A MALE-BIASED NATAL SEX-RATIO IN INBRED COLLARED LEMMINGS, DICROSTONYX GROENLANDICUS

A captive colony of collared lemmings (Dicrostonyx groenlandicus) from northern Alaska produced a male-biased sex ratio of 67% males for abo ut three generations. These lemmings have a pair of autosomes fused to the sex chromosomes. Thus, males have two copies of some (formerly au tosomal) sex-linked genes: One set is X-linked; the other can be descr ibed as Y-linked. Given such a karyotype, deleterious recessive allele s on the autosomal portion of the X chromosome are more resistant to s election than truly autosomal loci because they can be eliminated by h omozygosity only in females. The male-bias could have resulted from on e or more lethals carried on the formerly autosomal arm of the X chrom osome. As inbreeding coefficients approached 0.3, the lethal was appar ently homozygous in half of the homogametic (female) zygotes. This phe nomenon may explain the excess of males and XY females attributed to m eiotic drive in Dicrostonyx torquatus from Siberia. If under the natur al mating system, inbreeding depression limits fitness, then fusion of autosomal chromatin to the sex chromosomes could be an adaptation to reduce inbreeding depression in heterogametic individuals.