Pronounced genetic structure associated with differences in a reproductive trait and climatic barriers in Canadian populations of the western toad (Anaxyrus boreas)

Abstract

Identifying genetic groups within species is critical for inventorying biodiversity, understanding species distributions, and delineating conservation units. The western toad (Anaxyrus boreas) is one of the most widespread amphibians in western Canada and one of just two amphibian species to maintain an extensive range on both sides of the Canadian Rocky Mountains. Curiously, toads on the eastern side of the mountains have a vocal sac and produce a distinct advertisement call whereas toads on the western side of the mountains lack these traits. However, the extent to which these "Calling" and "Non-Calling" populations are genetically and ecologically distinct remains unclear. We used reduced representation sequencing (ddRAD-seq) to genotype individuals from across the Canadian portion of the species range. Combining population genomic analysis with the original phenotypic data used to delineate the Calling and Non-Calling populations and with predictions from ecological niche models, we show that Calling and Non-Calling populations of western toads represent distinct genetic groups separated by both differences in a major reproductive trait and by climate barriers. We additionally find evidence for a third, undescribed group in southwestern Alberta and southeastern British Columbia. Our results suggest Calling and Non-Calling populations should be managed as distinct groups and highlight the potential for there to be cryptic and strong genetic structure among northern populations of widespread species.