Genetic differentiation and diversity of two sympatric subspecies of Castilleja affinis; a comparison between the endangered serpentine endemic (spp. neglecta) and its widespread congener (ssp. affinis)

Edaphic endemic species are considered high conservation priority, as they are a unique and critical component of the ecosystems and are often restricted to small fragmented habitats. Castilleja affinis ssp. neglecta is a serpentine endemic species, known from six sites within the San Francisco Bay Area and is the focus of active restoration efforts. It grows sympatrically with the subspecies, C. affinis ssp. affinis, which is more widespread but differs in floral color and soil preference. In this study, we used morphometric measurements (three bract, ten floral, and two leaf measurements) and microsatellite markers to determine (1) how the two subspecies differ, (2) if there is evidence of hybridization and (3) quantify the genetic structure of known populations of C. affinis ssp. neglecta. We found that all 15 morphological measurements differed significantly between the subspecies and neutral genetic markers show strong genetic differentiation. Overall we found C. affinis ssp. neglecta populations had similar levels of genetic diversity and genetic differentiation between populations but higher inbreeding than compared to its more common congener. Two populations of C. affinis ssp. neglecta showed lower genetic differentiation from the C. affinis ssp. affinis populations, with some individuals showing considerable overlap in genotypic diversity, hence we cannot rule out historic or low levels hybridization in those populations. Castilleja affinis ssp. neglecta is a federally endangered species that would benefit from restoration efforts that aims to maintain genetic diversity while minimizing inbreeding in reintroduction efforts.