitness while
controlling for cytotype frequency (Husband 2000). Our study
also points to the possibility that ecotypic differentiation could
be occurring between cyotypes within mixed populations and
may be an important mechanism in the establishment and coexistence of novel polyploids with their diploid progenitors.
ulations. As in our study, they found tetraploids in sites that
were more open than those of diploids, which they attributed
to differences in flowering time and physiological requirements
for moisture and light. In contrast, van Dijk and Bakx-Schotman (1997) tested adaptive and nonadaptive explanations for
the range separation between diploid and tetraploid Plantago
media using a molecular approach. They concluded that the
phylogeny of diploid and tetraploid populations, based on
chloroplast DNA polymorphisms, supported a nonadaptive
explanation. The effects of habitat differentiation versus environment-independent selection on geographic relationships
between cytotypes likely vary among species. The factors that
cause such variation among species may include life-history
traits, mechanisms of polyploidy, time since chromosome doubling, and historical patterns of colonization of each cytotype.
Environment-dependent and -independent selection are not
mutually exclusive mechanisms maintaining geographic separation between cytotypes. Indeed, environment-independent
factors such as selection against triploid hybrids could operate
in concert with environmental selection for single-cytotype
patches in G. urceolata. At the same time, environment-specific
fitness differences between cytotypes may weaken minority cytotype exclusion and increase the likelihood that a cytotype
could spread in its preferred habitat, even when it is initially
rare (Fowler and Levin 1984; Felber 1991; Rodriguez 1996).
Our results support the hypothesis that environmentdependent selection is at least partially responsible in maintaining the geographic separation between diploid and tetra-
Acknowledgments
We are grateful to Gary Kauffmann from the Nantahalla
National Forest and Bill Wykle from Highlands Biological Station for assistance with identification. Highlands Biological
Station provided the use of their facilities. Uta Matthes, Richard Reader, and Isabelle Shmeltzer provided advice on the design and analysis of this study. We also thank Anurag Agrawal,
Doug Larson, Justin Ramsey, Jennifer Thaler, Sarah Wagner,
and anonymous reviewers for helpful comments on previous
versions of this manuscript. This project was supported by a
University of Guelph Undergraduate Research Assistantship
Award and a postgraduate award from the Natural Sciences
and Engineering Research Council of Canada (NSERC) to M.
T. J. Johnson and an NSERC operating grant to B. C. Husband.
Appendix A
Table A1
Raw Data on Cytotype, Species Richness, and Abundance for Four Diploid and Four Tetraploid Populations of Galax urceolata
Relative Abundance
Total Abundance
Sample
Cytotype 2x:
Population
a
b
c
d
e
Population
a
b
c
d
Population
a
b
c
d
Population
a
b
c
d
e
Richness
Herb
Shrub
Tree
Gu
Ar1
Kl
Sr
Ar2
8
9
5
10
9
1
9
0
3
3
44
9
44
18
30
17
18
23
23
10
0
0
0
67
0
9
44
91
50
47
2
0
7
0
0
30
0
2
6
23
12
0
4
9
0
8
11
9
10
0
0
0
0
28
12
9
38
14
21
15
13
0
0
0
0
7
17
22
0
0
17
0
0
32
0
56
24
11
10
10
10
0
0
0
0
70
100
104
4
39
40
35
41
0
0
0
0
0
0
0
25
1
0
0
0
4
1
1
50
17
17
16
6
14
51
7
10
0
4
66
91
85
2
57
7
41
9
23
13
0
0
0
0
0
15
2
1
50
4
0
0
0
0
0
5
7
1
0
4
Ns
Qp
Qr
Tc
0
6
0
0
0
12
0
4
13
10
0
11
0
0
0
0
0
0
0
0
0
10
0
0
7
0
0
0
7
5
7
0
0
5
7
0
0
0
0
0
0
15
0
2
0
0
0
2
8
0
9
0
3
0
0
0
0
0
0
0
29
0
11
0
0
14
0
11
0
0
14
0
0
0
0
0
0
0
9
8
8:
21:
31:
36:
14
4.9
11
0
62
JOHNSON ET AL.—HABITAT DIFFERENTIATION AND POLYPLOIDY IN GALAX
709
Table A1
(Continued )
Relative Abundance
Total Abundance
Sample
Cytotype 4x:
Population
a
b
c
d
Population
a
b
c
d
e
Population
a
b
c
d
Population
a
b
c
d
Richness
Herb
9
11
7
7
11
2
1
0
10
12
18
11
11
Shrub
Tree
Gu
Ar1
Kl
Sr
5
9
3
1
24
52
28
32
100
100
0
0
0
0
67
100
0
22
0
0
0
0
0
0
1
22
90
1
22
69
16
31
35
48
15
15
11
14
16
0
96
28
100
100
55
25
52
6
13
3
6
0
0
2
7
11
13
10
3
0
66
67
8
12
18
6
8
16
21
10
100
0
100
99
0
58
11
100
17
16
18
15
73
17
56
49
28
102
67
39
16
5
4
10
0
53
61
89
14
11
4
18
Ar2
Ns
Qp
Qr
Tc
0
2
0
16
0
0
0
0
0
0
0
3.1
0
6
7
0
0
12
21
3
3
19
0
0
0
7
0
0
14
6
0
7
0
0
6
13
7
27
7
6
7
7
9
7
6
7
0
9
0
6
13
8
6
0
63
8
0
0
25
13
5
0
13
0
19
10
0
6
19
10
13
6
0
20
0
6
0
0
25
0
1
21
7
1
0
23
6
20
25
0
0
0
0
0
0
0
25
0
0
0
0
0
0
60
0
50
9:
10:
17:
32:
Note. Abundance is reported as the total for three growth forms and as percent relative abundance for the nine common species. The nine
species are G. urceolata (Gu), Acer rubrum (Ar1) as a shrub, Kalmia latifolia (Kl), Smilax rotundifolia (Sr), A. rubrum (Ar2) as a tree, Nyssa
sylvatica (Ns), Quercus prinus (Qp), Quercus rubra (Qr), and Tsuga canadensis (Tc). All data are rounded to the nearest whole number.
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