Discussion
In our study, we aimed to address the interplay of sexual selection and pathogen presence on the evolution of resistance to a pathogen, P. entomophila . We found a signature of pathogen resistance in populations evolved under pathogen pressure for fourteen generations when compared to populations evolved without it. Surprisingly, despite only infecting males over the course of experimental evolution, resistance to pathogen was more prominent in females. We did not find any evidence that sexual selection can promote the evolution of resistance to the pathogen, contrary to the predictions of theory (Hamilton & Zuk, 1982; Westneat & Birkhead, 1998). We expected that the presence of sexual selection and pathogen pressure would act synergistically, resulting in a greater response to selection and therefore improved survival post-infection. We instead found an antagonistic interaction between the two in males, which could have possibly impeded the evolution of pathogen resistance.
Evolution of increased resistance of D. melanogaster to enteric infection and systemic infection has been seen in studies that have experimentally evolved fly populations with P. entomophila(Martins et al. , 2013; Gupta et al. , 2016). The study by Martins et al. (2013) imposed very strong selection on both sexes, with pathogen-induced mortality up to 77% in the initial generations. In our experiment, pathogen selection was only applied on males and was associated with much lower mortality (5-25% depending on the generation). This lower virulence likely resulted from a difference in the bacterial genotype and/or the initial Drosophila gene pool; the IV population is generally robust and harbors high levels of genetic variation. It is likely that the overall strength of selection for resistance was therefore considerably lower in our experiment, but yet still sufficient to generate a detectable response. A stronger response to selection might have been obtained with a more virulent pathogen, or if both males and females had been infected each generation. Infecting females introduces a difficulty, however, in that reductions in female mating rate and fecundity make maintenance of experimental populations more challenging, and any reductions in female choosiness due to infection would be expected to diminish the importance of sexual selection.
The fact that females from populations under pathogen pressure evolved higher resistance despite not experiencing direct selection supports a shared genetic basis for immunity between the sexes. Indeed, in line with this idea (Collet et al. , 2016; Connallon & Hall, 2016), adaptation to desiccation resistance in experimentally evolved populations of D. melanogaster was observed both in males and females even when selection was imposed only on males (Gibson Vegaet al. , 2020). Adaptation in our experiment may also be more evident in female post-infection survival simply because females show generally lower survival upon infection relative to males, which would make any evolved differences in survival easier to detect in females than males. Moreover, it is also plausible that alleles contributing to immunity that were favored in males under pathogen pressure had a larger effect size on resistance in females, making female resistance towards pathogen more detectable in this sex. We can exclude the possibility that selection did in fact act directly on females, for example by sexual or social transmission of the pathogen from males to females, because the pathogen was cleared by males by the time they encountered females.
In our study, we do not see any evidence that sexual selection aids the evolution of resistance to pathogen. Previous studies have attributed the lack of adaptation to novel environments to the negative effects of sexual conflict (Holland & Rice, 1999b; Rundle et al. , 2006). However, if sexual conflict negatively affected adaptation in our populations we would have expected to find that populations exposed to the pathogen each generation but not experiencing sexual selection (-SS +P) would show a stronger signal of adaptation to pathogen than those exposed to pathogen and experiencing sexual selection (+SS +P). While our results on male survival after infection align with this idea, there is no signal of a cost to sexual selection in female survival after infection, leaving it difficult to attribute any importance to sexual conflict in our experiment.
In conclusion, our study found that populations of D. melanogaster evolved resistance to the insect pathogen P. entomophila , but this was either not facilitated (in females) or hindered (in males) by sexual selection. We expect that the low mortality in our study compared to previous work on this pathogen (Guptaet al. , 2013, 2016; Martins et al. , 2013; Joye & Kawecki, 2019), in which the majority of infected individuals die, provided a level of biological realism. The pathogen was still virulent enough to induce downstream effects on male sexual success, suggesting that genetic variation conferring resistance to pathogen would provide a large target for sexual selection. In addition, because most males survived infection during the course of experimental evolution, this provided an opportunity for sexual selection to reinforce non-sexual selection by magnifying more subtle differences in pathogen resistance (e.g. differences in male condition or vigor that might emerge after weathering the infection). Despite a scenario that seems favorable for the detection of putative benefits of sexual selection—a relatively mild pathogen that might persist in natural host populations, that still yet influences mating success, in a host that harbors genetic variation for resistance—we found no such benefits.