Rhabdoviruses in Two Species of Drosophila: Vertical Transmission and a Recent Sweep

Abstract

Insects are host to a diverse range of vertically transmitted micro-organisms, but while their bacterial symbionts are well-studied, little is known about their vertically transmitted viruses. We have found that two sigma viruses (Rhabdoviridae) recently discovered in Drosophila affinis and Drosophila obscura are both vertically transmitted. As is the case for the sigma virus of Drosophila melanogaster, we find that both males and females can transmit these viruses to their offspring. Males transmit lower viral titers through sperm than females transmit through eggs, and a lower proportion of their offspring become infected. In natural populations of D. obscura in the United Kingdom, we found that 39% of flies were infected and that the viral population shows clear evidence of a recent expansion, with extremely low genetic diversity and a large excess of rare polymorphisms. Using sequence data we estimate that the virus has swept across the United Kingdom within the past ∼11 years, during which time the viral population size doubled approximately every 9 months. Using simulations based on our lab estimates of transmission rates, we show that the biparental mode of transmission allows the virus to invade and rapidly spread through populations at rates consistent with those measured in the field. Therefore, as predicted by our simulations, the virus has undergone an extremely rapid and recent increase in population size. In light of this and earlier studies of a related virus in D. melanogaster, we conclude that vertically transmitted rhabdoviruses may be common in insects and that these host–parasite interactions can be highly dynamic.

Identification of a Novel Rhabdovirus in Spodoptera frugiperda Cell Lines

“…Over 200 rhabdoviruses have been identified (31), and a majority of the plant rhabdoviruses and some vertebrate rhabdoviruses use an insect vector for transmission. However, only a few rhabdoviruses have been directly isolated from insects or insect cell lines; these belong to the order Diptera (such as sigma viruses in Drosophila and Muscina flies [26,32,33] and the Culex tritaeniorhynchus rhabdovirus [CTRV] in Culex mosquitoes [34]) and to the order Hemiptera (such as Taastrup virus in the leafhopper [25]). We report here the discovery of a novel rhabdovirus in the Spodoptera frugiperda Sf9 cell line, which is the first rhabdovirus found in the order Lepidoptera.…”

Vertically transmitted viral endosymbionts of insects: do sigma viruses walk alone?

“…through both eggs and sperm) [30,31]. Additionally, the trait of being paralysed by CO 2 that has been reported in Drosophila athabasca and C. quinquefasciatus is transmitted vertically in the biparental manner that is characteristic of sigma viruses [34,46].…”

“…The speed with which sigma viruses can spread is illustrated by recent work on DObsSV in populations of D. obscura [31]. By sequencing viral isolates collected from across the UK, it was found that the viral population shows clear evidence of a recent expansion, with low genetic diversity and an excess of rare polymorphisms.…”

“…Using this RNA sequence data, it was possible to reconstruct the viral population history, and estimate that the viral population size had doubled approximately every nine months, and within approximately the last 10 years it had swept across the UK to infect 39 per cent of flies. By measuring the transmission rate of the virus in the laboratory, it was found that such a spread could occur in this timescale even if the virus caused about a 10 per cent reduction in fecundity in infected hosts (figure 2) [31]. Although it is unclear, whether this virus was spreading through a previously uninfected host population or was replacing pre-existing strains, these results demonstrate that biparental transmission is a highly effective way for a virus to spread through host populations.…”

Merida virus, a putative novel rhabdovirus discovered in Culex and Ochlerotatus spp. mosquitoes in the Yucatan Peninsula of Mexico

“…A hallmark in the life cycle of insect-specific sigmaviruses is the exclusively vertical transmission through eggs and sperm (Longdon et al, 2011). Currently, no data are available to decide to what extent horizontal or vertical transmission contribute to the maintenance of MERDV in nature.…”