Karyotypic Variation in Peromyscus Boylii (Rodentia: Cricetidae) from Mexico and Corresponding Taxonomic Implications

Abstract

Schmidly, David J., and Gilbert L. Schroeter (Department of Wildlife and Fisheries Sciences and Department of Biology, Texas AM the principal differences are in the numbers of large and medium-sized biarmed autosomes which range from 2-10. Three chromosome races are recognizable in P. boylii; they are distributed parapatrically with respect to one another, and present evidence suggests that limited gene exchange occurs among them. The karyotype for topotypes of P. b. simulus from Nayarit, Mexico, is markedly different from all other forms of P. boylii, suggesting that this taxon is a distinct species. Chromosomal polymorphism of a non-Robertsonian type is seen in certain populations of P. boylii and may be the result of intergradation between the different chromosome races. The polymorphism involves a variable number of large biarmed autosomes with certain individuals in a population possessing a heteromorphic pair of autosomes. The karyotypes of P. oaxacensis and P. evides, both members of the boylii species group, are also described for the first time. [Karotypic variation; Peromyscus boylii.] The brush mouse, Peromyscus boylii, is distributed from Honduras northward to Oregon and Utah and eastward to Texas and New Mexico. This species has the largest geographic distribution of any in the boylii species group (Hooper, 1968) and is among the most common of Mexican mice. Currently, 15 subspecies are recognized with the greatest differentiation occurring on the highlands of Mexico. Chromosomal data from 80 specimens of P. boylii, mostly from the western and southwestern United States, revealed significant geographic variation in the karyotypes of these populations (Lee, Schmidly, and Huheey, 1972). However, only 14 specimens of boylii were examined from western Mexico (Durango, Sonora, and Michoacan) and none was examined from northeastern Mexico. Moreover, all populations sampled by Lee et al. (1972) were monomorphic with regard to their chromosomal complement; for that reason, they suggested that P. boylii is chromosomally conservative. However, since different populations of P. boylii display such karyotypic diversity, it might be expected that populations do exist which are chromosomally polymorphic. Our subsequent data reveal the existence of an extensive intrapopulational chromosomal polymorphism of a non-Robertsonian type in Mexican populations of P. boylii. The polymorphism has been found in populations of the subspecies P. b. levipes, P. b. beatae, and P. b. spicilegus from the Mexican states of Queretaro, Hidalgo, Veracruz, and Durango. Furthermore, certain populations of P. boylii from Queretaro, San Luis Potosi, Tamaulipas, and Nayarit possess karyotypes distinct from other populations of this species. Chromosomal variation in P. boylii is herein reviewed in light of certain taxonomic problems in this