The impact of biotic interactions and abiotic stressors on reproductive traits

Reproduction operates within a dynamic ecological and social context. Understanding variation in reproductive traits and their evolution requires examining how abiotic conditions and the genetic social environment shape reproductive outcomes. High temperatures can have a range of negative sublethal effects on organisms. Critically, heat stress can reduce fertility, negatively impacting population fitness. Assessing thermal sensitivity of fertility across taxa is thus vital for predicting species’ responses to rising temperatures. However, plastic behavioural changes – such as shifts in mate choice and mating rate – may buffer these effects, and the evolutionary potential of reproductive traits could facilitate adaptation over generations. Examining how heat stress affects fertility and reproductive behaviour, alongside trait genetic architecture, is therefore key to predicting how the thermal environment influences reproduction and population dynamics. Additionally, the expression or ultimate outcome of many reproductive traits depends on an interaction between the sexes, meaning both direct and indirect genetic effects (IGEs) shape their variation. Thus, studying how the genomes of both sexes interact is essential for explaining trait evolution and identifying genes underlying trait variance. By manipulating the thermal or genetic social environment, this thesis explores how abiotic stressors and biotic interactions shape reproductive traits. Chapter I investigated the genetic architecture of sperm ejection timing, a post-copulatory sexually selected trait, using isofemale lines of Drosophila melanogaster. Both direct and IGEs contributed to phenotypic variance, with sex-specific patterns suggesting a role in sexually antagonistic coevolution. Using adapted genomic frameworks, I also identified allelic variants with strong additive effects and introduced a novel approach to detect genes influencing trait variance via IGEs. Chapter II examined how temperature affects nuptial gift behaviour in monandrous D. subobscura. Heat stress reduced male gift-giving and caused male sterility, suggesting courtship changes reflect fertility. Females were also less likely to accept a gift from, and mate with, a heat-stressed male. Such pre-copulatory discrimination may buffer heat-induced sterility costs on population viability. Heat-stressed males also gave fewer gifts to heat-stressed females, indicating temperature-mediated mutual mate choice. Building on these findings, Chapter III explored whether heat-induced male sterility could drive selection on male fertility resilience and a shift from monandry to polyandry. Male heat stress increased both sterility and female remating propensity, consistent with facultative polyandry as a fertility assurance strategy. Genetic variation in both traits indicates evolutionary potential and could facilitate mating system evolution under warming conditions. Chapter IV examined sex-specific temperature effects on reproduction in Pieris napi. Heat stress caused near-complete male sterility – with no evidence of recovery – and reduced female mating success, highlighting the need to incorporate the consequences of heat stress on reproduction across taxa into climate change prediction models. Using diverse species and novel approaches, this thesis offers new insights into how environmental variation influences reproductive outcomes, with key implications for sexual selection, sexual conflict, and population viability.