Female sticklebacks count alleles in a strategy of sexual selection explaining MHC polymorphism

The origin and maintenance of polymorphism in major histocompatibility comp lex (MHC) genes in natural populations is still unresolved(1). Sexual selec tion, frequency-dependent selection by parasites and pathogens, and heteroz ygote advantage have been suggested to explain the maintenance of high alle le diversity at MHC genes(2-4). Here we argue that there are two (non-exclu sive) strategies for MHC-related sexual selection, representing solutions t o two different problems: inbreeding avoidance and parasite resistance. In species prone to inadvertent inbreeding, partners should prefer dissimilar MHC genotypes to similar ones. But if the goal is to maximize the resistanc e of offspring towards potential infections, the choosing sex should prefer mates with a higher diversity of MHC alleles. This latter strategy should apply when there are several MHC loci, as is the case in most vertebrates(2 ,5). We tested the relative importance of an 'allele counting' strategy com pared to a disassortative mating strategy using wild-caught three-spined st icklebacks (Gasterosteus aculeatus) from an interconnected system of lakes. Here we show that gravid female fish preferred the odour of males with a l arge number of MHC class-IIB alleles to that of males with fewer alleles. F emales did not prefer male genotypes dissimilar to their own.