Population-genetic studies of Japanese macaques: a review

Abstract

The results of population-genetic studies on the Japanese macaque (Macaca fuscata) conducted during about twenty years since the establishment of Department of Variation Research, Primate Research Institute, Kyoto University, were summarized.In the Japanese macaques morphological variations observable in coat color, dermatoglyphics and skeletal formation were inadequate as genetic markers for population-genetic study because their genetic determination were uncertain and/or the frequencies of variants were too low. So, we used the blood protein variations detectable by multilocus electrophoresis of enzymes and non-enzymatic proteins in sera and erythrocytes of the macaques as genetic markers. We collected more than 3, 000 blood samples from more than 50 natural troops of the Japanese macaque in its whole distribution range, and more than 30 independent genetic loci were electrophoretically screened. The proportion of polymorphic loci (Ppoly) in the troops were 13.66% in average and the mean heterozygosity (H) were calculated from the allele frequencies as 0.0215 in average. These values indicate that the genetic variability level in the Japanese macaques is markedly lower in comparison with those of other mammalia and even with those of other macaque species.The author found that the relationship between sexual rank and sexual activity of adult males in a troop could be approximately expressed by the geometrical series with a common ratio less than 1 and that the numbers of male and female adults were about 20% and 30%, respectively, of the number of living individuals of the troop. From a mathematical formulation considering these facts, the effective size of a troop could be estimated as about 1/3 of its census size. Then, the genetically effective size of a population of Japanese macaques can be estimated as about one third of its census population size.Starting from the formula of gene-frequency distribution in the population under the assumption of irreversible mutations, Nei (1977) formulated the estimation of average rate of structural mutation of proteins; and using the frequency values of the rare alleles detected in the Japanese macaques and a South-American Indian population, he could estimate the structural mutation rate as about 2×10-6 per locus per generation. Statistical tests of allele and genotype frequencies revealed that almost all the protein variations observed in the Japanese macaques were practically neutral to natural selection. Therefore, most of the changes of frequency, extinction and fixation of structural mutant alleles of the macaques are postulated to be determined mainly by selectively neutral mutations and random genetic drift.Geographical distribution of genetic variations of the Japanese macaque was not uniform in the whole species but the variants occurred only in limited areas. Assuming the selective neutrality of segregating alleles and the two-dimensional stepping-stone model of population structure (Kimura & Weiss, 1964), the genetic migration rate between local demes per generation could be estimated to average less than inverse of average effective deme size. Here, the local deme is not a troop itself, but it consists of several troops tightly connected with each other by frequent exchanges of reproductive males. Analyses of correlation between geographic and genetic distances between troops revealed that the gene constitutions of two troops apart more than 100km on an island could be regarded as practically independent of each other. These results suggest that the population structure of the Japanese macaque species has a tendency to split into a number of local subpopulations in which the effect of random genetic drift is prevailing.