Permissiveness of lepidopteran hosts is linked to differential expression of bracovirus genes
Text by Michael Strand
Polydnaviruses in the genus Bracovirus are large double-stranded DNA viruses that have evolved a mutualistic (beneficial) association with a group of insects called parasitoid wasps. These wasps fully rely on bracoviruses to parasitize hosts while bracoviruses fully rely on wasps for vertical transmission. Bracoviruses evolved ~100 million years ago from an ancestor related nudiviruses and baculoviruses, which are viruses that also infect insects. Cospeciation thereafter gave rise to thousands of bracovirus-carrying wasp species that each parasitize a narrow range of hosts. This study focused on one of these wasp species, Microplitis demolitor, and assessed features of M. demolitor bracovirus (MdBV) that contribute to its narrow host range. The results we report indicate that viral gene expression is a key factor in why M. demolitor successfully parasitizes one host species but fails to successfully parasitize another, closely related insect.
Viruses commonly evolve restricted host ranges but underlying molecular mechanisms are poorly understood. The impetus for this study was simply to gain insights into the restricted host range of MdBV and its associated wasp. At the same time, we also were interested in taking advantage of certain features of bracovirus biology to gain insights into the biology of nudiviruses and baculoviruses, which usually also have narrow host ranges. Most of the host range literature emphasizes the role of virus entry mechanisms and host immune genes as host range determinants. Contrary to this literature, results showed that MdBV infected and its genome similarly persisted in the host species we studied. In contrast, the large differences we observed in viral gene expression suggest adaptations related to how transcription is regulated are more important host range determinants than is generally recognized in the literature. The biggest challenges we faced in conducting the study were experimental design. Since so many factors and traits can influence virus function, sequentially examining key features of potential importance and presenting the logic of what was done and why in writing the study took considerable time. The unusual biology of polydnaviruses also presents challenges in writing because they are not generally familiar to virologists.
Pictured is the wasp in this study, injecting virus into the permissive host Chrysodeixus includens. Image of wasp was taken by Jena Johnson.
Introducing the authors
Pictured from left to right: Kavita Bitra, Gaelen Burke and Michael Strand.
About the research
Kavita Bitra, Gaelen R. Burke, Michael R. Strand
Virology, Volume 492, May 2016, Pages 259–272