In a soon-to-be-published review in Nature, Marylin Roossnick of Cornell University examines the often overlooked mutualisms of certain viruses. Though typically denounced as pathogens, viruses have been shown to be beneficial in many different organisms, illustrating true biological versatility. Roossnick describes numerous viral symbiotic relationships including those found in insects, plants, beneficial viruses in mammalian disease, and phage-bacteria interactions.
The discussion on polydnaviruses of endoparasitoid wasps was one of the many highlights of the review. Many endoparasitoid braconids and ichneumonids wasps lay their eggs inside living insect larvae. In order for wasp eggs to successfully hatch from inside a caterpillar, for instance, certain wasps have co-opted species-specific endosymbiotic viruses (bracoviruses and ichnoviruses, respectively) that suppress the hold caterpillar's innate immune system. The virus is delivered into the caterpillar upon oviposition. Subsequent expression of viral genes thwart the caterpillar's immune-response that would naturally trigger encapsulation, blockade any flow of nutrients, and kill the eggs.
This review demonstrates the remarkable co-evolution between viruses and the cells they infect. Despite their prominent role as pathogens, viruses can also be mutualistic, even necessary to an organism's survival. These viruses have the ability to confer remarkable benefits from egg survival in parasitoid wasps to mammalian placental evolution to tolerance in drought and cold conditions for plants.
Roossinck M. The good viruses: viral mutualistic symbioses. Nature Reviews Microbiology 9, 99-108 (February 2011) | doi:10.1038/nrmicro2491
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