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 (Thomomys bottae) along the eastern flank of the Sangre de Cristo Mountains in Colorado and New Mexico is examined by comparative karyology, electromorphic 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 greater than 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 are compared with four other T. bottae contact previously investigated. These five 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 approximately 1 km to 200 km, with the nature of the environmental setting at a being more predictive of cline width than gross measures of genetic differentiation (e.g., electromorphic distance values and chromosome numbers). [Pocket gophers; Thomomys; hybridization; chromosomes; electrophoresis; morphometrics; cline models.] During the past 15 years, we have gathered information on the dynamics of genetic, phenetic, and ecologic interactions in hybrid zones of pocket gophers of the Thomomys bottae group (Fig. 1). The philosophy behind these studies has rested on the view that detailed analyses of character variation, at geographic sites where phenotypically and/or genetically differentiated populations are in contact, provide the best means to evaluate the types of changes that may be involved in speciation in the group. Thus, the Patagonia Mountains (for review see Patton, 1973), where the species T. bottae and T. umbrinus hybridize but produce only F1, largely sterile offspring, emphasized the potential role of meiotic imbalance in mediating hybrid success in chromosomally differentiated forms. Other parapatric zones within T. bottae, involving both chromosomal and electromorphic differentiation (White-Sacramento Mts. [Patton et al., 1979]; Rio Grande [Smith et al., 1983]) or solely electromorphic differences (Colorado River [Smith and Patton, 1980]), emphasize that not all types of chromosomal changes will affect reproductive performance of hybrids, that hybridization potential is unrelated to the level of electromorphic differentiation, and that genic introgression may reflect only the local ecology and physiography of given regions (reviewed by Patton, 1981). In this study genetic and morphologic information are evaluated for T. bottae character clines along a narrow peninsular distribution abutting the Sangre de Cristo Mountains of Colorado and New Mexico (Fig. 1). This situation involves the addition of up to 12 extra chromosomal elements to the diploid number, yet population samples lack electromorphic differentiation. This is an unusual system of cytological differentiation,