Population genetic implications of post-glacial migration in Carex cryptolepis (Cyperaceae)

1. Hypotheses Populations of Carex cryptolepis occupying formerly glaciated regions will exhibit reduced genetic diversity. Populations of C. cryptolepis will also be highly differentiated. 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), Nei’s unbiased genetic identity, and Wright’s 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 Carexviridula 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.