The Sigma Viruses of Drosophila

The sigma virus of Drosophila melanogaster (DMelSV) was discovered accidentally over 70 years ago owing to it causing infected flies to become paralysed and die on exposure to CO2. Recently, five other species of Drosophila and a species of Muscidae fly have also been found to be infected with sigma viruses, and together these viruses form a major clade of rhabdoviruses that is a sister clade to the dimarhabdoviruses. In those cases where the transmission of these viruses has been investigated, sigma viruses are transmitted purely vertically through infected eggs or sperm. In natural populations of insects the spread of these viruses can be very rapid, and has led to flies evolving resistance to sigma viruses. Two resistance genes have been identified in D. melanogaster, one of which is involved in autophagy. Discovery of the sigma virus of Drosophila melanogaster (DMelSV) The sigma virus of Drosophila melanogaster (DMelSV) was discovered by chance in 1937, when French researchers found that some flies became paralysed and died when exposed to CO2 during routine anesthetization, rather than recovering as normal (L’Heritier and Teissier, 1937). They found that the trait was transmitted vertically from parent to offspring, but was independent of the host chromosomes. It could also be transferred by injecting haemolymph from flies that were sensitive to CO2 into non-sensitive flies, suggesting that the trait was caused by an infectious agent. The trait was later attributed to the presence of a virus-sized particle, which they named sigma (L’Heritier, 1948). The bullet-shaped morphology, antigenic profile and partial genome sequences subsequently identified DMelSV as a rhabdovirus (Berkalof et al., 1965; Teninges, 1968; Calisher et al., 1989; Teninges et al., 1993). The sigma virus clade D. melanogaster is not the only species to be infected with a sigma virus. By screening species for CO2 sensitivity, six additional species of flies were found to be infected with sigma viruses (Longdon et al., 2010, 2011c). These viruses infect five species of Drosophila – D. affinis, D. obscura, D. tristis, D. immigrans and D. ananassae – and one member of the Muscidae, Muscina stabulans. These viruses have been tentatively named as DAffSV, DObsSV, DTriSV, DImmSV, DAnaSV and MStaSV, respectively (using the standard Drosophila species name abbreviation and SV for sigma virus). As these new viruses have been only recently discovered we know very little about their biology, so the majority of this chapter describes the sigma virus of D. melanogaster (DMelSV). Together the sigma viruses form a clade of viruses that infect Diptera (flies), that are closely related to the dimarhabdoviruses (Bourhy et al., 2005); a clade containing the Ephemerovirus and Vesiculovirus clades of rhabdoviruses (Fig. 8.1 and Fig. 3.2) (Longdon et al., 2010, 2011c). The sigma virus clade contains a greater amount of genetic divergence between its members than seen in four out of the six accepted rhabdovirus genera, suggesting that they are a diverse and major group of