Conservation Genomics of Beach Spectacle Pod (Dithyrea maritima (Davidson) Davidson; Brassicaceae)
Dithyrea maritima (Davidson) Davidson (Brassicaceae), or beach spectaclepod, is a dune specialist endemic to coastal dunes from central California, USA, to northwestern Baja California, MX. Individuals of this perennial herb spread by rhizomes, forming a diffuse colony of ramets. Flowering typically occurs from April to July, with the apex of the ramet elongating to form a dense raceme of fragrant flowers. Corollas are white, cream, or sometimes pink to purple tinged. The species produces a two-chambered fruit that resembles spectacles; each round half of the fruit contains a single seed. It is listed as Threatened under the California Endangered Species Act. Threats to extant occurrences include OHV activity, marine mammal activity, and non-native plants; it is also among the most threatened of coastal California plants with respect to climate change. Studied occurrences of the self-incompatible D. maritima have low seed set despite seemingly adequate pollinator service. Previous studies demonstrated that manual outcrosses boost seed production, suggesting that knowledge of the distribution of genotypes on the landscape is a critical first step toward any number of recovery actions. In this study, we include 30 individuals from each of eleven sampling locations spanning the range of the taxon from Morro Bay, CA, USA to San Quintin, Baja CA, MX. We used double digestion RADseq to prepare libraries for high-throughput sequencing. We place the population genomic patterns into the context of regional biogeography. We conclude with recommendations for managing the species given the on-going threats of climate change and lack of seed set.