Population genetic implications of postglacial migration in Carex cryptolepis Cyperaceae

1
Population genetic implications of post-glacial
migration in Carex cryptolepis (Cyperaceae)
Nathan Derieg and Leo P. Bruederle Department of
Biology, University of Colorado at Denver and
Health Sciences Center, Denver, CO 80217
Introduction Pleistocene glaciations greatly
influenced the modern flora and fauna of Eastern
North America. With the onset of the Wisconsin
Glaciation, climate change induced shifts in
species distributions, and continental ice sheets
overran those populations that where not
extirpated by changing environmental conditions
(Pielou 1991). The effects varied some species
persisted relatively close to the ice front,
while others seem to have survived in southern
refugia.
Discussion As expected, populations of C.
cryptolepis exhibit relatively low levels of
genetic diversity and are highly differentiated.
Inbreeding can result in a loss of alleles, but
similar levels of inbreeding in C. cryptolepis
and C. lutea indicates an additional process
acted to reduce genetic diversity in C.
cryptolepis. The decrease in number of
polymorphic loci and allelic variation at
polymorphic loci may have resulted from founder
effects in post-glacially established
populations. The high degree of population
differentiation in C. cryptolepis relative to C.
lutea may reflect limited gene flow among
populations established after the last glacial
maximum however, incomplete sampling of broadly
distributed species can inflate observed levels.
Retreat of the continental ice sheets allowed
dispersal into glaciated regions from the
refugial populations. Range expansion of this
sort is expected to result in low genetic
diversity and a large degree of population
differentiation (Ibrahim et al. 1996).
While none of the populations included in this
study were likely refugial, further sampling
along the southern distributional margin, e.g.,
southern Ohio, may identify such populations.
Populations from higher latitudes will help
characterize the extent and pattern of genetic
diversity reduction in C. cryptolepis.
Additionally, further sampling will clarify the
intraspecific phylogeography of C. cryptolepis
within the context of post-glacial migration.
Carex cryptolepis Mack. is a broad endemic
distributed across glaciated areas of
northeastern North America, occuring on sandy or
organic substrates with neutral or acidic pH and
low calcium content (Fig. 1) (Crins and Ball
1989). Carex cryptolepis is a self compatible
perennial with caespitose, or clump forming,
growth habit (Fig. 2). A small number of
populations are found south of the last glacial
maximum (LGM), e.g., the Edge of Appalachia
Preserve in southern Ohio.
Results Allozyme data for ten populations
indicates C. cryptolepis maintains low levels of
genetic diversity relative to the closely related
C. lutea, a narrow endemic from unglaciated North
Carolina (e.g., P 3.9 versus P 21.1)
(Table 2). Expected heterozygosity and observed
heterozygosity were both lower in C. cryptolepis
(He 0.007, Ho 0.004) than C. lutea (He
0.051, Ho 0.029). Statistically significant
deviations from Hardy-Weinberg Equilibrium were
correlated with large positive fixation indices.
Mean inbreeding within populations (f 0.49) was
similar to that observed in C. lutea (f 0.44).
Populations of C. cryptolepis were more
differentiated (FST 0.86 versus FST 0.40).
CONTML analysis supports recognition of C.
cryptolepis as a distinct species and suggests
the presence of intraspecific lineages (Fig. 3).
Allozyme analysis was performed to assess the
impact of post-glacial migration on the levels
and apportionment of genetic diversity within and
among populations of C. cryptolepis.
Methods Soluble enzymatic proteins were extracted
from 346 individuals representing ten populations
of C. cryptolepis (Table 1) (Bruederle and
Fairbrothers 1986). Carex lutea, putative sister
taxon to C. cryptolepis, was utilized for
comparisons. Samples were stored at -70oC in the
UCDHSC Plant Systematics Lab. Allozyme
electrophoresis was conducted using 11 starch
gels and three gel-electrode buffer systems
thirteen substrate specific stains resolved 18
putative loci (Bruederle and Fairbrothers 1986
Bruederle and Jensen 1991 Kuchel and Bruederle
2000). Observed allozyme phenotypes for each
individual were interpreted as genotypes
following Bruederle and Fairbrothers (1986), with
loci and alleles named following standard
nomenclature.
GDA 1.1 (Lewis and Zaykin 2002) was used to
generate descriptive statistics, including
proportion of polymorphic loci (P), mean number
of alleles per locus (A), mean number of alleles
per polymorphic locus (Ap), observed
heterozygosity (Ho), expected heterozygosity
(He), Neis unbiased genetic identity, and
Wrights F-statistics. The CONTML function of
PHYLIP version 3.65 (Felsenstein 2005) was used
to construct a phylogenetic hypothesis for C.
cryptolepis.
Figure 3. CONTML tree illustrating tentative
intraspecific and interspecific relationships for
Carex cryptolepis (green box) and C. lutea (gold
box). The relationships within C. cryptolepis
are poorly resolved by the current data set, but
serve to guide further sampling efforts.
Table 2. Summary of genetic diversity
statistics, including proportion of polymorphic
loci (P), mean number of alleles per locus (A),
mean number of alleles per polymorphic locus
(Ap), observed heterozygosity (Ho), expected
heterozygosity (He), and population
differentiation (FST).
Works Cited Bruederle, L.P., and D.E.
Fairbrothers. 1986. Allozyme variation in
populations of the Carex crinita complex
(Cyperaceae). Systematic Botany 11
583-594. ------------------, and U. Jensen.
1991. Genetic differentiation of Carex flava and
Carex viridula in West Europe (Cyperaceae).
Systematic Botany 16 41-49. Crins, W.J. and P.W.
Ball. 1989. Taxonomy of the Carex flava complex
(Cyperaceae) in North America and Northern
Eurasia. II. Taxonomic treatment. Canadian
Journal of Botany 67 1048-1065. Felsenstein, J.
2005. PHYLIP (Phylogeny Inference Package)
version 3.6. Distributed by the
author. Department of Genome Sciences, University
of Washington, Seattle. Ibrahim, K.M., R.A.
Nichols, and G.M. Hewitt. 1996. Spatial
patterns of genetic variation generated
by different forms of dispersal during range
expansion. Heredity 77 282-291. Kuchel, S.D.,
and L.P. Bruederle. 2000. Allozyme data support
a Eurasian origin for Carex viridula
subsp. viridula var. viridula (Cyperaceae).
Madrono 47 147-158. Lewis, P. O., and D. Zaykin.
2001. Genetic Data Analysis computer program
for the analysis of allelic data. Version 1.0
(d16c). Free program distributed by the authors
over the internet from http//lewis.eeb.uconn.edu/
lewishome/software.html Pielou, E.C. 1991.
After the ice age the return of life to
glaciated North America. University of
Chicago Press, Chicago, IL.
Table 1. Populations of Carex cryptolepis
sampled for this study.
Acknowledgments Council Awards for Graduate
Student Research provided funding for this
research.
Figure 1. Distribution of Carex cryptolepis
Mack. (Cyperaceae) in North America, showing
populations already analyzed (green dots) and
populations to be sampled (blue dots). The
southern maximal extent of ice cover during the
Wisconsin Glaciation is marked by the red line
(modified from Crins and Ball 1989).
Figure 2. Clump-forming growth form of Carex
cryptolepis.