A fundamental challenge faced by all organisms is the risk of infection by pathogens that can significantly reduce their fitness. The evolutionary dynamic between hosts and pathogens is expected to be a coevolutionary cycle, as pathogens evolve by increasing their level of virulence and hosts respond by increasing their level of resistance. The factors that influence the dynamics of adaptation by pathogen and host in response to one another are not well understood. Social insects live in dense colonies in high-pathogen soil environments, making them an ideal model system to study the factors influencing the evolution of pathogen resistance. In this thesis work, I investigated several alternative hypotheses to explain patterns of host resistance to entomopathogenic fungi in the harvester ant genus Pogonomyrmex: that high resistance is associated with high environmental pathogen loads, that local adaptation leads to increased resistance to coevolved pathogen populations, that life history tradeoffs increase allocation to resistance in harsher environments, and that increased genetic diversity caused by interspecific hybridization enhances inherent resistance. First, I characterized patterns of spatial variation in abundance and diversity of fungal pathogens among habitats of Pogonomyrmex species. I found 17 genera of fungi in the soil, six of which were entomopathogenic. Lower precipitation habitats, where P. rugosus occurs, had the lowest diversity, while the highest was experienced by the H lineage, one of two hybrid populations. When actual infection rates of field-caught workers were compared, the mesic-habitat P. barbatus was infected significantly more often. These results suggest that habitat does plays a role in fungal diversity, and that species are exposed to more entomopathogens may be more likely to get infected. Second, I tested experimentally whether hybridization and or habitat differences play a role in pathogen resistance by testing the effect of soil type and species identity on infection rates in pupae of the two species and their hybrids. This experiment showed P. rugosus ants had the highest inherent resistance to infection, supporting the life history tradeoff hypothesis. This suggest that Pogonomyrmex ants species are allocating their resources differently according to their environment, with more stressful environment leading to less investment in reproduction and more in protection against pathogens. Overall our study shows that environment plays a role in differences in infection risk, while genetic effects such as hybridization may not play a role in pathogen resistance.

The difference in evolutionary interests of males and females can select for traits that favour an individual??s fitness at the expense of their mate??s lifetime fitness. Despite the widespread occurrence of this sexual conflict over reproductive interactions, however, research to date has largely focused on the fitness costs imposed on females by manipulative males. Empirical evidence is particularly sparse for how mating can also be costly for males, the genetic structure of traits involved in reproductive interactions, and how sexual conflict can modify sexual selection in general. My aim was to explore the broader evolutionary consequence of sexual conflict and male-female interactions. In the nuptial-feeding Australian ground cricket, Pteronemobius sp., I used an experimental evolution approach to explore how diet and sexual conflict interact to determine the costs of mating. In the Australian black field cricket, Teleogryllus commodus, I used molecular and quantitative genetic approaches to characterise the fitness consequences and genetic basis of spermatophore attachment, a trait at the centre of inter-locus sexual conflict, and then related this to both condition and male attractiveness. Finally, in T. commodus, I quantified how sexual conflict alters the sexual selection acting on male sexual traits and how this in turn shapes genetic architecture and the persistence of additive genetic variance. My results demonstrate the complex nature of reproductive interactions between males and females. Importantly, I show that diet can mediate the expression of sexual conflict in a mating system and shape the evolution of male life-span. I also show that reproductive interactions influence the fitness benefits that both male and females obtain from mating in ways that are not predicted by current theory and that much of the potential for such traits to co-evolve is via a common genetic association with condition. Finally, I demonstrate that sexual conflict can profoundly modify the process and outcome of sexual selection, thereby influencing how additive genetic variation is maintained in a suite of male sexual traits. These results highlight the need for a greater integration of sexual conflict and sexual selection theory as the evolutionary potential and significance of sexual conflict may currently be underestimated.

The difference in evolutionary interests of males and females can select for traits that favour an individual??s fitness at the expense of their mate??s lifetime fitness. Despite the widespread occurrence of this sexual conflict over reproductive interactions, however, research to date has largely focused on the fitness costs imposed on females by manipulative males. Empirical evidence is particularly sparse for how mating can also be costly for males, the genetic structure of traits involved in reproductive interactions, and how sexual conflict can modify sexual selection in general. My aim was to explore the broader evolutionary consequence of sexual conflict and male-female interactions. In the nuptial-feeding Australian ground cricket, Pteronemobius sp., I used an experimental evolution approach to explore how diet and sexual conflict interact to determine the costs of mating. In the Australian black field cricket, Teleogryllus commodus, I used molecular and quantitative genetic approaches to characterise the fitness consequences and genetic basis of spermatophore attachment, a trait at the centre of inter-locus sexual conflict, and then related this to both condition and male attractiveness. Finally, in T. commodus, I quantified how sexual conflict alters the sexual selection acting on male sexual traits and how this in turn shapes genetic architecture and the persistence of additive genetic variance. My results demonstrate the complex nature of reproductive interactions between males and females. Importantly, I show that diet can mediate the expression of sexual conflict in a mating system and shape the evolution of male life-span. I also show that reproductive interactions influence the fitness benefits that both male and females obtain from mating in ways that are not predicted by current theory and that much of the potential for such traits to co-evolve is via a common genetic association with condition. Finally, I demonstrate that sexual conflict can profoundly modify the process and outcome of sexual selection, thereby influencing how additive genetic variation is maintained in a suite of male sexual traits. These results highlight the need for a greater integration of sexual conflict and sexual selection theory as the evolutionary potential and significance of sexual conflict may currently be underestimated.