Abstract
The major histocompatibility complex (MHC) is a polymorphic gene family associated with immune defence, and it can play a role in mate choice. Under the genetic compatibility hypothesis, females choose mates that differ genetically from their own MHC genotypes, avoiding inbreeding and/or enhancing the immunocompetence of their offspring. We tested this hypothesis of disassortative mating based on MHC genotypes in a population of great frigatebirds (Fregata minor) by sequencing the second exon of MHC class II B. Extensive haploid cloning yielded two to four alleles per individual, suggesting the amplification of two genes. MHC similarity between mates was not significantly different between pairs that did (n = 4) or did not (n = 42) exhibit extra-pair paternity. Comparing all 46 mated pairs to a distribution based on randomized re-pairings, we observed the following (i): no evidence for mate choice based on maximal or intermediate levels of MHC allele sharing (ii), significantly disassortative mating based on similarity of MHC amino acid sequences, and (iii) no evidence for mate choice based on microsatellite alleles, as measured by either allele sharing or similarity in allele size. This suggests that females choose mates that differ genetically from themselves at MHC loci, but not as an inbreeding-avoidance mechanism.
Highlights
A fundamental goal in evolutionary biology is to gain a better understanding of forces that create and maintain genetic variation in natural populations
Comparing all 46 mated pairs to a distribution based on randomized re-pairings, we observed the following (i): no evidence for mate choice based on maximal or intermediate levels of major histocompatibility complex (MHC) allele sharing (ii), significantly disassortative mating based on similarity of MHC amino acid sequences, and (iii) no evidence for mate choice based on microsatellite alleles, as measured by either allele sharing or similarity in allele size
Our data on MHC and microsatellite genotypes suggest that mate choice favours partners with divergent MHC amino acid sequences, in a manner beyond simple mean of true pairs; p = 0.672 (a) 1000 number of simulations 2.5% 97.5%
Summary
A fundamental goal in evolutionary biology is to gain a better understanding of forces that create and maintain genetic variation in natural populations. To analyse the magnitude of differences between alleles, we summed the number of amino acid differences from pairwise combinations of alleles in mated birds [25,40] The average of this score for the 46 known pairs was compared with the distribution of scores generated from 10 000 simulations of 46 random pairings selected from the same 92 individuals, just as with the allele-sharing analysis [25,40]. Because the allele-sharing value of actual pairs was slightly, but not significantly, lower than the mean from the randomizations, we used a variance test to assess the possibility that females choose mates for an intermediate level of MHC dissimilarity. To test for disassortative mating based on MHC allele sequence similarity (rather than simple identity), we tested whether the sum of pairwise amino acid differences between mated pairs was extremely large (or small) relative to randomized pairings. The true genetic sires of extra-pair offspring were not identified, given that this breeding population may contain more than 10 000 mate-seeking males [46]
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