SUMMARYIn the varying lemming, only some of the females exhibit a conventional sex chromosome constitution, XX. Many fertile females as well as all males have one X chromosome. The Y chromosome was not revealed by the methods employed. It has been shown that the sex of XO specimens depends on a genetic factor which displays two alternative forms. The XO lemmings possessing the rn-form develop as males; those carrying the f-form develop as females. This factor is most likely X-linked. The XX females are heterozygous for this factor or homozygous for the rn-form. In progeny of the XO and heterozygous XX females an excess of daughters is observed, the secondary sex ratio being O67 and O75 respectively. Two or three mutational events must have given rise to this complex system of sex determination. The possible adaptive significance of the relevant system is discussed.

In the varying lemming, numerous fertile XY females occur regularly due to the X-linked mutation (X*). Their frequency both in natural populations and Jaboratory colonies turned out to be about twice higher than that expected under random segregation of heterochromosomes in both sexes. It has been shown in experiments that an excess of XY females resulted from a preferential segregation of the Y chromosome in males. Segregation distortion is not produced by selective embryonal mortality. Meiotic drive of the Y chromosome also causes a significant decrease of sex ratio. Although in the varying lemming meiotic drive is rather weak (the segregation ratio of the Y being 0.54-0.59), it seems to contribute essentially to the evolutionary spread and the maintenance of the XK mutation in populations. The example of Dicrostonyx and probably of other microtines also demonstrates the possible role of meiotic drive in the regulation of the population sex ratio in mammals.

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Lemmings of the genus Lemmus display a very moderate degree of karyotype diversity. In L. lemmus from the Kola Peninsula, L. sibiricus sibiricus from the Pechora inlet coast and the Laptev Sea coast, L.s. portenkoi from the Vrangell Island and L. amurensis from Southern Yakutia the gross chromosome morphology and C-banding pattern proved to be identical (2n = 50, NF = 50). The chromosome set of L.s. chrysogaster (the Chukotka Peninsula) consists of 23 pairs of acrocentrics and 2 pairs of subtelocentrics (2n = 50, NF = 54), and so it is identical to the karyotype of North American Lemmus. As to spatial structure of populations, population dynamics, degree of inbreeding and uniformity of habitats throughout the area, Lemmus is basically similar to the lemmings of another genus, Dicrostonyx. In the latter, however, an extensive chromosome diversity has been found. Therefore, the ecological and population characteristics mentioned above are suggested not to be essential for the rate of chromosome evolution in lemmings.

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