Genetical Research

The effect of subdivision on variation at multi-allelic loci under balancing selection

a1 Institute of Cell, Animal and Population Biology, University of Edinburgh, Edinburgh EH9 3JT, UK
a2 Laboratoire de Génétique et d'Ecologie Végétales, Université Libre de Bruxelles, 1850 Chaussée de Wavre, B-1160 Brussels, Belgium


Simulations are used to investigate the expected pattern of variation at loci under different forms of multi-allelic balancing selection in a finite island model of a subdivided population. The objective is to evaluate the effect of restricted migration among demes on the distribution of polymorphism at the selected loci at equilibrium, and to compare the results with those expected for a neutral locus. The results show that the expected number of alleles maintained, and numbers of nucleotide differences between alleles, are relatively insensitive to the migration rate, and differentiation remains low even under very restricted migration. However, nucleotide divergence between copies of functionally identical alleles increases sharply when migration decreases. These results are discussed in relation to published surveys of allelic diversity in MHC and plant self-incompatibility systems, and to the possibility of inferring ancient population genetic events and processes. In addition, it is shown that, for sporophytic self-incompatibility systems, it is not necessarily true in a subdivided population that recessive alleles are more frequent than dominant ones.

(Received June 2 1999)
(Revised November 3 1999)

c1 Corresponding author. Department of Ecology and Genetics, University of Aarhus, Ny Munkegade, Building 540, DK-8000 Aarhus C., Denmark.