Cotesia plutellae Bracovirus Genome and Its Function in Altering Insect Physiology

Polydnavirus is a group of animal DNA virus mutually associated with some ichneumonoid wasp. Its relatively large size of genome has been considered as a major source of the parasitoid function to manipulate developmental and immunological processes of target parasitized insects. Cotesia plutellae bracovirus (CpBV) is a polydnavirus derived from C. plutellae, which parasitizes the diamondback moth, Plutella xylostella. Parasitized P. xylostella exhibits altered physiological symptoms in development and immune reactions. Though several other parasitic factors such as ovarian proteins, venom, and teratocytes are identified, CpBV has been more focused on elucidating various host physiological alterations occurring due to the parasitism, which has driven the CpBV genome project. CpBV attains a typical bracovirus structure by its single unit membrane envelope, in which multiple nucleocapsids are enclosed. Its genome DNAs are segmented and located on the genome of C. plutellae. Its replication begins at adult tissue development during pupal stage. An apparent genome size is 471 kb estimated from 27 segments separated on 5% agarose gel. A current work on the genome has been completely sequenced 24 genomic segments and analyzed their genomic structure. The aggregated genome size is 351, 299 bp long and exhibits an average GC content of approximately 34.6%. Average coding density is about 32.3% and 125 putative open reading frames are predicted. Though more than half (52.5%) of predicted genes are annotated as hypothetical, the annotated CpBV genes share amino acid sequence homologies with those of other bracoviral genomes. The annotated genes are classified into the known bracoviral families, in which a family of protein tyrosine phosphatase is the largest including 36 ORFs, suggesting a significant role during parasitization. In addition, 8 and 7 ORFs encode Iκβ-like and EP1-like, respectively. Some predicted genes are known only in Cotesia-associated bracoviral genomes. Finally, two homologous genes, CpBV15α/β, are unique in CpBV genome, which are not matched to any other known polydnaviral genes. Their homology with malarian circumsporozoite toxin and eukaryotic translation inhibition factors suggests their function in host translation inhibitory factor. This review discusses CpBV genes on their putative physiological functions based on the molecular interactions between the host-parasite.