Role of labellar and ovipositor taste sensilla of Drosophila suzukii in host recognition: a morpho-functional and behavioural approach.

Drosophila suzukii is an invasive and destructive crop pest that originated in Southeast Asia. Unlike most other Drosophila species which attack only decaying or rotten fruits, this species has the ability to infest healthy fruits because of its serrated ovipositor used to pierce the skin of fruits and lay eggs in them. This saw-like ovipositor represents one key adaptation, but other traits, such as fruit recognition mediated by the olfactory and/or gustatory systems, are also implicated. Chemoreception is vital for all animals: by means of chemosensory organs, animals can locate mates, suitable oviposition sites and even discriminate palatable and beneficial foods from toxic and harmful substances in their environment. Aim of this thesis was to give a contribution to the general understanding of chemoreceptive mechanisms. In particular, two main aspects of chemoreception in insects were investigated, divided in two separate sections. The first section is a morpho-functional study of the sensilla housed in the ovipositor of D. suzukii, the role of which is still unknown. The results show that the ovipositor of D. suzukii presents 10 single pore sensilla that respond to gustatory stimuli such as carbohydrates (sucrose, fructose and glucose), bitter compounds (nicotine and caffeine) and organic acids (ascorbic acid). Behavioural no-choice and multi-choice trials showed that the flies lay a higher number of eggs on substrates containing sugars than those with bitter or acid compounds. Our results suggest for the first time, in a Drosophila species, a chemosensory role for the ovipositor sensilla and their possible involvement in the choice of the oviposition sites. Aim of the second study was to evaluate peripheral sensitivity and palatability to different carbohydrates and assess their nutritional value, in adult insects of D. suzukii, by means of an electrophysiological and behavioural approach. The spike activity was recorded from the labellar ltype sensilla stimulated with metabolizable mono- and disaccharides (maltose, sucrose, trehalose, glucose, fructose) and non-metabolizable sugars (arabinose, sucralose); the response to maltose and sucrose was stronger than to trehalose and to all monosaccharides, and that to sucralose was lower than to all other sugars. The palatability of the same sugars was evaluated by recording the proboscis extension reflex (PER). The palatability to sugars tested was: maltose = sucrose >trehalose = fructose = arabinose = glucose > sucralose. The nutritional value of the carbohydrates was assigned by means of survival trials. Flies fed on a diet containing maltose or trehalose lived longer lifespan than flies on sucrose: this suggests a higher nutritional value for the first two disaccharides. Flies fed on a diet containing sucralose or arabinose had a shorter lifespan than fructose and glucose and this suggests a lower nutritional value for the former two monosaccharides. Sugars that evoke a stronger response are also those that promote a higher activity of PER, thus suggesting that the more stimulant is a sugar the more appetitive it is for flies. Flies fed on diets containing metabolizable sugars (maltose, sucrose, trehalose, glucose and fructose) live longer than those fed on diets with non-metabolizable sugars (arabinose and sucralose). The survival capability is longer for flies fed with disaccharides, in particular maltose, than fed with monosaccharides.