Genome Sequence Of Tsetse Polydnavirus: Insights Into Symbiotic Virus Evolution

Tsetse flies are the sole vectors of African trypanosomiasis in humans (sleeping sickness) and animals (nagana). The whole genome sequence of the tsetse fly Glossina morsitans morsitans revealed the presence of putative bracoviral sequences (n = 310) widely spread in the genome. These sequences are similar to those identified in parasitic braconid wasps. Bracoviruses (family Polydnaviridae) encode proteins that lower host immunity allowing for the development of parasitoid larvae in the host. The study aimed to determine the presence, prevalence and genetic diversity of Polydnaviruses (PDVs) across five tsetse fly species genomes (Glossina austeni, Glossina brevipalpis, Glossina fuscipes fuscipes, Glossina morsitans morsitans and Glossina pallidipes) found in East Africa. Four genes were unique to PDVs and served as good evolutionary models. These genes are components of mobile elements called Mavericks that are found in the PDVs genomes. They include DNA polymerase B2, Parvovirus coat protein, retroviral-like integrase and poxvirus A32 protein. The genes were observed to be present in multiple copies. G. austeni had the highest number (n = 18) and G. brevipalpis the least (n = 6). Phylogenetic reconstruction of each gene revealed two major clades which represent the two types of Mavericks. Selection pressure acting on these genes and their flanking regions was evaluated using dN/dS ratio. The study confirmed the presence of PDVs in tsetse fly genomes. The four PDV associated genes used as evolutionary modes were observed to be under varying magnitudes of purifying selection except for the poxvirus A32 which was under positive selection. These genes were showed to be inserted at conserved regions and co-evolve at similar rates with the host genomes.