Chromosomes and Taxonomy of the Ground Squirrel Subgenus Ictidomys

Chromosomes were analyzed from the marrow of three species of ground squirrels of the genus Spermophilus , subgenus Ictidomys . Samples of 8 Spermophilus mexicanus parvidens , 10 S. tridecemlineatus tridecemlineatus and 1 S. tridecemlineatus pallidas have a diploid number of 34 and karyotypes that are generally similar but distinctive at the species level. In contrast, samples of 6 S. spilosoma marginatus and 6 S. spilosoma canescens have a diploid number of 32 and a karyotype that differs more from that of the other two species than from each other. Karyotype differences were found within S. spilosoma and are thought to represent subspecific variation. Taxonomic conclusions based on chromosomal characters agree closely with those based on characters of gross morphology and indicate that all three species are correctly classified as members of the same subgenus. In addition, S. mexicanus and S. tridecemlineatus appear most closely related, while S. spilosoma is considered divergent because of its karotype, baculum and pelage.