Conservation genomics of the Mogollon Narrow-headed gartersnake (Thamnophis rufipunctatus) and Northern Mexican gartersnake (Thamnophis eques megalops)

The ability of populations to persist and adapt to abiotic and biotic changes is reliant on genetic diversity. When connectivity across a species landscape is disrupted, the levels and distribution of genetic diversity can rapidly deteriorate as a result of genetic drift, leading to increased inbreeding and reduced adaptive potential. Therefore, understanding the distribution and degree of genetic variation within imperiled populations provides important information for conservation management and recovery strategies, especially when paired with translocation and repatriation programs. Here, we used genome-wide nuclear markers to study the population structure and genetic diversity from tissue samples collected between 2010 and 2016 of two threatened species of gartersnakes inhabiting the lower Colorado River Basin in the United States: Mogollon Narrow-headed gartersnake (Thamnophis rufipunctatus) and Northern Mexican gartersnake (Thamnophis eques megalops). Our specific objectives were to determine how populations inhabiting the lower Colorado River Basin were related to sister species and southern populations along the Sierra Madre Occidental in Mexico, to determine how genetic variation is partitioned among drainage basins in the lower Colorado River Basin, and to provide estimates of genetic diversity and effective sizes of sampled sites to aide species-specific conservation management of these threatened gartersnakes. For both species, we found that populations along the lower Colorado River Basin are highly differentiated from sister species and southern populations located further south in Mexico, and exhibit reduced genetic diversity relative to populations along the Sierra Madre Occidental. Within the lower Colorado River Basin, genetic analyses revealed highly structured genetic groups for both species of gartersnakes that point to shared contemporary and historical drivers of differentiation. We found that most sites throughout the lower Colorado River Basin have low genetic diversity and effective population sizes below the threshold required to retain adaptive potential. However, these trends were especially pronounced for T. rufipunctatus. If genetic management and translocation strategies are adopted in the future, these population genetic results can be used to highlight sites of particular concern and locate the most genetically similar sites for translocation or re-establishment efforts. Such measures could help curb further population genetic change, alleviate problems associated with low genetic diversity, and strengthen the adaptive potential across the range of these two gartersnake species.