Phylogeography of eastern leatherwood (Dirca palustris L.) resolved by chloroplast sequencing and microsatellite genotyping

Abstract

The broad distribution of eastern leatherwood (Dirca palustris L.), from Florida to Canada, contrasted with its patchy restriction to rich, mesic forests and the paucity of vectors for dispersal of its seeds, represents a classic example of Reid’s paradox of plant migration. It long has been presumed that temperate forest species of eastern North America persisted in southern glacial refugia near the Gulf Coast, after which many dispersed nearly 2000 km to contemporary northern range limits. The rates of migration necessary to account for present distributions exceed those suggested by contemporary observations of seed dispersal. I used ecological niche modeling, chloroplast sequencing, microsatellite genotyping, and phenotypic investigations to answer questions about the historical and contemporary distribution of populations and genes of D. palustris across the landscape of eastern North America. Ecological niche modeling supported my hypothesis that D. palustris persisted within midlatitudinal refugia during the Last Glacial Maximum, and the distribution of chloroplast haplotypes supported the prehistoric presence of individuals at such latitudes. Deep diversification of chloroplast haplotypes indicated a complex history of population isolation, migration, and admixture over Pleistocene glaciations. I used next-generation sequencing for de novo isolation of highly polymorphic microsatellite loci from D. palustris. Microsatellite genetic analyses provided further evidence for mid-latitudinal refugia. Results from both markers were indicative of general dispersal limitation with instances of long-distance dispersal, especially to the Upper Midwest and Great Lakes Region; I suggest that an active vector likely was important for post-glacial range expansion of D. palustris. I provide genetic and phenotypic evidence for a new subspecies of D. palustris that is restricted in distribution to