Inbreeding and strong population subdivision in an endangered salamander

Studies of genetic population structure and genetic diversity are often critical components of endangered species conservation and management plans. Genetic studies are thus particularly important for amphibians, which are in global decline. We studied genetic variation and population structure among 276 individuals from approximately half of the known localities of the endangered Sonora tiger salamander, Ambystoma mavortium stebbinsi, using ten microsatellite loci. Allelic diversity was generally low (2.7 alleles per locus per population) and overall observed heterozygosity (0.191) was significantly lower than expected (0.332). Most populations showed significant departures from Hardy-Weinberg equilibrium, which are likely due to inbreeding. In addition, evidence of recent bottlenecks was suggested by shifted allele frequency distributions in 5 of 16 populations, and ratios of allele number to allele size range (M) values lower than critical values in all populations. A high degree of genetic subdivision (theta = 0.133) was found over all populations, and nearly all pairwise population combinations were genetically subdivided. Thus, gene flow is limited even over small distances, perhaps because high desert grassland throughout the study area limits the efficacy of inter-pond movement of salamanders. Further, population sizes and gene flow of Sonora tiger salamanders are likely compromised by several contemporary ecological threats, including: frequent die-offs due to an infectious virus, introductions of non-native species, and continuing cattle grazing. Overall, these genetic data support the endangered status of the Sonora tiger salamander and suggest the subspecies exists in small, inbred populations.