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

Draft nuclear genome and complete mitogenome of the Mediterranean corn borer,Sesamia nonagrioides, a major pest of maize

“…Given the high nucleotide identity between the wasp and moth sequences (95.4–99.4%) and the deep divergence time between hymenopterans and lepidopterans (>300 million years; Misof et al 2014 ), we infer that this helitron-like sequence has been recently transferred between S. nonagrioides and C. sesamiae . This event adds up to the list of helitrons reported to have undergone HT between parasitoid wasps and lepidopterans ( Thomas et al 2010 ; Guo et al 2014 ; Coates 2015 ; Heringer et al 2017 ; Han et al 2019 ). Whether these transfers were facilitated by the integration of wasp DNA circles in germline genomes of lepidopterans larvae during parasitism is an interesting possibility that deserves further investigation.…”

Drosophila parasitoid wasps bears a distinct DNA transposon profile

“…Another interesting point is that even knowing that Drosophila genomes have one of the best-characterized mobilomes [ 62 ], we could not find any traces of potential horizontal transfer events between wasps and flies, which supports our previous findings that Drosophila parasitoid wasps, and Drosophila hosts do not exchange TEs through HT, and that VLPs injected by these wasps are not an important TE vector [ 29 ]. This is in contrast with several horizontal transfer events mediated by Lepidoptera parasitoid wasps and their VLPs [ 25 – 27 , 63 ]. These contradictory findings suggest that each vector-parasite relationship has different characteristics that either allow or act as a barrier for HT between species and highlights the importance of taking into account non-model organisms when extrapolating results on the evolution of TEs.…”

Giant Transposons in Eukaryotes: Is Bigger Better?

“…1 B ). Furthermore, Helitrons were found in polydnaviruses, sometimes comprising entire viral segments ( Thomas and Pritham 2015 ; Heringer et al. 2017 ).…”

“…Equally dependent on Rep function are Helitrons , which can also be transmitted by large DNA viruses ( Thomas et al. 2010 ; Coates 2015 ; Heringer et al. 2017 ).…”