Chromosomes and Systematics of American Ground Squirrels of The Subgenus Spermophilus

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Abstract
Chromosome counts, karyotypes and possible mechanisms of karyotype evolution are presented for seven of the eight American species comprising the subgenus Spermophilus. Within S. townsendi, the subspecies vigilis, mollis and townsendi have a 2n of 46, 38 and 36, respectively. S. washingtoni, 2n = 36, has a karyotype that differs from that of S. townsendi townsendi. The chromosomes of S. richardsoni also exhibit intraspecific variation; the subspecies elegans and nevadensis have a 2n of 34 and similar karyotypes, whereas richardsoni has 36 chromosomes. S. undulatus and S. armatus also have 34 chromosomes and autosomes that are indistinguishable from those of S. richardsoni elegans and nevadensis. However, on the basis of X and Y chromosome morphology, S. armatus can be differentiated from S. undulatus and S. richardsoni. S. columbianus and S. beldingi have diploid numbers of 32 and 30 respectively, and each has a distinctive karyotype. Karyotypes suggest a close relationship between three subspecies of S. townsendi and S. washingtoni. A second group of closely related species consists of S. undulatus, S. armatus and S. richardsoni. The first two of these three species and two subspecies of S. richardsoni, elegans and nevadensis all have identical autosomes. Evidence from this study of the chromosomes supports other lines of evidence which suggest that the subgenus Spermophilus contains two species groups, one composed of S. townsendi and S. washingtoni and the second consisting of S. richardsoni, S. undulatus, S. armatus, S. columbianus and S. beldingi. A revision of interspecific relationships within the subgenus Spermophilus is presented.