Abstract
THE wood lemming (Myopus schisticolor Lilljeborg), a small rodent inhabiting mossy forests of northern coniferous areas of Eurasia, is unique with respect to sex ratio, showing a definite prevalence of females, and with respect to the occurrence of two types of female, one producing daughters only, the other producing progeny of both sexes1,2. We thought that these phenomena might be associated with chromosome constitution, and as the chromosomes of this species were poorly known, we have made a detailed study of its karyotype. Some additional observations, pertinent both to the occurrence of chromosome variation and to reproduction, will be reported elsewhere. We report here two remarkable findings: (1) the existence of two types of fertile female with different somatic sex chromosome constitutions—the orthodox female complement, XX, and the male complement, XY, and (2) evidence for selective non-disjunction in XY females, leading to the formation of X-type egg cells only.
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