Fifty years of hybridization: Introgression between the arizona toad (Bufo microscaphus) and woodhouse's toad (B. woodhousii) along Beaver Dam Wash in Utah

Recent guidelines for conservation of species subject to hybridization strongly urge long-term studies to determine the extent and the direction of hybridization and introgression, especially in relation to human-induced habitat alterations. This study extends previous work on hybridization between the Arizona Toad (Bufo microscaphus) and Woodhouse's Toad (B. woodhousii) along Beaver Dam Wash by evaluating morphological and genetic status of four populations. Populations occur at sites from the high elevation headwaters to the hybrid zone at the confluence with the Virgin River in extreme southwestern Utah and extreme northwestern Arizona. Hybrid indices for individuals at the confluence shifted from predominantly microscaphus-like in 1949-1953 samples, to predominantly woodhousii-like in 1991-1992 samples, and back to microscaphus-like in 2001 samples. Nuclear and mitochondrial markers identified 49 individuals from the confluence site as parental types, F1 hybrids, or reciprocal backcrossed hybrids. Although observed and expected frequencies of individuals in each of six cytonuclear categories were similar, numbers of hybrids with the woodhousii cytotype were significantly greater than those with the microscaphus cytotype. By contrast, hybrid indices of toads upstream (45-97 km) from the confluence were predominantly microscaphus-like in the 2001 samples, similar to earlier reports. Nonetheless, individuals with woodhousii mtDNA and microscaphus nuclear markers were found at sites 45.0 and 64.4 km, but not 96 km, upstream from the confluence of Beaver Dam Wash and the Virgin River. These results indicate that the confluence site is a hybrid swarm, and that introgression of woodhousii mtDNA into putatively pure microscaphus populations occurs for a considerable distance upstream along Beaver Dam Wash.