Levels and partitioning of genetic variation of northeastern populations of diamondback terrapin (Malaclemys terrapin)

The diamondback terrapin (Malaclemys terrapin) is a mid-sized turtle that serves as a keystone predator in salt marsh ecosystems of eastern North America. The terrapin has historically faced population declines due to habitat loss and overharvesting, which has resulted in its listing under multiple jurisdictions across the northern part of its range. To characterize levels and partitioning of terrapin genetic variation throughout the northeast region, we used restriction site-associated DNA sequencing (RADseq). We analyzed genetic variation among 116 individuals sampled across 18 sites. Within-population genetic diversity was relatively low (He = 0.080–0.122), and we observed a strong negative correlation between diversity and latitude. Furthermore, levels of genetic differentiation were moderate (pairwise FST = 0.00–0.19), with the mean pairwise FST of each population exhibiting a strong positive correlation with latitude. Together, these results are consistent with a model of serial colonization from a Pleistocene refugium in the mid-Atlantic. Spatial genetic variation was best explained by a landscape model that considered migration to be limited to coastal habitats, where northern range-edge populations maintained comparatively low genetic diversity and were more genetically distinct than populations to the south—consistent with their greater geographic isolation. Admixture analyses revealed weak genetic clustering, with the distribution of genetic clusters reflecting the combined historical effects of isolation-by-distance and human-mediated translocations. Regional efforts to restore terrapin habitat or reintroduce captive individuals should consider patterns of historic gene flow, cognizant of the relatively distinct and isolated populations at the northeastern range edge.