Wood lemming (Myopus schisticolor) populations are characterized by female biased sex ratios and cyclic variations in population size. Both of these characteristics are assumed to reduce genetic variation and thus affect the evolutionary adaptation of the species. We addressed these questions by studying the genetic structure of a wood lemming population from eastern Finland by isozyme markers during a 21-year period, which corresponds to 40-50 generations. Contingency tests showed that genotypic proportions conformed to Hardy-Weinberg equilibrium in each of the four sampling years. Among the temporal replicates, allele frequencies differed most by 0.14 and were not significant. Genetic variation was also stable and fairly high with a mean observed heterozygosity of H ¼ 0.057. Variability in the Heinävesi population was higher than previously reported in wood lemming. The difference was mainly caused by variation at a phosphoglucomutase locus that was monomorphic in earlier studies. Significant linkage disequilibrium was observed in three of the comparisons but the disequilibrium did not appear consistently in all years. This pattern was also evidenced by the variance components, which indicated that selection favoured for specific allele pairs only in few subsamples. Heredity (2005) 94, 443-447.

Eskelinen O., Sulkava P. and Sulkava R. 2004. Population fluctuations of the wood lemming Myopus schisticolor in eastern and western Finland. Acta Theriologica 49: 191-202. From 1982 to 2003 we studied fluctuations in populations of the wood lemming Myopus schisticolor (Liljeborg, 1844) in the Heinävesi (eastern Finland) and Keuruu regions (western Finland) by counting field signs and dead animals in standardized field surveys. We compared the population fluctuations of lemmings to those of other voles, owls and small mustelids in these regions. The lemming population in Heinävesi fluctuated regularly in 3-year cycles and in synchrony with the field vole popu-lation. Populations of owls also fluctuated in synchrony with wood lemming and field vole populations. In the Keuruu region, oscillations in the wood lemming population were irregular, and neither lemming and vole populations nor lemming and owl populations were correlated. Although direct mechanistic evidence is lacking, specialist predators such as weasels and owls probably cause the cyclicity in the field vole and wood lemming populations in the Heinävesi area. On the other hand, scarcity of high-quality habitats, unfavourable winter weather conditions and generalist predators may prevent the development of cyclicity in the wood lemming population of Keuruu.