Shotgun sequencing of Physaria filiformis

Understanding genetic diversity and structure in a rare species is critical for prioritizingboth in situ and ex situ conservation efforts. One such rare species is Physariafiliformis (Brassicaceae), a federally threatened, winter annual plant species. Thespecies has a naturally fragmented distribution, occupying three different soil typesspread across four disjunct geographical locations in Missouri and Arkansas. Thegoals of this study were to understand: (1) whether factors associated withfragmentation and small population size (i.e., inbreeding, genetic drift or geneticbottlenecks) have reduced levels of genetic diversity, (2) whether genetic variation isstructured as a result of its naturally fragmented range or the different soil types itoccupies, and (3) how much extant genetic variation of P. filiformis is currentlypublicly protected to develop conservation strategies to protect its genetic diversity. We genotyped individuals from 20 populations of P. filiformis from across itsgeographical range and one population of Physaria gracilis for comparison andanalyzed genetic diversity and structure. Populations of P. filiformis showedcomparable levels of genetic diversity to its congener, except a single population innorthwest Arkansas showed evidence of a genetic bottleneck and two populations inthe Ouachita Mountains of Arkansas showed lower genetic variation, consistent withgenetic drift. Populations showed isolation by distance, indicating that migration isgeographically limited, analyses of genetic structure grouped individuals into sevengeographically structured genetic clusters, and comparative AMOVA analyses showedthat geographic location/spatial separation was the most important factor affectinggenetic structure. At least one population is protected for all genetic clusters exceptone in north-central Arkansas, which should therefore be prioritized for protection.Populations in the Ouachita Mountains were genetically divergent from the rest of P.filiformis ; future morphological analyses are needed to identify whether it meritsrecognition as a new, extremely rare, cryptic species.