High genetic divergence characterizes populations of the endemic plant Lithophragma maximum (Saxifragaceae) on San Clemente Island

Narrowly-ranging species frequently harbor less genetic variability relative to widespread relatives and face graver extinction threats due to the heightened impacts of stochastic events on ecological and genetic diversity. In this study, we examined the impact of historical and current threats to the maintenance of genetic variation in Lithophragma maximum (Saxifragaceae), a perennial herb endemic to San Clemente Island, California. This species exists as small populations confined to canyons along 4 km of the southeast coastline of the island. In 15 populations analyzed with 10 microsatellite markers, we identified an average of 2.05 alleles per locus and 58.7% polymorphic loci. Significant departures from Hardy–Weinberg equilibrium existed in six populations; five of these exhibited heterozygote deficiency. Bayesian inference of genetic structure indicated a significant amount of structure among populations and canyons and infrequent gene flow even over very short distances. We also identified a significant and positive correlation between genetic and geographic distances, indicative of isolation by distance. There was no evidence of recent bottlenecks in any of the sampled populations, but older bottlenecks were detected in two populations. These results suggest that populations of L. maximum have historically been small and isolated, which is likely due to the rugged habitat in which this species occurs and limited pollen and seed dispersal. Given the high degree of structure observed across populations, we suggest that conservation efforts should focus on preserving populations in multiple canyons, maintaining large population sizes to preserve genetic variation, and controlling the spread of invasive species in areas where L. maximum occurs.