Comparative population genetics of aquatic turtles in the desert

In a desert-spring ecosystem, the desert can be a formidable barrier to dispersal for species with sensitivity to desiccation. In the desert-spring ecosystem of Cuatro Ciénegas, México, three endemic turtle species, all of conservation concern, have overlapping ranges but different requirements for aquatic habitat. By using existing genetic markers and generating new ones using next-generation sequencing, I compared the population genetic structure of two of these endemic turtles. The aquatic slider turtle, Trachemys taylori, exhibited significant isolation by distance and genetic differentiation among populations. However, stronger genetic structure was identified for the obligate aquatic softshell turtle, Apalone atra. For Apalone, no correlation of genetic differentiation with geographic distance was observed, as most populations appear genetically distinct from one another. This evidence, combined with allelic and heterozygosity patterns, suggests that the desert matrix is a sufficient barrier to dispersal for the softshell turtles but less of a barrier for the more terrestrial slider turtles. It appears that softshell turtles from each pond may have been relatively isolated from other ponds since the basin became substantially drier near the beginning of the Holocene. This strong population genetic structure of the two aquatic species is in contrast to the near panmixia of the third endemic, the previously studied semi-aquatic box turtle, Terrapene coahuila. Separately, the issue of hybridization of the endemic turtles with invasive congeners has been raised as a conservation concern, but detailed genetic assessment of potential hybridization has not been performed. In this study, hybridization of the endemic slider with its invasive congener was assessed and not detected with genetic data. Also, it appears that different color morphs of Apalone in the basin, which are currently designated as separate species, are not genetically distinct at the loci examined in this work.