Range-wide population genetics reveals how disjunctions, admixture, and translocations influence conservation of Physaria globosa (Brassicaceae)

Population genetic analyses of endangered species can directly inform management strategies to conserve genetic diversity and increase long-term survival. Physaria globosa, an endangered mustard, has a geographically restricted and disjunct distribution across river systems in Indiana, Kentucky, and Tennessee, USA. In this study, we assessed whether genetic structure was shaped by the species' disjunct distribution, whether genetic diversity has been affected by bottlenecks, drift, or inbreeding, and whether translocations based on mixed sources can retain genetic variation. We genotyped 332 individuals at 20 microsatellite loci from 14 natural and 2 translocated populations from across the species' range. The genetic structure of natural populations showed two major geographic groups with little to no connectivity between them: a northern group in Indiana and Kentucky (IN-KY) and a southern group in Tennessee (TNE-TNW). Genetic diversity was highly variable among and within geographic regions; however, several local populations showed reduced genetic diversity, inbreeding, or recent bottlenecks, consistent with demographic decline and ongoing pressures from habitat degradation and competition from invasive plants. The two translocated populations showed clear genetic mixing of source populations but showed depletion of private alleles. Our results suggest that conservation actions for P. globosa should prioritize the protection of multiple populations within each major geographic region. Combining material from mixed sources appears to be a viable approach to increase genetic diversity and restore gene flow, but we recommend that translocations combine material from within the same geographic regions to preserve the distinct evolutionary histories of the main geographic groups.