CHROMOSOMAL AND MITOCHONDRIAL-DNA VARIATION IN 4 LABORATORY POPULATIONS OF COLLARED LEMMINGS (DICROSTONYX)

Genetic differentiation among populations and speciation in Dicrostonyx is hypothesized to have resulted from either allopatric divergence in glacial refugia during the Wisconsin or sympatric processes uncorrelated with refugial isolation. We examined chromosomal and mitochondrial DNA variation in four laboratory colonies, representing three species, in a preliminary evaluation of these hypotheses. Chromosomal variation is extensive among populations, diploid numbers ranging from 38 to 50. Autosomal variation appears to be due primarily to Robertsonian rearrangements and additions of super-numerary chromosomes, and is geographically unpatterned. Sex chromosome morphology is geographically structured and correlated with proposed southern and northern refugia. Restriction fragment analysis of mitochondrial DNA revealed two ancient, divergent genotypic assemblages, corresponding to geographic distributions of sex chromosomes. Autosomal variation, and any resulting reproductive isolation, probably is recent and uncorrelated with refugial history, whereas divergence of sex chromosomes and disparate mitochondrial assemblages likely predate the Wisconsin.