Gene Expression Profiling of Odorant Binding Proteins in the Tsetse fly Glossina brevipalpis

Abstract

Glossina brevipalpis is a vector of trypanosomes that cause Animal African Trypanosomiasis. Chemoreception in tsetse flies is critical for identifying larvipositioning sites, hosts and mates. Odorant binding proteins (OBPs) are thought to mediate insect chemoreception by shuttling odours to receptors on olfactory sensory neurons. Little is known about the roles of OBPs in tsetse fly chemoreception. The genome of G. brevipalpis has been sequenced and 28 OBPs identified. This study aimed at profiling the expression levels of G. brevipalpis OBP genes at different starvation periods and developmental stages. Glossina morsitans morsitans OBP genes retrieved from VectorBase were queried against G. brevipalpis proteome. Putative G. brevipalpis OBP genes were then searched against Drosophila melanogaster proteome in FlyBase. Multiple sequence alignment of G. brevipalpis OBPs identified six conserved cysteines for most of the OBPs. Phylogenetic analysis of G. brevipalpis, G. m. morsitans and D. melanogaster OBPs showed that Glossina OBPs clustered closely. Wild G. brevipalpis collected from Shimba Hills National Park in Kwale County were fed on rabbits and starved for 2, 24, 48, 72 and 96 hours. Larvae, pupae and tenerals (newly emerged unfed flies) were also obtained. From the adults and tenerals, a pool of ten pairs of antennae plus one head were used for RNA extraction. Subsequently, one larva and one pupa were also used for RNA extraction. The RNA extracted was used for cDNA synthesis. Conventional PCR was used to screen the 28 putative G. brevipalpis OBP genes and the amplified genes were quantified by qRT- PCR. Glossina brevipalpis OBP8 was highly expressed in the differentially starved adults while two (GbrOBP2 and GbrOBP7) were found to be significantly expressed in the larval and pupal stages. Expression of OBPs in the adult flies confirmed their involvement in olfaction while expression in larva and pupa may suggest their involvement in non-olfactory processes. This study is the first to show the expression levels of OBPs at the starvation and developmental stages of G. brevipalpis. The findings suggest their roles in tsetse chemoreception and give insights on development of specific and environmentally friendly control strategies.