Population genetics of introduced bullfrogs, Rana (Lithobates) catesbeianus, in the Willamette Valley, Oregon, USA

The American bullfrog, Rana (Lithobates) catesbeianus, is endemic to eastern North America, but has been introduced to approximately 40 countries on four continents and is considered one of the hundred worst invasive alien species in the world. Here, we investigated the genetics of invasive bullfrogs in the Willamette Valley, Oregon, USA, where bullfrogs are widespread and abundant to determine: (1) the minimum number of bullfrog introductions; (2) the native source population(s); and (3) whether genetic variation is reduced compared to source populations. To answer these questions, we analyzed partial sequences of the mitochondrial cytochrome b gene for 251 bullfrogs from the Willamette Valley and the native range. We found that bullfrogs from the Mississippi River basin and Great Lakes region were introduced at least once to the Willamette Valley. Genetic variation measured as haplotype diversity (h) and nucleotide diversity (pi (n) ) was not significantly different between Willamette Valley and source populations. Our results were in contrast to a recent genetic analysis of invasive bullfrog populations in Europe, which found that genetic variation in European bullfrog populations was much lower than in source populations. European bullfrogs also originated from different source populations than Willamette Valley bullfrogs. The difference in genetic composition between Willamette Valley and European bullfrogs is likely due to differences in their invasion histories and may have implications for the potential of bullfrogs in these different regions to adapt and expand.