Genetic diversity and gene flow among recovery regions of the federally endangered Peninsular bighorn sheep (Ovis canadensis nelsoni), California, USA
Peninsular bighorn sheep (Ovis canadensis nelsoni, PBS) are a federally endangered metapopulation of desert bighorn sheep endemic to the Peninsular Ranges of southern California, extending to the U.S./Mexico border. This metapopulation declined to <300 animals prior to listing in 1999, and is currently managed within a series of recovery regions approximating ewe home range groups. The goal of this project was to examine genetic diversity and spatial structure within the metapopulation and determine the extent of gene flow among selected recovery regions. Genetic data was generated by amplifying 39 microsatellite loci and a 515 bp fragment of the mtDNA control region from blood samples collected from sheep (n= 165) captured in seven recovery regions from 1992 to 2013.
STRUCTURE analysis of microsatellite genotypes clustered sheep into two distinct genetic groups (north vs. south). Significant pairwise FST estimates among sheep from seven recovery regions (0.03 to 0.12) generally supported the presence of two genetic groups, with the possibility of additional substructure in the north (pairwise FST = 0.03 to 0.04, amova P = 0.02). Mitochondrial DNA analyses revealed five distinct haplotypes and indicated a similar pattern of north vs. south population structure (pairwise ΦST = 0.27 to 0.50). Considerable microsatellite diversity was found within both northern and southern groups (mean HO = 0.496; allelic richness = 3.47; FIS = 0.02), comparable to published accounts for desert bighorn sheep in other regions. We identified first-generation migrants of both sexes using GENECLASS2, but significantly lower assignment indices (AIc) among rams (Mann-Whitney U-test: P < 0.01) suggested dispersal was primarily male biased. Despite past population declines and ongoing deterministic threats, PBS have retained substantial genetic variation and gene flow among regions. Future conservation efforts should not only focus on sustaining population numbers, but also maintaining functional connectivity so the recovering metapopulation can expand into available habitat throughout the Peninsular Ranges.