THE EVOLUTION OF CHEMORECEPTION IN THE INVASIVE PEST DROSOPHILA SUZUKII AND OTHER ARTHROPODS

Chemoreception is the process that allows animals to respond to the chemical stimuli in their environment. In insects, this is mediated by specialized neurons expressing a variety of dedicated receptors and carrier proteins: olfactory (OR), gustatory (GR) and Ionotropic (IR) receptors, odorant binding (OBP) and chemosensory (CSP) proteins. How the evolution of these genes correlates with adaption to new ecological niches is still a debated topic; when these genes arose during arthropods evolution is also an unresolved question. To tackle the first of these issues I have studied these gene families in Drosophila suzukii Matsumura (Diptera: Drosophilidae), an invasive pest that, unlike other Drosophila, oviposits in fresh fruits. I have initially contributed in curating D. suzukii’s genome and transcriptome, and then annotated its entire repertoire of chemosensory genes. Analysis of these genes on a 14 Drosophila phylogenetic framework revealed that ORs, OBPs, and GRs are characterized by high turnover rates and uneven distribution of duplication and gene loss events on the phylogeny: these peculiar evolutionary patterns are consistent with a change in selective pressures. D. suzukii is characterised by loss of function of key ORs that bind volatiles typically released by fermenting substrates, providing with a rare example of ecological adaptation due to genes loss. I further present my work in annotating and studying genes involved in the metabolism and perception of an aggregating pheromone that may play an active role in D. suzukii’s peculiar biology. To inquire the origin of chemosensory genes in insects and other arthropods, I have screened various recently released genomes: I identified multiple lineage-specific expansions of GRs in each of the arthropod clades, an interesting loss of GRs in most crustaceans, and the first genomic evidence of ORs in the Palaeoptera. My postgraduate work has contributed in identifying receptors and ligands that will be used in downstream D. suzukii control applications, and has shed some light in the evolution of chemosensory genes in arthropods.