Population genetic structure of the endangered Butler's Gartersnake (Thamnophis butleri): Does the Short-headed Gartersnake (Thamnophis brachystoma) exist in Canada?

Understanding population genetic structure is fundamental to conservation of endangered species. It is particularly important when working with species that are morphologically conserved because strong genetic divisions could represent cryptic species. Butler's Gartersnake (Thamnophis butleri (Cope, 1889)) is an endangered species in Canada, having a fragmented distribution and being restricted to southwestern Ontario. Furthermore, it is difficult to distinguish morphologically from a closely related species, the Short-headed Gartersnake (Thamnophis brachystoma (Cope, 1892)). We use mitochondrial DNA (mtDNA) and seven microsatellite DNA loci to evaluate the genetic structure of Canadian T. butleri populations and to test for the presence of T. brachystoma in one of these populations. All individuals had the same mtDNA haplotype, and there was no evidence of multiple, syntopic genetic clusters, thereby rejecting the hypothesis that T. butleri and T. brachystoma co-exist in Canada. Two different model-based assignment tests using microsatellite DNA data suggest that there are four to five genetically distinct clusters of T. butleri (F_ST from 0.12 to 0.20). We provide the first population genetic study of T. butleri in Canada and refute the presence of T. brachystoma. Our results may provide guidance on recovery strategies for this species and identify areas to target fine-scale genetic analyses.