Listing of endemic subspecies under the Endanger Species Act implies evolutionary or ecological uniqueness. The insular cotton rat (Sigmodon hispidus insulicola) is one such candidate for listing. Despite the taxonomic distinction between the island (restricted to Sanibel and Pine islands, Florida) and mainland subspecies (S. h. hispidus), we do not know the degree of isolation of, nor the relative genetic status of, the insular form. We asked, what is the degree of divergence among island and between island and mainland populations of cotton rats? Does the level of inferred gene flow reflect long-term isolation, and is gene flow between island and mainland forms symmetric? To address these questions, we genotyped 248 cotton rats at 13 loci, and sequenced the cytochrome b mitochondrial gene for 166 of these. We tested alternative gene flow models between mainland and island populations using Bayes factor analysis based on migration estimated using Migrate-n, and explored patterns of divergence among mainland and insular populations.
Results/Conclusions
Sigmodon hispidus insulicola from Sanibel Island were differentiated at mitochondrial haplotypes (no shared haplotypes with mainland or Pine Island) and nuclear DNA reflected a combination of population bottleneck (low diversity on island relative to mainland) and long-term isolation (monophyletic haplotypes and unique genotypes). Pine Island cotton rats were also distinct in their multi-locus nuclear profiles, but were not monophyletic in mtDNA and shared one common haplotype with mainland cotton rats. This pattern may reflect more recent isolation of Pine Island, a larger matrilineal Ne, or more recent matrilineal gene flow (the latter supported by Bayes factor analysis of mtDNA gene flow, Prob = 1.0). Pine and Sanibel islands were as differentiated from one another as either were to the mainland. There is a clear genetic demarcation between Sanibel and mainland Sigmodon, suggesting an independent evolutionary history. Pine Island appears to have had a more recent, or repeated genetic introgression from mainland sources, but is still isolated enough to have a similarly distinct microsatellite profile. While our data can give insight into the demographic history of island populations, we cannot infer adaptive divergence, which is ultimately the more important type of variation to protect at the intra-specific level.