We have found sigma viruses that infect several Drosophila species, Mediterranean fruit flies and a butterfly are all vertically transmitted (see here and here). Unlike bacterial symbionts, sigma viruses are transmitted vertically through both sperm and eggs, so are able to spread through populations despite being costly to infected flies.
Four out of the five sigma viruses tested to date have recently spread through their host populations (e.g. Figure 2). This could be due to selective sweeps of an advantageous mutation through an exiting viral population or the spread of new viruses from a different species or population through an uninfected population. It seems that vertically transmitted rhabdoviruses may be common in insects, and they can have very dynamic interactions with their hosts.
One of the best-studied naturally occurring parasites of Drosophila is the sigma virus of Drosophila melanogaster (DMelSV). DMelSV is a negative sense single stranded RNA virus in the family Rhabdoviridae. DMelSV is particularly cool as it is transmitted purely vertically through both eggs and sperm, and also causes infected flies to become paralysed and die on exposure to carbon dioxide (so it is easy to screen for infected flies).
We have found that sigma viruses are common pathogens of Drosophila and other diptera, and represent a new genus of rhabdoviruses (Figures 1 and 3). Based on our findings – and reports of carbon dioxide sensitivity in other species – we suggested these viruses are widespread in dipterans, if not insects as a whole (see here and here). Our recent work has confirmed this: we have found sigma-like viruses in a range of different insects (Lepidoptera, Hemiptera and a diverse array of Diptera).