Ethnicity, gene flow, and population subdivision in Limón, Costa Rica

In this paper we examine the effects of ethnicity on the gene flow between two groups living in Limón, Costa Rica. Our main interest is to determine if ethnicity has acted as a barrier to the exchange of genes, and if the groups have remained distinct genetically. We report the admixture estimates, F(st) values, and inbreeding coefficients of the two samples. The data consist of blood samples and surnames obtained from 375 individuals. The subjects' two surnames were analyzed to determine the ethnicity of their parents (individuals carry their father's and mother's first surnames). We used the formula of Crow and Mange ([1965] Eugen Q 12:199-203) to compute F(t), F(n), and F(r) with the surnames. Admixture estimates were computed for both groups using the computer program ADMIX.PAS kindly provided by Jeffrey Long. The estimates for the Hispanic-Limonense group are M1 = 0.5866 European, M2 = 0.3383 Amerindian, and M3 = 0.0751 African ancestry. For the Afro-Limonense group, the admixture estimates indicate M1 = 0.1047 European, M2 = 0.1357 Amerindian, and M3 = 0.7595 African ancestry. The F(st) values are F(st) = 0.00558 for the Hispanic group and F(st) = 0.05137 for the Afro-Limonense group. These F(st) values indicate that the Afro-Limonense group has experienced more genetic drift than has the other group, possibly as a result of its long history of isolation in Costa Rica. Indeed, when plotted along a scaled eigenvector R matrix of Caribbean gene frequencies, the two Limonense groups did not cluster with each other. Thus we conclude that the two ethnic groups have remained distinct breeding populations.