Low genetic diversity and evidence of population structure among subspecies of Nerodia harteri, a threatened water snake endemic to Texas

Nerodia harteri is a threatened small-bodied water snake that occupies one of the most restricted ranges of any snake within the continental United States. It is found closely associated with rivers and tributaries in the Colorado and Brazos river basins, which flow through north-central Texas. Nerodia harteri has been at the center of debate owing to conflicts between conservation efforts and the construction of dams that change or destroy its preferred habitat. Additionally, its taxonomic status has also been under contention with some authors recognizing two subspecies, the Brazos water snake (N. h. harteri) and the Concho water snake (N. h. paucimaculata), whereas other authors consider each separate species. Despite its relatively recent discovery during the 1940s, N. harteri has been the subject of several ecological studies, yet no population genetic assessment of either subspecies has been performed to date. We first evaluated the phylogenetic placement of both subspecies among other North American Nerodia using partial sequence data from the mitochondrial gene cytochrome-b. We then tested for population subdivision among four rivers encompassing the range of N. harteri and tested for the presence of admixture between river basins using mitochondrial sequence data (920?bp of cyt-b) and five cross-species amplified microsatellite loci. We found low mitochondrial haplotype diversity represented by two unique haplotypes in each river basin, which were separated by no more than four nucleotide changes. Nuclear loci showed low genetic diversity and population structuring within and among river basins. We did not find conclusive evidence of admixture between basins, and we support the presence of two separate evolutionarily significant units and two separate management units corresponding to each major river basin. Given increasing natural and anthropogenic threats, we recommend continued ecological and genetic monitoring of both subspecies.