PATTERNS OF GEOGRAPHIC VARIATION IN KARYOTYPE IN THE POCKET GOPHER, THOMOMYS BOTTAE (EYDOUX AND GERVAIS).

Abstract

Variation in the chromosome complement resulting from both Robertsonian and non-Robertsonian mechanisms has been described for numerous mammalian taxa. In most cases such analyses have been restricted to levels at or above that of the species. Little attention has been paid to detailed variation within and between populations over the geographic range of any particular species even in cases where detailed geographical studies have been made. It is at this level, however, that an understanding of the role chromosomal changes play in evolutionary divergence will best be achieved. The pioneering work of Matthey (1966, and elsewhere) on the African murid genus, Mus (Leggada) indicates both the importance and the potential of the population approach to an understanding of chromosomal divergence. The real challenge to the investigator in this field is the elucidation of the precise selective bases which regulate interand intra-population variation rather than the continued cataloguing of examples of taxa displaying chromosomal variation of one sort or another. Unfortunately mammalian species are not blessed with short generation times nor are most species suitable for controlled laboratory breeding and experimentation. Inferences regarding the adaptive significance of chromosomal changes within such species must come, therefore, from correlations between the observed type and geographical extent of the variants and extrinsic (e.g., environmental and historical) and/or intrinsic (e.g., genetic) parameters determining the habitus of the forms in question. The most karyotypically diverse group of mammals so far discovered are the pocket gophers of the genus Thomomys. Within this genus, one species, T. bottae (Eydoux and Gervais), is known to display a greater degree of chromosomal variability than any other mammal. This animal ranges in distribution from coastal California to the front range of the Rocky Mountains, and from southern Oregon to northern Sinaloa in Mexico (Hall and Kelson, 1959; Anderson, 1966). Populations from approximately one-half of the total range have been karyotyped and more than 40 separate karyotypes have been recognized (Patton and Dingman, 1970; Patton, 1970; Berry and Baker, 1971). Moreover, and of fundamental importance to an understanding of the patterns and mechanisms of chromosomal evolution in the species, detailed information is available on specific biological attributes of gophers, including life history, habitat preferences, distribution patterns, physiology, functional morphology, and limited information on paleontological history. The present paper is thus an attempt to interrelate these aspects of the biology of gophers with the causative agents and resulting patterns of geographic variation in karyotype in T. bottae.