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

In this paper we examine the effects of ethnicity on the gene flow between two groups living in Limon, Costa Rica. Our main interest is to determine i f ethnicity has acted as a barrier to the exchange of genes, and if the gro ups have remained distinct genetically. We report the admixture estimates, F-st values, and inbreeding coefficients of the two samples. The data consi st of blood samples and surnames obtained from 375 individuals. The subject s' 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-x with the surnames. Admixture estimates were computed for both grou ps using the computer program ADMIX.PAS kindly provided by Jeffrey Long. Th e estimates for the Hispanic-Limonense group are M1 = 0.5866 European, M2 = 0.3383 Amerindian, and M3 = 0.0751 African ancestry. For the Afro-Limonens e 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 gro up. These F-st values indicate that the Afro-Limonense group has experience d 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 scale d eigenvector R matrix of Caribbean gene frequencies, the two Limonense gro ups did not cluster with each other. Thus we conclude that the two ethnic g roups have remained distinct breeding populations. (C) 2001 Wiley-Liss, Inc .