A horizontally transferred autonomous Helitron became a full polydnavirus segment inCotesia vestalis

Abstract

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 by 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 Cotesia 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.

Systematic annotation ofHelitron-like elements in eukaryote genomes using HELIANO

“…For example, Helitrons have been estimated to span around 6% of the little brown bat genome and about 4% of the silkworm genome (20,21). Numerous reports revealed that Helitrons can capture genes and lead to horizontal transfer and genome shuffling, making them significant sources for genome dynamics and evolution (18,(22)(23)(24). While their evolutionary significance is undebated, HLEs are still tricky to identify efficiently because they do not create target site duplication (TSD) upon transposition and lack classical structural features (1,8).…”