Genetic Structure of Human Populations on Tobishima Island:

III. Analysis of Genetic Differentiation in Relation to the Degree, Pattern, and Cause

Abstract

Blood samples of a total of 389 inhabitants from Tobishima Island in Yamagata Prefecture, locating in the north-western part of Japan, were examined for 23 blood polymorphic systems, for the purpose of evaluating the degree and pattern of genetic differentiation among the three constituent villages (KA, NA and HO) and to identifying the factors mainly responsible for the differentiation. A remarkable degree of genetic variability was found to exist among the villages, being especially conspicuous in the ABO and Gm systems. The inter-village differentiation was evaluated to be 1.17% of the total genetic variation according to Nei's coefficient of gene diversity, indicating the degree by far greater than that seen among the general populations off three distant districts of Japan, Tohoku, Kinki and Kyushu, and appreciably greater than that among the Ainu, Kinki and Sakishima populations. As for the differentiation pattern, two of the villages (KA and HO) were similar to each other in the array of allele frequencies, whereas the other one (NA) was rather more similar to the general Japanese than to them. However, the majority of the systems exhibited a more or less unbalanced variation pattern among the villages. Characteristic deviations of the Tobishima populations from the general Japanese population were noted in not a few allele frequencies, being notable especially in a decrease of JK^a and an increase of PGMvar1., some of which might be common to the northern areas of Japan. Based on the results obtained, it is suggested that random genetic drift, gene flow, inter-village migrations and probably founder effect have contributed as the main factors to the production of genetic differentiation among the villages on Tobishima. The present study has significance in providing an excellent evidence of demonstrating that gene flow has also played an important role in an ongoing micro-evolutionary process within a small area of Japan.