Coding of tsetse repellents by olfactory sensory neurons: towards the improvement and the development of novel tsetse repellents

Abstract

Tsetse flies are the biological vectorsof human and animal trypanosomiasisand hence representant medical and veterinary importance. The sense of smell plays a significant role in tsetse and its ecological interaction, such as finding blood meal source, resting, and larvicidal sitesand for mating. Tsetse olfactory behaviour can be exploited for their management;however, olfactory studies in tsetse flies are still fragmentary. Here in my PhD thesis, usingscanning electron microscopy, electrophysiology, behaviour, bioinformatics and molecular biology techniques,I have investigated tsetse flies(Glossina fuscipes fuscipes)olfaction using behaviourallywell studied odorants, tsetse repellent by comparing with attractant odour. Insect olfaction is mediated by olfactory sensory neurons (OSNs), locatedin olfactory sensilla,which are cuticular structures exposed to the environment through pore and create a platform for chemical communication. In the sensilla shaft the dendrite of OSNs are housed, which are protected bycalled the sensillum lymph produced by support cells andcontains a variety of olfactory proteins, including the odorant binding protein (OBP) and chemosensory proteins (CSP). While on the dendrite of OSNs are expressedolfactory receptors. In my PhD, studies I tried to decipher the sense of smell in tsetse fly. In the secondchapter,I demonstrated that G. f. fuscipesis equippedwith diverseolfactory sensilla, that various from basiconic, trichoid and coeloconic. I also demonstrated, there is shape, length,number difference between sensilla types and sexual dimorphism. There is a major difference between male and female, while male has the unique basiconic sensilla, club shaped found in the pits, which is absentfrom female pits. In my thirdchapter,I investigated the odorant receptors which are expressed on the dendrite of the olfactory sensory neurons (OSNs). G. f. fuscipeshas 42ORs, which were not functionally characterised. I used behaviourally well studied odorants, tsetse repellents, composed of four components blend. I demonstrated that tsetse repellent isalso a strong antifeedant for both G.pallidipesand G. f. fuscipesusing feeding bioassays as compared to the attractant odour, adding the value of tsetse repellent. However, the attractant odourenhanced the feedingindex. Using DREAM (deorphanization of receptors based on expression alterations of mRNA levels). I found that in G. f. fuscipes, following a short in vivo exposure to the individual tsetse repellent component as well as an attractant volatile chemical, OSNs that respond to thesecompoundsaltered their mRNA expression in two opposite direction, significant downregulation and upregulation in their number of transcripts corresponding to the OR that they expressed and interacted with odorant. Also, I found that the odorants with opposite valence already segregate distinctly at the cellular and