Robertsonian Polymorphism in the Cotton Rat, Sigmodon Fulviventer

Karyotypes of 28 fulvous-bellied cotton rats, Sigmodon fulviventer, from Arizona and New Mexico were analyzed. Intrapopulational variation in diploid numbers of 30, 29, and 28 was found. This polymorphism is ascribed to Robertsonian changes: centric fusion or fission involving two pairs of acrocentric autosomes or one pair of metacentric autosomes, respectively. Possible mechanisms are discussed for the origin of this polymorphism and its evolutionary significance. The relatively high frequency of each diploid type is contrasted with the condition in certain other species of mammals. Tentative remarks concerning chromosomal relationships among four kinds of Sigmodon are presented.