Pathways to perception : sexual differentiation and pheromone communication in parasitoid wasps

Insects have evolved genetic pathways that determine whether they develop as a male or female. These pathways also shape physical and behavioural differences in their pheromone communication systems to facilitate how they share and process information. Research into how these pathways regulate these differences has primarily been limited to the fruit fly Drosophila melanogaster, which utilizes a so-called dedicated labelled-line system to perceive and process pheromones. Hymenoptera – ants, bees and wasps – have developed a combinatorial coding system, where a single chemical can trigger various sensory neurons to enable these insects to perceive complex pheromones. However, it is not yet fully understood how the sex-determination pathway actually regulates this pheromone-processing mechanism.The aim of my thesis was to elucidate the genetic mechanisms by which sex determination regulates pheromone communication in parasitoid wasps by investigating how key sex-determination genes influence pheromone production and perception in the parasitoid model species Nasonia vitripennis.In Chapter 2, I provide a detailed review of current scientific literature and focus on the distinct olfactory systems insects have evolved to detect and process pheromones. A comparison is made between Diptera, Lepidoptera and Hymenoptera. Conclusions are drawn on how their unique antennal lobes and glomeruli develop to process odours as well as on the causal effects of genes of the sex-determination pathway.Chapters 3 and 4 focus on differences between males and females in the olfactory system of N. vitripennis and how two sex-determination genes, Transformer and Doublesex, influence these differences. I analysed how males and females develop different sensilla on their antennae and how their glomeruli are organized in the antennal lobe. I conclude that these male-female differences arise from modifications in sensory neurons in the antennae. Antennal sensory neurons express the olfactory receptor co-receptor, Orco, a conserved chemosensory gene known to play an important role in forming the glomeruli in other Hymenoptera. My experiments confirm that Orco is required for adult glomerular formation in N. vitripennis.In Chapters 5 and 6, I look at how the sex-determination pathway regulates differences between male and female pheromones. My experiments show that, in females, Transformer is crucial for producing the unique mix of specific pheromones called cuticular hydrocarbons, CHCs. These sex-specific differences in CHCs are driven by the regulation of genes responsible for pheromone production. I analysed a group of these genes that are regulated by Doublesex, called fatty acyl-CoA reductases. My experiments show that these genes play a crucial role in the chemical pathway that creates CHCs and are essential for producing sex-specific pheromone profiles.In Chapter 7, I interpret the key findings of my thesis and contextualize them within the existing body of knowledge on insect sexual differentiation. I explain how the sex-determination pathway shapes complex pheromone systems and highlight the importance that sex-determining genes have in producing the extensive range of pheromones seen in parasitoid wasps. I elaborate on the implications and future directions of my research within the areas of evolutionary biology, neuroethology and biological control.