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(e-mail: andrew.ashton@duke.edu). ................................................................. Female sticklebacks count alleles in a strategy of sexual selection explaining MHC polymorphism Thorsten B. H. Reusch*², Michael A. Ha ¨ berli*². Peter B. Aeschlimann*² & Manfred Milinski* * Max Planck Institute for Limnology, Department of Evolutionary Ecology, Postfach 165, 24306 Plo ¨n, Germany ² These authors contributed equally to this work .............................................................................................................................................. The origin and maintenance of polymorphism in major histo- compatibility complex (MHC) genes in natural populations is still unresolved 1 . Sexual selection, frequency-dependent selection by parasites and pathogens, and heterozygote advantage have been suggested to explain the maintenance of high allele diversity at MHC genes 2–4 . Here we argue that there are two (non-exclusive) strategies for MHC-related sexual selection, representing solu- tions to two different problems: inbreeding avoidance and para- site resistance. In species prone to inadvertent inbreeding, partners should prefer dissimilar MHC genotypes to similar ones. But if the goal is to maximize the resistance 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 count- ing’ strategy compared to a disassortative mating strategy using wild-caught three-spined sticklebacks (Gasterosteus aculeatus) from an interconnected system of lakes. Here we show that gravid female fish preferred the odour of males with a large number of MHC class-IIB alleles to that of males with fewer alleles. Females did not prefer male genotypes dissimilar to their own. Sexual selection, the preference of certain mating partners over others, is ubiquitous among animals 6,7 . The choosing sex may be able to increase the attractiveness of offspring, or gain direct benefits such as parental care 8 . Another function of mate choice is to increase the fitness of offspring by eitherchoosing ‘good genes’, or avoiding incompatible ‘bad genes’—for example in matings with close kin. Particularly suited for testing the idea of choosiness with respect to genes is the MHC, a multigene family that is important in control- ling the vertebrate immune system by presentation of self and foreign peptides to T cells 2 . MHC alleles confer specific resistance against pathogens and parasites. Therefore, mate choice should increase the fitness of offspring by maximizing the heterozygosity at MHC loci 1,4,9,10 , allowing a wider spectrum of pathogens to be recognized during early infection. The focus of previous studies was disassortative mating, that is, the preference of dissimilar males or females as a mating part- ner 11–13 , as a means of inbreeding avoidance, or in order to increase the heterozygosity at MHC loci 3,14 . In the context of sexual selection, it has not been taken into account that most vertebrates possess several MHC loci 5 . As a result, there are many possible combina- tions of alleles at different loci 2 . The chances of choosing a partner with identical MHC haplotypes become very unlikely because the combination of alleles at multiple loci renders the expected like- lihood of existing MHC genotypes very small, even under linkage disequilibrium. A mechanism of sexual selection focusing on the distinction between similar and dissimilar MHC genotypes becomes inefficient when increasing parasite resistance is impor- tant. Females should rather choose partners that maximize the number of different MHC alleles in their offspring 3 . We studied populations of the three-spined stickleback (Gasterosteus aculeatus) where we identified high MHC diversity for partial sequences of MHC class-IIB loci, coding for the peptide- binding region. At an estimated six loci 15 , we identified 24 distinct sequences in only eight fish from a system of interconnected populations, and many more alleles were identified on the basis of single-strand conformation polymorphisms (SSCP) in a total of 144 fish. We also observed marked differences in the number of different alleles per individual fish, varying between two and eight detectable alleles across all loci (Fig. 1). For one location (Scho ¨hsee) we calculated that the chance a female has of mating with an MHC- identical male is only 1% if she mates at random. But the probability of choosing a mate with fewer alleles is 46% under random expectations (see Methods). Therefore, we predicted that females Number of MHC class-IIB alleles 2 7 3 4 5 6 8 10 20 30 40 Number of fish Figure 1 Frequency distribution of the number of MHC class-IIB alleles (peptide-binding region) detectable by SSCP in 144 fish from one population (Scho ¨ hsee). The mean number of MHC alleles 6 s.e. was 5:8 6 0:13. © 2001 Macmillan Magazines Ltd