from Ben Longdon, Lena Wilfert and Francis M Jiggins writing in Rhabdoviruses: Molecular Taxonomy, Evolution, Genomics, Ecology, Host-Vector Interactions, Cytopathology and Control:
The sigma virus of Drosophila melanogaster (DMelSV) was discovered accidentally over seventy years ago due 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.
Further reading: Rhabdoviruses: Molecular Taxonomy, Evolution, Genomics, Ecology, Host-Vector Interactions, Cytopathology and Control