Genetic Variation in Thomomys bulbivorus, an Endemic to the Willamette Valley, Oregon

Abstract

Populations of Thomomys bulbivorus, a pocket gopher endemic to the Willamette Valley, were sampled at irregular intervals along a north-south transect from Hillsboro, Washington Co., to Monroe, Benton Co., Oregon; this produced 217 specimens from 10 localities. Starch-gel electrophoresis was used to assess intra- and interpopulation genetic variation. Of 43 presumptive loci, 11 were polymorphic. T. bulbivorus exhibited a genetic pattern of limited inbreeding within populations (F(IS) = 0.138) and much differentiation among populations (F(ST) = 0.232) coupled with a high level of heterozygosity (0.048). This pattern is characteristic of species of Thomomys, except T. townsendii, with more extensive geographic distributions than T. bulbivorus. As reported for Geomys, the level of intraspecific heterozygosity in Thomomys appears to be unrelated to the extent of the geographic distribution of the species. Our results are consistent with the hypothesis of a balance between random genetic drift and gene flow. The pattern of differentiation among current populations of T. bulbivorus, as revealed by cluster analysis of Rogers' genetic similarity coefficients, reflects a cataclysmic event affecting the entire geographic distribution of the species ca. 13,000 years ago.