Population structure and conservation genetics of the Oregon spotted frog, Rana pretiosa

Abstract

The Oregon spotted frog (Rana pretiosa) is one of the most threatened amphibians in the Pacific Northwest. Here we analyzed data from 13 microsatellite loci and 298 bp of mitochondrial DNA in frogs collected from 23 of the remaining R. pretiosa populations in order to (1) assess levels of genetic diversity within populations of R. pretiosa, (2) identify the major genetic groups in the species, (3) estimate levels of genetic differentiation and gene flow among populations within each major group, and (4) compare the pattern of differentiation among R. pretiosa populations with that among populations of R. cascadae, a non-endangered congener that also occurs in Oregon and Washington. There is a strong, hierarchical genetic structure in R. pretiosa. That structure includes six major genetic groups, one of which is represented by a single remaining population. R. pretiosa populations have low genetic diversity (average H e = 0.31) compared to R. cascadae (average H e = 0.54) and to other ranid frogs. Genetic subdivision among populations is much higher in R. pretiosa than in R. cascadae, particularly over the largest geographic distances (hundreds of kilometers). A joint analysis of migration rates among populations and of effective sizes within populations (using MIGRATE) suggests that both species have extremely low migration rates, and that R. pretiosa have slightly smaller effective sizes. However, the slight difference in effective sizes between species appears insufficient to explain the large difference in genetic diversity and in large-scale genetic structure. We therefore hypothesize that low connectivity among the more widely-spaced R. pretiosa populations (owing to their patchier habitat), is the main cause of their lower genetic diversity and higher among-population differentiation. Conservation recommendations for R. pretiosa include maintaining habitat connectivity to facilitate gene flow among populations that are still potentially connected, and either expanding habitat or founding additional ‘backup’ populations to maintain diversity in the isolated populations. We recommend that special consideration be given to conservation of the Camas Prairie population in Northern Oregon. It is the most geographically isolated population, has the lowest genetic diversity (H e = 0.14) and appears to be the only remaining representative of a major genetic group that is now almost extinct. Finally, because the six major groups within R. pretiosa are strongly differentiated, occupy different habitat types, and are geographically separate, they should be recognized as evolutionarily significant units for purposes of conservation planning.