The results of many years work on the ecology of the fieldmouse (Apodenus sylveticus) present a sound background to a study of its ecological genetics. Gel electrophoresis and orbital bleeding techniques were used to pursue this.\ud A preliminary estimation of the degree of genetic variability in this species was made through studies on ten loci in 250 animals from eight populations. Similar levels of heterozygosity were observed to those found in the house mouse and two widespread polymorphisms were discovered at the lactate dehydrogenase regulator (Ldr) and phosphoglucomutase (PMG_2 ) loci. The eight populations were similar in gene frequencies at both.\ud To discover the degree of change which occurs in a population over a period of time, two sites were visited three times at six-monthly intervals. They showed great constancy in gene frequencies at both the PGM_2 and Ldr Ioci when large samples were available. A third site was sampled bimonthly for two years. No real change in gene frequency was observed at Ldr but at PGM_2 significant changes were found during one winter when numbers were high. As survival rates were low, these results suggest natural selection.\ud The most important single factor influencing survival in overwintering fieldmice is food supply. The central role that phosphoglucomutase plays in glycogen metabolism suggested that the enzyme itself might be subject to selection and experiments were designed to test this. Liver glycogen levels were measured in animals that had been starved overnight.\ud When analysed according to PGM_2 genotype a highly significant difference was found between the levels in aa animals and those in ac and cc genotypes. This pattern was the same as that found in the survival rates of a large overwintering natural population. It was concluded that these genotypes are not neutral with respect to selection
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