Monitoring Insect Transposable Elements in Large Double-Stranded DNA Viruses Reveals Host-to-Virus and Virus-to-Virus Transposition.

The mechanisms by which transposable elements (TEs) can be horizontally transferred between animals are unknown, but viruses are possible candidate vectors. Here, we surveyed the presence of host-derived TEs in viral genomes in 35 deep sequencing data sets produced from 11 host-virus systems, encompassing nine arthropod host species (five lepidopterans, two dipterans, and two crustaceans) and six different double-stranded (ds) DNA viruses (four baculoviruses and two iridoviruses). We found evidence of viral-borne TEs in 14 data sets, with frequencies of viral genomes carrying a TE ranging from 0.01% to 26.33% for baculoviruses and from 0.45% to 7.36% for iridoviruses. The analysis of viral populations separated by a single replication cycle revealed that viral-borne TEs originating from an initial host species can be retrieved after viral replication in another host species, sometimes at higher frequencies. Furthermore, we detected a strong increase in the number of integrations in a viral population for a TE absent from the hosts' genomes, indicating that this TE has undergone intense transposition within the viral population. Finally, we provide evidence that many TEs found integrated in viral genomes (15/41) have been horizontally transferred in insects. Altogether, our results indicate that multiple large dsDNA viruses have the capacity to shuttle TEs in insects and they underline the potential of viruses to act as vectors of horizontal transfer of TEs. Furthermore, the finding that TEs can transpose between viral genomes of a viral species sets viruses as possible new niches in which TEs can persist and evolve.

Assessing the Impact of a Viral Infection on the Expression of Transposable Elements in the Cabbage Looper Moth (Trichoplusia ni).

Most studies of stress-induced transposable element (TE) expression have so far focused on abiotic sources of stress. Here, we analyzed the impact of an infection by the AcMNPV baculovirus on TE expression in a cell line (Tnms42) and midgut tissues of the cabbage looper moth (Trichoplusia ni). We find that a large fraction of TE families (576/636 in Tnms42 cells and 503/612 in midgut) is lowly expressed or not expressed at all [≤ 4 transcripts per million (TPM)] in the uninfected condition (median TPM of 0.37 in Tnms42 and 0.46 in midgut cells). In the infected condition, a total of 62 and 187 TE families were differentially expressed (DE) in midgut and Tnms42 cells, respectively, with more up- (46) than downregulated (16) TE families in the former and as many up- (91) as downregulated (96) TE families in the latter. Expression log2 fold changes of DE TE families varied from -4.95 to 9.11 in Tnms42 cells and from -4.28 to 7.66 in midgut. Large variations in expression profiles of DE TEs were observed depending on the type of cells and on time after infection. Overall, the impact of AcMNPV on TE expression in T. ni is moderate but potentially sufficient to affect TE activity and genome architecture. Interestingly, one host-derived TE integrated into AcMNPV genomes is highly expressed in infected Tnms42 cells. This result shows that virus-borne TEs can be expressed, further suggesting that they may be able to transpose and that viruses may act as vectors of horizontal transfer of TEs in insects.