No evidence of positive assortative mating for genetic quality in fruit flies

In sexual populations, the effectiveness of selection will depend on how gametes combine with respect to genetic quality. If gametes with deleterious alleles are likely to combine with one another, deleterious genetic variation can be more easily purged by selection. Assortative mating, where there is a positive correlation between parents in a phenotype of interest such as body size, is often observed in nature, but does not necessarily reveal how gametes ultimately combine with respect to genetic quality itself. We manipulated genetic quality in fruit fly populations using an inbreeding scheme designed to provide an unbiased measure of mating patterns. While inbred flies had substantially reduced reproductive success, their gametes did not combine with those of other inbred flies more often than expected by chance, indicating a lack of positive assortative mating. Instead, we detected a negative correlation in genetic quality between parents, i.e. disassortative mating, which diminished with age. This pattern is expected to reduce the genetic variance for fitness, diminishing the effectiveness of selection. We discuss how mechanisms of sexual selection could produce a pattern of disassortative mating. Our study highlights that sexual selection has the potential to either increase or decrease genetic load.