Isolation of a natural DNA virus of Drosophila melanogaster, and characterisation of host resistance and immune responses

Abstract

Drosophila melanogaster has played a key role in our understanding of invertebrate immunity. However, both functional and evolutionary studies of host-virus interaction in Drosophila have been limited by a dearth of native virus isolates. In particular, despite a long history of virus research, DNA viruses of D. melanogaster have only recently been described, and none have been available for experimental study. Here we report the isolation and comprehensive characterisation of Kallithea virus, a large double-stranded DNA virus, and the first DNA virus to have been reported from wild populations of D. melanogaster. We find that Kallithea virus infection is costly for adult flies, reaching high titres in both sexes and disproportionately reducing survival in males and movement and late fecundity in females. Using the Drosophila Genetic Reference Panel, we quantify host genetic variance for virus-induced mortality and viral titre and identify candidate host genes that may underlie this variation, including Cdc42-interacting protein 4. Using full transcriptome sequencing of infected males and females, we examine the transcriptional response of flies to Kallithea virus infection, and describe differential regulation of virus-responsive genes. This work establishes Kallithea virus as a new tractable model to study the natural interaction between D. melanogaster and DNA viruses, and we hope it will serve as a basis for future studies of immune responses to DNA viruses in insects.\n\nAuthor SummaryThe fruit fly Drosophila melanogaster is a useful model species to study host-virus interaction and innate immunity. However, few natural viruses of Drosophila have been available for experiments, and no natural DNA viruses of Drosophila melanogaster have been available at all. Although infecting flies with viruses from other insects has been useful to uncover general immune mechanisms, viruses that naturally infect wild flies could help us to learn more about the coevolutionary process, and more about the genes that underlie the host-virus interaction. Here we present an isolate of a DNA virus (named Kallithea Virus) that naturally infects the model species Drosophila melanogaster in the wild. We describe the basic biology of infection by this virus, finding that both male and females flies die from infection, but females are more tolerant of infection than males, while laying lay fewer eggs than uninfected females. We quantify genetic variation for virus resistance in the flies, and we use RNA sequencing to see which genes are expressed in male and female flies in response to infection. These results will form the basis for further research to understand how insects defend themselves against infection by DNA viruses, and how DNA viruses can overcome antiviral defence.