Four Contacts Between Ranges of Different Chromosome Forms of the Thomomys Talpoides Complex (Rodentia: Geomyidae)

Thaeler, C. S., Jr. (Department of Biology, New Mexico State University, Las Cruces, 88003) 1974. Four contacts between ranges of different chromosome forms of the Thomomys talpoides complex (Rodentia:Geomyidae). Syst. Zool. 23:343-354.-Contacts between the ranges of four pairs of different chromosome forms of pocket gophers of the Thomomys talpoides complex were studied in an effort to gain insight into the relationship between karyotype differences and level of evolutionary divergence. All contacts are located in the mountainous region of western Colorado. Three different results were found. At one contact no evidence of hybridization between the two chromosome forms was detected by an examination of karyotypes. At two other contacts limited hybridization occurs between the chromosome forms. The fourth contact studied is a broad zone of intergradation (20 miles or more in width) in which more or less unrestricted interbreeding occurs. A discussion of each contact area is presented and I conclude that frequently, though not always, karyotype differences in pocket gophers are indicative of species-level differences. [Thomomys; chromosomes; speciation.] Within the last ten years our knowledge of the karyotypes of mammals has been greatly increased. One of the features of this increased knowledge has been the discovery of numerous karyotypic differences among populations which are considered to represent a single species (for examples, Baker and Lopez, 1970; Berry and Baker, 1972; Davis et al., 1971; Nadler, 1966, 1968; Nelson-Rees et al., 1968; Patton, 1973; Patton and Dingman, 1968, 1970; Raicu et al., 1968; Shellhammer, 1967; Sutton and Nadler, 1968; and Wahrman et al., 1969). These findings raise questions with regard to current classifications. To what extent are such chromosome differences features of intraspecific variation and to what extent may such differences indicate unrecognized species? The answers to these questions are more than queries concerning the accuracy of current taxonomy. Rather, the significance of chromosome variation in the process of evolution and especially in the process of speciation can be accurately appraised only in the context of a correct understanding of the relationship between different chromosome forms. The chromosome variation which occurs in the pocket gopher, Thomomys talpoides (Richardson), offers an opportunity to examine this problem. Karyotypes from four different populations of this complex in Colorado were reported previously (Thaeler, 1968a). The karyotype representing each of these populations was distinct and each had a different diploid number of chromosomes (44, 48, 56 and 60). Subsequently, a program of extensive collecting of additional material was carried out in an effort to discover the systematic relationships among these karyotype forms. This effort has revealed considerably more variation than initially reported and also has furnished sufficient data to establish an understanding of the distribution of each of the chromosome forms now known. Thus far 15 distinct chromosome forms can be recognized and each is known from several (2 to 25) different localities. With an increased knowledge of the distribution of each chromosome form it has been possible to investigate the systematic relationship between several pairs