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Abstract
Sexual selection can increase rates of adaptation by imposing strong selection in
males, thereby allowing efficient purging of the mutation load on population fitness
at a low demographic cost. Indeed, sexual selection tends to be male-biased throughout
the animal kingdom, but little empirical work has explored the ecological sensitivity
of this sex difference. In this study, we generated theoretical predictions of sex-specific
strengths of selection, environmental sensitivities and genotype-by-environment interactions
and tested them in seed beetles by manipulating either larval host plant or rearing
temperature. Using fourteen isofemale lines, we measured sex-specific reductions in
fitness components, genotype-by-environment interactions and the strength of selection
(variance in fitness) in the juvenile and adult stage. As predicted, variance in fitness
increased with stress, was consistently greater in males than females for adult reproductive
success (implying strong sexual selection), but was similar in the sexes in terms
of juvenile survival across all levels of stress. Although genetic variance in fitness
increased in magnitude under severe stress, heritability decreased and particularly
so in males. Moreover, genotype-by-environment interactions for fitness were common
but specific to the type of stress, sex and life stage, suggesting that new environments
may change the relative alignment and strength of selection in males and females.
Our study thus exemplifies how environmental stress can influence the relative forces
of natural and sexual selection, as well as concomitant changes in genetic variance
in fitness, which are predicted to have consequences for rates of adaptation in sexual
populations.