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Genotype-by-environment interactions (G x Es) describe genetic variation for phenotypic plasticity, such that the relative performance of genotypes varies across environments. These interactions have been studied in the context of natural selection for decades, but research interest in the evolutionary consequences of G x Es in sexual traits is more recent. Theory suggests that G x Es in sexual traits could be of fundamental importance to the operation of sexual selection across heterogeneous environments, but empirical research lags behind the theory. In this thesis, I review the current literature on the role of G x Es in sexual selection and identify areas for further research. Using cuticular hydrocarbons (CHCs) in the fruit fly Drosophila simulans as a model system for sexual selection, I examine G x Es in trait expression and quantify the effect of these G x Es in terms of sexual signal reliability and the coevolution of male and female sexual traits. To do so, I use a combination of quantitative genetics and laboratory environmental manipulations. First, I demonstrate that male CHC profile is subject to sexual selection through female mate choice and find some variation in patterns of mate choice across diets and temperatures (Chapter 3). Next, I identify G x Es in male and female CHC expression across diets and temperatures, although G x Es in male CHC profile across temperatures are weak (Chapter 4). I find that G x Es in male CHC expression can cause sexual signal unreliability, as predicted by theory, since male CHCs do not reliably signal heritable aspects of male attractiveness across diets and temperatures (Chapter 5). I also find G x Es in some aspects of female mate choice across temperatures (Chapter 6). In spite of the evidence for signal unreliability and variation in female mate choice across environments, I show that the overall outcome of mate choice is unaffected by G x Es, such that the same male genotypes are attractive across diets and temperatures (Chapters 5 and 6). From my results, it seems likely that females assess male attractiveness based on multiple male sexual signals, so that whilst male CHCs influence mate choice, CHC profile does not necessarily correlate well with overall male attractiveness. I discuss the implications of these results for the evolution of sexual traits and the genetic covariance between male and female sexual traits across environments. The research in this thesis highlights the importance of multivariate studies of sexual selection across environments for a more complete understanding of the evolution of sexual traits.

Conflicts between the sexes over mating and reproduction are common in many taxa. In Drosophila melanogaster sexual conflict over mating frequency occurs because male fitness is maximised by a higher mating frequency than that which maximises female fitness. In addition, females incur survival and reproductive costs from mating frequently. The female cost of mating is mediated by male accessory gland proteins (Acps), which are transferred to females in the seminal fluid of males during mating. Theory predicts that sexual conflict can lead to antagonistic coevolution between the sexes in which males evolve traits that benefit males but harm females and females evolve traits to minimise the extent of male-induced harm. This coevolution may be rapid and has the potential to promote speciation. In this thesis I used Drosophila melanogaster as a model organism to investigate the evolution and genetics of sexual conflict. In Chapters 3 and 4 I used selection lines in which the level of sexual conflict had been experimentally manipulated. Chapter 3 describes how females in the selection lines adapted to the level of sexual conflict by evolving different levels of resistance to male-induced harm. Chapter 4 shows that the selected females did not differ in pre-mating resistance to males, suggesting instead that the resistance to male harm had evolved via post-mating changes. Chapter 5 describes how a single Acp, the sex peptide (SP), which reduces female receptivity and increases egg production after first matings, also causes female mating costs. Chapter 6 confirms that SP benefits males by increasing their post-mating reproductive success. Thus, SP benefits males but harms females suggesting that the SP gene underlies sexual conflict in D. melanogaster. Finally, Chapter 7 summarises the thesis and discusses future directions for investigating sexual conflict in Drosophila.

Over the last century, sexual selection has grown from a controversial theory into a vast field of theoretical and empirical research. Although Darwin outlined two major mechanisms within his theory, male-male competition and female mate choice, the latter has promoted a wealth of research by virtue of its complexity. Despite decades of research into how female preferences and sexually selected traits have evolved, there is still little consensus as to why females prefer the males they do. Preferences are thought to evolve from either direct selection on the preference, as females themselves benefit directly from mating with a preferred male, or through indirect selection on the preference via offspring fitness. In all cases however, female preferences should compensate for the costs of discriminating between potential mates, if they are to remain overall beneficial. The fitness benefits of mating with preferred males were investigated here using the fruitfly Drosophila simulans, employing a range of behavioural, phenotypic and quantitative genetic approaches. The findings presented here indicate that female Drosophila simulans do not gain directly from mating with a preferred male. Multiple mating can increase fecundity, although costs from male harassment can reduce the net benefit. They also indicate that females may benefit indirectly from mating with attractive males as attractiveness is heritable and sons of preferred males are themselves preferred. There is also evidence that attractive males are successful in both the pre- and post-copulatory sense, as preferred males are better sperm competitors than less-preferred males. However, although there appear to be benefits from preferred males via their sons, there appear to be no benefits from males via their daughters’ fitness. These findings collectively indicate that female preferences in Drosophila simulans are driven by indirectly selected benefits (via Fisherian sons), and that females benefit directly from mating multiply.