Selective pressures on MHC class II genes in the guppy (Poecilia reticulata) as inferred by hierarchical analysis of population structure.

Genes of the major histocompatibility complex, which are the most polymorphic of all vertebrate genes, are a pre-eminent system for the study of selective pressures that arise from host-pathogen interactions. Balancing selection capable of maintaining high polymorphism should lead to the homogenization of MHC allele frequencies among populations, but there is some evidence to suggest that diversifying selection also operates on the MHC. However, the pattern of population structure observed at MHC loci is likely to depend on the spatial and/or temporal scale examined. Here, we investigated selection acting on MHC genes at different geographic scales using Venezuelan guppy populations inhabiting four regions. We found a significant correlation between MHC and microsatellite allelic richness across populations, which suggests the role of genetic drift in shaping MHC diversity. However, compared to microsatellites, more MHC variation was explained by differences between populations within larger geographic regions and less by the differences between the regions. Furthermore, among proximate populations, variation in MHC allele frequencies was significantly higher compared to microsatellites, indicating that selection acting on MHC may increase population structure at small spatial scales. However, in populations that have significantly diverged at neutral markers, the population-genetic signature of diversifying selection may be eradicated in the long term by that of balancing selection, which acts to preserve rare alleles and thus maintain a common pool of MHC alleles.

Heterozygosity and orange coloration are associated in the guppy (Poecilia reticulata).

The good-genes-as-heterozygosity hypothesis predicts that more elaborate male sexual ornaments are associated with higher levels of heterozygosity. Recent theoretical work suggests that such associations are likely to arise in finite, structured populations. We investigated the correlation between multilocus heterozygosity (MLH), which was estimated using 13 microsatellite loci, and male coloration in a wild population of guppies (Poecilia reticulata), a model species in sexual selection research. We found that MLH was a significant predictor of the relative area of orange spots, a trait that is subject to strong female preference in this species. Neither the relative area of black spots nor the number of black or orange spots was significantly correlated with MLH. We found no statistical support for local effects (i.e. strong effects of heterozygosity at specific markers), which suggests that relative orange spots area reflects genome-wide heterozygosity.