A REEVALUATION OF CHROMOSOMAL VARIATION IN POPULATIONS OF PEROMYSCUS MANICULATUS ALONG AN ELEVATIONAL GRADIENT.

Abstract

In a recent report, Macey and Dixon (1987) presented data suggesting a relationship between chromosomal variation and environmental differences. Individuals representing three subspecies of the deer mouse Peromyscus maniculatus were collected along a latitudinal transect which comprised an elevational gradient ranging from 1,070 m to 4,110 m. Karyotypic analysis revealed an apparent positive correlation between increasing altitude and decreasing proportion of animals homozygous for the acrocentric form of chromosome 6. The occurrence of individuals heterozygous at chromosome 6 (acrocentric/submetacentric) was interpreted as evidence of gene flow between the subspecies sampled. Although the data in Macey and Dixon (1987) do seem to show a general correlation between the relative number of acrocentric-6 homozygotes and elevation, the conclusions made by these authors on the basis of chromosome 6 in P. maniculatus are in need of reevaluation. First, these authors focused their attention on an autosome that is highly polymorphic for pericentric inversions within deer mouse populations, and yet they assumed a hybrid origin for chromosomally heterozygous individuals. Additionally, the different inversion morphs of chromosome 6 occur in a wide variety of habitats throughout North America. These authors concentrated only on the pattern along their transect and ignored evidence from other studies that contradicts their conclusions on the relationship between chromosomal morphology and environmental conditions. Finally, and most importantly, Macey and Dixon (1987) observed an apparent correlation between altitude and chromosomal phenotype and assumed a cause-and-effect relationship; it is invalid to make such assumptions from circumstantial evidence. Chromosomal banding data reveal extensive pericentric-inversion polymorphism within P. maniculatus and two closely related species P. sitkensis and P. oreas (Greenbaum et al., 1978; Dixon et al., 1980; Clark, 1983; Pengilly et al., 1983; Greenbaum and Reed, 1984; Van Fleet, 1985; Gunn, 1986, 1988; Gunn and Greenbaum, 1986; Hale, 1986; Macey and Dixon, 1987; Hale and Greenbaum, 1988). The considerable chromosomal variability observed both among and within populations of P. maniculatus is largely attributable to pericentric-inversion polymorphisms identified in chromosomes 6, 7, 10, 14, 15, and 19. Karyotypic data from various subspecies across the continental United States indicate that populations are frequently polymorphic for pericentric inversions of chromosome 6