Infections by Transovarially Transmitted DMelSV in Drosophila Have No Impact on Ovarian Transposable Element Transcripts but Increase Their Amounts in the Soma

Transposable elements (TEs) are genomic parasites, which activity is tightly controlled in germline cells. Using Sindbis virus, it was recently demonstrated that viral infections affect TE transcript amounts in somatic tissues. However, the strongest evolutionary impacts are expected in gonads, because that is where the genomes of the next generations lie. Here, we investigated this aspect using the Drosophila melanogaster Sigma virus. It is particularly relevant in the genome/TE interaction given its tropism to ovaries, which is the organ displaying the more sophisticated TE control pathways. Our results in Drosophila simulans flies allowed us to confirm the existence of a strong homeostasis of the TE transcriptome in ovaries upon infection, which, however, rely on TE-derived small RNA modulations. In addition, we performed a meta-analysis of RNA-seq data and propose that the immune pathway that is triggered upon viral infection determines the direction of TE transcript modulation in somatic tissues.

Identification of Regulatory Host Genes Involved in Sigma Virus Replication Using RNAi Knockdown in Drosophila

The Drosophila melanogaster sigma virus, a member of the Rhabdoviridae family, specifically propagates itself in D. melanogaster. It contains six genes in the order of 3ʹ-N–P–X–M–G–L-5ʹ. The sigma virus is the only arthropod-specific virus of the Rhabdoviridae family. Sigma-virus-infected Drosophila may suffer from irreversible paralysis when exposed to a high CO2 concentration, but generally, no other symptoms are reported. A recent study reported that host gene expression in immune pathways was not changed in sigma-virus-infected Drosophila, which does not necessarily suggest that they are not involved in virus–host interactions. The present study aimed to identify host genes associated with sigma virus replication. Immune pathways JAK-STAT and IMD were selected for detailed study. The results showed that the genome copy number of the sigma virus increased after knocking down the immune pathway genes domeless and PGRP-LC in Drosophila S2 cells. The knocking down of domeless and PGRP-LC significantly up-regulated the expression of the L gene compared to the other viral genes. We propose that the immune pathways respond to sigma virus infection by altering L expression, hence suppressing viral replication. This effect was further tested in vivo, when D. melanogaster individuals injected with dsdome and dsPGRP-LC showed not only an increase in sigma virus copy number, but also a reduced survival rate when treated with CO2. Our study proved that host immunity influences viral replication, even in persistent infection. Knocking down the key components of the immune process deactivates immune controls, thus facilitating viral expression and replication. We propose that the immunity system of D. melanogaster regulates the replication of the sigma virus by affecting the L gene expression. Studies have shown minimal host–virus interaction in persistent infection. However, our study demonstrated that the immunity continued to affect viral replication even in persistent infection because knocking down the key components of the immune process disabled the relevant immune controls and facilitated viral expression and replication.

Sigma Virus (DMelSV) Incidence in Lines of Drosophila melanogaster Selected for Survival following Infection with Bacillus cereus

The immune response of Drosophila melanogaster is complex and involves both specific and general responses to parasites. In this study we tested for cross-immunity for bacteria and viruses by scoring the incidence of infection with the vertically transmitted Sigma virus (DMelSV) in the progeny of a cross between females transmitting DMelSV at high frequencies and males from lines subjected to three selection regimes related to resistance to Bacillus cereus. There was no significant difference in transmission of DMelSV among selection regimes, though results suggest that the B. cereus selected lines had lower rates of infection by DMelSV. We found a significant difference in viral infection with respect to the sex of the progeny, with males consistently less likely to be infected than females. Given a finite energy budget, flies that have experienced immune system challenge may show alterations in other life history traits. Later eclosing progeny were also less likely to be infected than earlier eclosing progeny, indicating a relationship with development time. Finally, there was a significant interaction between the timing of collection and the sex of the progeny, such that later eclosing males were the most resistant group. Increased development time is sometimes associated with increased energy acquisition; from this perspective, increased development time may be associated with acquiring sufficient resources for effective resistance.

Host-switching by a vertically transmitted rhabdovirus in Drosophila

A diverse range of endosymbionts are found within the cells of animals. As these endosymbionts are normally vertically transmitted, we might expect their evolutionary history to be dominated by host-fidelity and cospeciation with the host. However, studies of bacterial endosymbionts have shown that while this is true for some mutualists, parasites often move horizontally between host lineages over evolutionary timescales. For the first time, to our knowledge, we have investigated whether this is also the case for vertically transmitted viruses. Here, we describe four new sigma viruses, a group of vertically transmitted rhabdoviruses previously known in Drosophila . Using sequence data from these new viruses, and the previously described sigma viruses, we show that they have switched between hosts during their evolutionary history. Our results suggest that sigma virus infections may be short-lived in a given host lineage, so that their long-term persistence relies on rare horizontal transmission events between hosts.