Phylogenetic, population genetic, and morphological analyses reveal evidence for one species of Eastern Indigo Snake (Drymarchon couperi)

Accurate species delimitation and description are necessary to guide effective conservation management of imperiled species. The Eastern Indigo Snake (Drymarchon couperi) is a large species in North America that is federally-protected as Threatened under the Endangered Species Act. Recently, two associated studies hypothesized that Drymarchon couperi is two species. Here, we use diverse approaches to test the two-species hypothesis for D. couperi. Our analyses reveal that (1) phylogenetic reconstruction in previous studies was based entirely on variance of mitochondrial DNA sequence data, (2) microsatellite data demonstrate significant population admixture and nuclear gene flow between mitochondrial lineages, and (3) morphological analyses recover a single diagnosable species. Our results are inconsistent with the two-species hypothesis, thus we reject it and formally place Drymarchon kolpobasileus into synonymy with D. couperi. We suggest inconsistent patterns between mitochondrial and nuclear DNA may be driven by high dispersal of males relative to females. We caution against species delimitation exercises when one or few loci are used without evaluation of contemporary gene flow, particularly species with strong sex-biased dispersal (e.g., squamates) and/or when results have implications for ongoing conservation efforts.