The good viruses: viral mutualistic symbioses

Viruses have traditionally been thought of as pathogens, but many confer a benefit to their hosts and some are essential for the host life cycle.The polydnaviruses of endoparasitoid wasps have evolved with their hosts to become essential. Many of the viral genes are now encoded in the host nucleus.Endogenous retroviruses are abundant in many genomes of higher eukaryotes, and some have been involved in the evolution of their hosts, such as placental mammals.Some mammalian viruses can protect their hosts from infection by related viruses or from disease caused by completely unrelated pathogens, such as bubonic plague.Viruses can protect their hosts by killing off competitors, as is seen with the killer viruses in yeasts.A fungal virus confers thermal tolerance to a plant in a complex symbiosis involving its fungal host and the plant that the fungus colonizes.Several acute plant viruses confer conditional mutualism by enhancing drought tolerance in plants.Insect viruses have numerous mutualistic relationships with their hosts; in addition, viruses play parts in bacterium–insect mutualisms.