Chromosomal Differentiation of Hybridizing Ground Squirrels (Spermophilus mexicanus and S. tridecemlineatus)

Abstract

The ground squirrels Spermophilus mexicanus and S. tridecemlineatus represent species that might be considered to be in an early stage of speciation. They have a broad distribution: S. tridecemlineatus occurs from central Canada to northwestern and eastern Texas, and S. mexicanus occurs throughout parts of the southwestern United States and northern Mexico (Hall, 1981). Two subspecies of S. tridecemlineatus (S. t. arenicola and S. t. texensis) contact S. mexicanus in western Texas and southeastern New Mexico. The area of contact between S. mexicanus and S. t. arenicola is confined to eastern New Mexico and northwestern Texas and that of S. mexicanus and S. t. texensis to northern and eastern Texas (Zimmerman and Cothran, 1976). Standard karyotypes indicate that some introgression is occurring between these two taxa in areas of contact; they produce backcross individuals as well as Fl's (Zimmerman and Cothran, 1976). Because chromosomal rearrangements may play a role in speciation (White, 1978), identification of the degree of chromosomal differences and of the types of rearrangements involved are important to understanding the significance of genetic interactions occurring between these two species. An assessment of standard karyotypic data is insufficient as such data have underestimated the number and types of rearrangements distinguishing closely related taxa (Baker and Bickham, 1980; Haiduk et al., 1981). Therefore, our study was designed to examine chromosomal differentiation between S. mexicanus and S. tridecemlineatus with the use of chromosomal G- and C-banding techniques. Standard karyotypes were prepared following the method of Lee (1969); preparations were examined from 100 specimens of each species including individuals from every population listed in Cothran (1982). G- and C-banded chromosomal preparations were made from fibroblast tissue cells cultured from ear biopsies