Polymorphism and selection at exon 2 of the class II beta chain of the major histocompatibility complex in Forpus passerinus

Female mate choice has been proposed to be a major force acting to maintain the polymorphism of the MHC, the most genetically variable genes known. I genotyped seventy-two individuals from a free-living population of the parrot, Forpus passerinus, in order to determine whether disassortative mating is the mode of selection acting at this locus. Both the high rate of nonsynonymous amino acid changes at the peptide binding regions of exon 2 of the class II beta gene and the unprecedented number of alleles found in this population provided strong evidence for the operation of selection. Despite this, a chi-square test revealed no evidence of disassortative mating for this sample, X2=0.569, n.s. I propose that natural selection, not sexual selection, must be operating at this locus to maintain the degree of variation found in 270 base-pairs of this gene.