The effect of parasites on sex differences in selection

Abstract

The life history strategies of males and females are often divergent, creating the potential for sex differences in selection. Deleterious mutations may be subject to stronger selection in males, owing to sexual selection, which can improve the mean fitness of females and reduce mutation load in sexual populations. However, sex differences in selection might also maintain sexually antagonistic genetic variation, creating a sexual conflict load. The overall impact of separate sexes on fitness is unclear, but the net effect is likely to be positive when there is a large sex difference in selection against deleterious mutations. Parasites can also have sex-specific effects on fitness, and there is evidence that parasites can intensify the fitness consequences of deleterious mutations. Using lines that accumulated mutations for over 60 generations, we studied the effect of the pathogenic bacterium Pseudomonas aeruginosa on sex differences in selection in the fruit fly Drosophila melanogaster. Pseudomonas infection increased the sex difference in selection, but may also have weakened the intersexual correlation for fitness. Our results suggest that parasites may increase the benefits of sexual selection.

Figure 1

Conceptual basis for studying the effect of parasites on sex-specific selection. There is empirical evidence for synergistic effects of parasites and mutations (A), sex-specific effects of mutations (B), and sex differences in immunity and the effect of parasites on fitness (C), leading to the prediction that parasites and mutations may interact in a sex-specific manner, i.e. a three-way interaction.

Figure 2

Effect of infection on fitness decline (ΔM) in each sex. The dashed line indicates no change in fitness under mutation accumulation. Values above the line indicate reduced fitness relative to controls. The sex difference in selection, ψ, increased when flies were infected with bacteria.

Figure 3

Intersexual correlations for arcsine-transformed mutant W values in uninfected flies and flies infected at levels D1 and D2, with linear regression lines. We observed a significant positive intersexual correlation in uninfected flies but not in infected flies; the correlation coefficients do not differ significantly from one another.