Contact Zones and the Genetics of Differentiation in the Pocket Gopher Thomomys bottae (Rodentia: Geomyidae)

Abstract

The genetic interaction between clinally varying populations of pocket gophers (T. bottae) along the eastern flank of the Sangre de Cristo Mountains in Colorado and New Mexico [USA] is examined by comparative karyology, electromorphetic analysis of proteins and colorimetric and morphometric analyses of morphology. Diploid number varies clinally along the transected populations from 76 in the south to 88 in the north. The karyotypic change is unaccompanied by electromorphic divergence as all populations studied share > 95% similarity, although similar clines in pelage brightness and mean morphometric coefficient of variation are evident. Individuals with intermediate diploid numbers suffer no apparent fitness deficit. The patterns of genetic interaction at the Sangre de Cristo contact are compared with 4 other T. bottae contact zones previously investigated. These 5 situations include differentiated parental forms which span the range of known genetic (karyological and electromorphic) divergence in the T. bottae species group, and involve both instances of reproductive isolation and extensive introgressive hybridization. Cline widths for T. bottae zones vary from .apprx. 1-200 km, with the nature of the environmental setting at a contact being more predictive of cline width than gross measures of genetic differentiation (e.g., electromorphic distance values and chromosome numbers).