An Analysis of Natural Hybridization between the Pocket gophers, Thomomys bottae and Thomomys umbrinus, in Arizona

Natural hybridization between Thomomys bottae and T. umbrinus at a contact locality in the Patagonia Mountains of southern Arizona is documented utilizing chromosomal and exomorphological characteristics. The hybridization is unbalanced in favor of F1 production with little evidence of backcrossing. Moreover, there is no indication of genic introgression between the two species. A post-mating reproductive barrier due to meiotic imbalance results in nearly complete sterility in male hybrids and a significantly lowered reproductive potential in female hybrids. Partial development of a pre-mating barrier based on differential parental habitat selection and behavioral differences is suggested, and reasons why such secondary barriers have not developed fully are discussed. Ecological relationships between T. bottae and T. umbrinus at this locality suggest that the niche of the former species is best considered a subset of the latter.