For the 15 subspecies of Peromyscus maniculatus (2n = 48) examined, there was no karyotypic pattern characteristic of a subspecies population. In 13 subspecies, the number of biarmed chromosomes clustered around a single mode. Overlapping ranges were observed in geographically adjacent subspecies, and clinal tendencies were suggested both within and among the subspecies, which seemed to correlate with climate, elevation, and latitude. In two subspecies, P. m. rufinus and P. m. oreas, dual modes were evident, which implied genetic separation within the subspecies populations. P.m. rufinus of southeastern Arizona may be a relict part of the species or possibly a part of another species population. The two cytological types in P.m. oreas are cytologically similar to either adjacent coastal or mountain subspecies. Cytological preparations of five subspecies stained for heterochromatin (C-bands) revealed a uniquely stained Y chromosome and heavily stained short arms among the biarmed elements. The addition of heterochromatin may be one way this evolutionarily plastic species has altered the chromosome pattern in addition to or in conjunction with pericentric inversions and reciprocal translocations. From a limited number of specimens, it is predicted that G-banding will become a powerful tool in future analysis of cytogenetic relationships among individuals, populations, subspecies, and species of Peromyscus.
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