Formation of a recent hybrid zone offers insight into the geographic puzzle and maintenance of species boundaries in musk turtles

Abstract

Speciation is the result of an accumulation of reproductive barriers between populations, but pinpointing the factors involved is often difficult. However, hybrid zones can form when these barriers are not complete, especially when lineages come into contact in intermediate or modified habitats. We examine a hybrid zone between two closely related riverine turtle species, Sternotherus depressus and S.peltifer, and use dual-digest RAD sequencing to understand how this hybrid zone formed and elucidate genomic patterns of reproductive isolation. First, the geographical extent and timing of formation of the hybrid zone is established to provide context for understanding the role of extrinsic and intrinsic reproductive isolating mechanisms in this system. The strength of selection on taxon-specific contributions to maintenance of the hybrid zone is then inferred using a Bayesian genomic cline model. These analyses identify a role for selection inhibiting introgression in some genomic regions at one end of the hybrid zone and promoting introgression in many loci at the other. When selective pressures necessary to generate outliers to the genomic cline are considered with the geographical and temporal context of this hybrid zone, we conclude that habitat-specific selection probably limits introgression from S.depressus to S.peltifer in the direction of river flow. However, selection is mediating rapid, unidirectional introgression from S.peltifer to S.depressus, which is probably facilitated by anthropogenic habitat alteration. These findings indicate a potentially imminent threat of population-level genomic extinction for an already imperiled species due to ongoing human-caused habitat alteration.