Genetic Variation and Structure in an Endemic Island Oak, Quercus tomentella, and Mainland Canyon Oak, Quercus chrysolepis

Premise of research. Quercus tomentella is a tree species endemic to the California Channel Islands and Isla Guadalupe. Given its distribution across six widely separated islands, significant genetic structure would be expected, despite the propensity of oaks for long-distance pollen dispersal. In comparison, its close mainland relative, Quercus chrysolepis, has a more continuous range and fewer barriers to gene flow. Methodology. We sampled Q. tomentella from all the islands in its range (N = 345) and Q. chrysolepis from five mainland sites and on the islands where it occurs (N = 100) and genotyped the trees using eight polymorphic microsatellite loci. Genetic differentiation within and between species was examined using genetic distances, analysis of molecular variance, Bayesian clustering (both spatial and nonspatial approaches), a neighbor-joining tree, and genetic discontinuities indicative of barriers to gene flow. We also looked for evidence of population bottlenecks. Pivotal results. A high level of clonality was found in Q. tomentella on Santa Catalina Island and Santa Rosa Island, but genetic variability was high in both species and at all sites, including the tiny surviving population on Isla Guadalupe. Genetic distance measures were significant between most populations of both species. The most surprising result is that the two species were not clearly differentiated, and genetic clusters identified through both spatial and nonspatial analyses were shared between species. Conclusions. The island endemic Q. tomentella and the widespread Q. chrysolepis are not well-differentiated species. Further work is needed to clarify the relationships within and among these species. Insular populations of Q. tomentella are genetically diverse and distinct; the remaining population found on Isla Guadalupe warrants protection and management to support recruitment.