Diversity across the border: Genetic study of a highly disjunct occurrence of the U.S. federally-endangered plant species Physaria thamnophila, Brassicaceae (Zapata bladderpod) discovered in Mexico

Taxonomy, genetics and biogeography each make key contributions to biological conservation. However, integrating these disciplines to obtain a coherent account of the status of a taxon of concern not always straightforward. This is the case for the cross-border endemic plant Physaria thamnophila (Brassicaceae). This US federally-listed endangered species is restricted to a set of unique geological sites just north of the Rio Grande (Rio Bravo del Norte) river in south Texas, USA. A single highly-disjunct occurrence of this species is found on a geologically and ecologically distinct site 260 km to the south, in Tamaulipas, Mexico. In this work, we quantify the genetic differentiation between the U.S. and Mexican populations using four microsatellite markers and sequences from three nuclear genes. In both sets of data, we find a high level of genetic divergence consistent with geographic isolation on a time scale of 1–2.5 million years. Further, we provide a hypothesis for the geological basis of this geographic isolation. Integrating our data with ecological, taxonomic and conservation considerations, we propose the sub-specific designation of Physaria thamnophila subsp. loretensis for the Mexican population. The evolutionary and conservation implications of this designation are presented.