A Horizontally Transferred Autonomous Helitron Became a Full Polydnavirus Segment in Cotesia vestalis.

Bracoviruses associate symbiotically with thousands of parasitoid wasp species in the family Braconidae, working as virulence gene vectors, and allowing the development of wasp larvae within hosts. These viruses are composed of multiple DNA circles that are packaged into infective particles, and injected together with wasp's eggs during parasitization. One of the viral segments of Cotesia vestalis bracovirus contains a gene that has been previously described as a helicase of unknown origin. Here, we demonstrate that this gene is a Rep/Helicase from an intact Helitron transposable element that covers the viral segment almost entirely. We also provide evidence that this element underwent at least two horizontal transfers, which appear to have occurred consecutively: first from a Drosophila host ancestor to the genome of the parasitoid wasp C. vestalis and its bracovirus, and then from C. vestalis to a lepidopteran host (Bombyx mori). Our results reinforce the idea of parasitoid wasps as frequent agents of horizontal transfers in eukaryotes. Additionally, this Helitron-bracovirus segment is the first example of a transposable element that effectively became a whole viral circle.

[Co-evolutionary strategies of interaction between parasitoids and polydnaviruses]. / Estrategias co-evolutivas de la interacción entre parasitoides y polidnavirus.

Polydnaviruses are genetic symbionts of wasp endoparasitoids belonging to the hymenopteran families Ichneumonidae (ichnoviruses) and Braconidae (bracoviruses). They exist as proviruses integrated in the wasp's chromosomal genome, which then excise and undergo replication during the stage of adult development of the wasp. During wasp oviposition into their caterpillar host, the fully formed virus particles are injected along the parasitoid's eggs into the host hemocoel, where the eggs hatch and undergo larval development. The primary function of the polydnavirus is to trigger host immunosuppression so that host hemocytes are prevented from encapsulating the parasitoid's eggs and/or larvae. Polydnavirus transcripts are expressed following parasitization and alter host hemocyte adhesive properties that prevents encapsulation; in some species, viral gene expression triggers host hemocyte apoptosis, thereby rendering the host immunosuppressed. This review summarizes the major features of polydnaviruses and provides a global view of their functions in the lepidopteran hosts of the parasitoid wasps that carry them both as integrated viral sequences in their genome and as free virus to function physiologically in host regulation following parasitization of the host.