The evolution of immune genes in tsetse flies (Glossina) and insights into tsetse-symbiont-trypanosome interactions

Tsetse flies (genera Glossina) are the sole biological vectors of African Trypanosoma species, the infectious agents of African Trypanosomiasis. Vector control is a key inhibitor of disease transmission; however, long-term control measures are economically and ecologically unsustainable and therefore, alternatives must be explored. In this thesis we aim to explore the evolution of three important immune genes: attacin-A (AttA), Defensin (Def) and Toll-like receptor 2 (TLR2), in relation to symbionts and parasitic interactions. This could in turn lay the foundations for genetic control methods

The successful identification of novel attacin orthologues confirmed the previous descriptions of attacin clusters within the Glossina genome, while a single novel defensin orthologue was identified in each of the six Glossina genomes. A total of six TLRs were confirmed within the Glossina genome, and three additional TLRs were potentially identified, though these are unconfirmed. The evolutionary history of the attacin cluster remains undetermined, however concerted evolution likely impacts the evolution of AttA, while Def and TLRs are governed by strict Darwinian selection.

A wild population sample of Glossina morsitans morsitans illustrated differing levels of nucleotide variation in each gene, Def being the least polymorphic (n = 8) and TLR2 being the most (n = 22). All genes indicated a recent population expansion event and deviations from neutrality, indicative of population expansion and balancing selection. Genetic variation in both AttA and TLR2 was found to be maintained via purifying selection, while Def exhibited signs of the Red Queen arms race and balancing section. Trypanosome infection rates were unexpectedly high (69.35%), consisting of mixed species infections. Advantageous Def variants were observed to reduce infection rates within samples, while an observable relationship between TLR2 and symbiont variation, and infection rate requires further research.

The results within described the impacts of evolution and population change on immune genes and how the interactions with symbiont populations can influence trypanosome infection rates. This thesis indicates that an understanding of the evolution and interactions of the tsetse-symbiont-trypanosome triplet could be used to inform novel genetic control methods.