Introduction
Darwin posited that sexual selection plays an important role in improving non-sexual fitness, writing that, “the strongest and most vigorous males, or those provided with the best weapons, have prevailed under nature, and have led to the improvement of the natural breed or species” (Darwin, 1871). The modern version of this idea proposes that sexually selected traits in males reflect “good genes” (Fisher, 1930; Zahavi, 1975; Iwasa et al. , 1991; Houle & Kondrashov, 2002), explaining potentially costly female choice by indirect benefits received in the form of increased offspring fitness. Theory suggests expression of sexually-selected traits should evolve to become dependent on overall condition—which would maintain signal fidelity—leading to accelerated rates of adaptation (Lorch et al. , 2003) and more efficient purging of deleterious mutations (Whitlock & Agrawal, 2009).
In line with predictions of positive effects of sexual selection on population performance, sexual selection has been found to diminish the likelihood of population extinction (Jarzebowska & Radwan, 2010; Lumleyet al. , 2015). Experimental work in Drosophila has also shown that the presence of sexual selection accelerates the purging of deleterious alleles in experimental populations (Radwan, 2004; Holliset al. , 2009; Whitlock & Agrawal, 2009; Grieshop et al. , 2016). In several experiments, sexual selection facilitated adaptation to novel environmental challenges, including the evolution of desiccation resistance in D. melanogaster (Gibson Vega et al. , 2020), pesticide resistance in Tribolium castaneum (Jacombet al. , 2016), and adaptation to a novel diet inCallosobruchus maculatus (Fricke & Arnqvist, 2007). However, an arguably larger body of experimental work has found no role for sexual selection in improving non-sexual fitness. Multiple experimental evolution studies failed to find population-level net benefits of sexual selection when examining larval competitive ability, net reproductive rate, or female fecundity (Promislow et al. , 1998; Holland & Rice, 1999; Long et al. , 2009 respectively). Moreover, a large body of work has also failed to demonstrate a role of sexual selection in adaptation in novel environments (e.g. to higher temperatures (Holland, 2002) or a novel diet (Rundle et al. , 2006)). There is also no evidence that overall mutation load from the genome is reduced under heightened sexual selection (Hollis & Houle, 2011; Arbuthnott & Rundle, 2012) (although in environments that are spatially complex, this is not true and the predicted beneficial effects of sexual selection on mutation load are seen (Singh et al. , 2017)). Thus, taken together, the literature is equivocal about role of sexual selection in non-sexual fitness. This leaves an open question about whether the “good genes” mechanism plays a role in adaptation in general or even in specific scenarios, like during adaptation to pathogens or parasites, where this role has been predicted to be most evident but remains largely untested.
One potential explanation for these mixed results is that the non-sexual fitness of populations is normally elevated by competition for mates—that is, sexual selection in the broad sense does have adaptive value—but these benefits are counterbalanced by the negative effects of sexual conflict and therefore invisible in many experimental designs. Sexual conflict arises because of an evolutionary conflict of interests between the sexes (Parker, 1979; Hosken et al. , 2019) which can manifest in two ways. The first, interlocus sexual conflict, is characterized by selection favoring traits that increase male competitive success even when these traits are accompanied by harm to females. Interlocus sexual conflict can lead to the evolution of female resistance and sexually antagonistic coevolution (Holland & Rice, 1999a; Chapman et al. , 2003; Rice et al. , 2006), reducing mean population fitness (Bonduriansky & Chenoweth, 2009; Long et al. , 2009, 2012). In Drosophila, interlocus sexual conflict acts through antagonistic effects on female fecundity and survival (Rice, 1996; Chapman, 2006), especially on the most fecund females (Longet al. , 2009). Intralocus conflict, on the other hand, involves sexually antagonistic pleiotropic effects of polymorphisms at the same locus in males and females (Bonduriansky & Chenoweth, 2009; Van Doorn, 2009; Innocenti & Morrow, 2010)) that constrain males and females from reaching sex-specific optima (Chippindale, 2001; Hollis et al. , 2014, 2019). Either form of sexual conflict leads to a burden on populations that might overwhelm any positive effects of sexual selection for mean population fitness (Bonduriansky & Chenoweth, 2009; Long et al. , 2009, 2012).
Male-male competition and female choice have been proposed to be particularly consequential for evolution of pathogen resistance (Hamilton & Zuk, 1982; Folstad & Karter, 1992; Roberts et al. , 2004). Pathogens are a major evolutionary driver of the life histories of organisms (Price, 1980; Schmid-Hempel, 2005) due to their prevalence, diversity, and because they adapt to the host and represent a moving target for the immune system. According to the Hamilton-Zuk hypothesis (1982), sexual ornaments indicate immunity towards prevalent pathogens or parasites (Hamilton & Zuk, 1982; Martin, 1990). A number of studies in birds have indeed demonstrated phenotypic correlations between male parasite or pathogen load and the quality of sexual ornaments (Hamilton & Zuk, 1982; Martin, 1990; Balenger & Zuk, 2014) or female preference towards the males (Blount et al. , 2003; Hund et al. , 2020). Yet, whether this phenotypic correlation should be positive or negative is not unequivocally predicted by mathematical models; either may be predicted depending on details of the model assumptions (Getty, 2002). These phenotypic correlations between sexual ornaments and parasite/pathogen resistance do not necessarily predict whether sexually attractive fathers will sire resistant offspring; rather, this key element of the ”good genes” hypothesis is mediated by additive genetic correlations (Hamilton & Zuk, 1982). One way to test for this genetic correlation would be to track the evolution of resistance under controlled laboratory conditions (Kawecki et al. , 2012) where both the strength of sexual selection and pathogen pressure are manipulated. If there is an additive genetic correlation between sexually successful fathers and pathogen-resistant offspring, resistance should evolve more readily in populations where males also experience sexual selection.
Selection for improved immunity (including better physiological responses to immune challenges) in experimental populations has generally resulted in a robust and rapid response (Armitage & Siva-Jothy, 2005; Martins et al. , 2013; Joop et al. , 2014; Ferro et al. , 2019). Two studies that explored the effect of sexual selection on immunity by experimentally evolving populations with and without sexual selection have found that males and females diverge in their investment in innate immunity (measured as phenyloxidase activity; PO) (Hangartner et al. , 2015; Bagchi et al. , 2021). In both these studies (one on the flour beetle Tribolium castaneum and the other on the seed beetle Callosobruchus maculatus; (Hangartner et al. , 2015; Bagchi et al. , 2021 respectively)), females from polygamous populations had higher levels of PO than females from monogamous populations, with no effect on males from either of the two experimental regimes. The higher levels of PO in females from sexually selected populations did not influence pathogen clearance in either study, although in one of the studies higher PO activity was correlated with lower survival in females upon bacterial infection (Bagchi et al. , 2021). These studies indicate how sexual selection and sexual conflict can drive sex-specific differences in male and female immunity. This pattern is not without exceptions; a study on the yellow dung fly, Scathophaga stercoraria, did not report sex differences in PO levels in populations evolved with or without sexual selection (Hosken 2001). Hosken (2001) also found that monogamous populations had higher PO levels than polygamous populations, although here also this difference did not translate into differences in bacterial clearance after infection (Hosken, 2001). The above studies manipulated the presence or absence of either a pathogen or sexual selection. In the work reported here, we manipulated both pathogen and sexual selection in order to test for effects of the presence of each, as well as any interaction, on the evolution of pathogen resistance.
We carried out a 2-way factorial evolutionary experiment manipulating sexual selection and exposure to a pathogen. We let replicate populations of D. melanogaster evolve for 14 generations either under controlled monogamy or random polygamy (i.e., with or without sexual selection (Hollis & Houle, 2011)), each generation exposing males to either an intestinal pathogen (a gram-negative bacteriumPseudomonas entomophila ) or a sham treatment. In our experimental design, we only exposed males to the pathogen and allowed the males to interact with females after one day of exposure to the pathogen (we verified that males had cleared the bacteria from their gut at this timepoint, and thus did not infect the females). With this design, we aimed to increase the opportunity for sexual selection to act via differential mating success of males differentially coping with infection. We aimed to address several interconnected questions.
First, and most simply, do D. melanogaster populations exposed to the pathogen as adults evolve resistance, measured as survival after infection, over a short timescale? Resistance to P. entomophilahas been reported to evolve after only four generations of strong selection imposed by breeding from flies that survived a prior infection (Martins et al. , 2013). Second, if only one sex—in our design, males—experiences the pathogen, would evolved resistance to P. entomophila be detectable in the other sex? If evolved resistance is evident in both sexes, this would indicate a shared genetic basis. Third, would sexual selection lead to the evolution of differences in pathogen resistance even in the absence of pathogen? This would be predicted if there were an additive genetic correlation between male sexual traits and resistance that were expressed irrespective of pathogen exposure (Joye & Kawecki, 2019). A result supporting this prediction has been reported in Tribolium (Hangartner et al. , 2015) and Callosobruchus (Bagchi et al. , 2021); however, the conclusion was based on quantifying an aspect of immune response rather than resistance to an actual pathogen. Fourth, does sexual selection accelerate the evolution of resistance in populations exposed to the pathogen, and does it do so to a greater degree than would be expected based on the sum of effects of sexual selection and pathogen exposure acting alone? This positive interaction between the effects of pathogen and sexual selection would be expected if heritable variation in pathogen resistance influenced infected males’ sexual success.
The rationale of this study relied on the pathogen affecting the sexual success of males. Therefore, prior to the evolutionary experiment we tested if infection with P. entomophila affects competitive paternity share. Mortality in our laboratory population was much lower than is generally reported (Martins et al. , 2013; Faria et al. , 2015; Joye & Kawecki, 2019), but uninfected males had greater competitive paternity success than infected males. If genetic variation conferring resistance to P. entomophila has a similar positive effect on male competitive success after exposure to the pathogen, this scenario should provide an opportunity for female choice to amplify nonsexual selection and accelerate adaptation to pathogen.