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 about three generations. These lemmings have a pair of autosomes fused to the sex chromosomes. Thus, males have two copies of some (formerly autosomal) sex-linked genes: One set is X-linked; the other can be described as Y-linked. Given such a karyotype, deleterious recessive alleles on the autosomal portion of the X chromosome are more resistant to selection than truly autosomal loci because they can be eliminated by homozygosity only in females. The male-bias could have resulted from one or more lethals carried on the formerly autosomal arm of the X chromosome. As inbreeding coefficients approached 0.3, the lethal was apparently homozygous in half of the homogametic (female) zygotes. This phenomenon may explain the excess of males and XY females attributed to meiotic drive in Dicrostonyx torquatus from Siberia. If under the natural 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.