A while ago I mentioned the truly spectacular symbiosis between some parasitic wasps and their weaponized internal viruses. These wasps parasitize caterpillars and the like; and to subdue their prey, the wasps inject viruses into their prey, along with their own eggs. The viral immune evasion functions then block the caterpillar’s defenses, allowing the wasp’s eggs to replicate. The viruses, in turn, get free replication, because they’re actually part of the wasp genome – wasp genes, that assemble to look like viruses and act like viruses.
Back in the prehistoric days when I first talked about it (December ’07), it wasn’t clear whether these were all actually viruses that had become symbiotic and moved into the wasp genome, or whether they started out as wasp genes that became more and more virus-like:
A more parsimonious hypothesis would be that bracoviruses do not originate from any of the large genome viruses characterized to date. They may have been built up from a simple system producing circular DNA intermediates, such as mobile elements, within the wasp genome. The acquisition of a capsid protein, possibly of viral origin, around the circular DNA intermediates would have allowed infection of lepidopteran cells. Finally, virulence genes could have been acquired from the wasp genome at different times during evolution of bracovirus-bearing wasp lineages, thus explaining why CcBV genes encoding proteins with a predicted function resemble cellular genes. 1
Because I know my dedicated readers2 have been fretting about this since I mentioned it, I now am happy to report that the question is apparently settled:3 Bézier et al. searched through various wasp genomes and found the genes associated with the virus-like particles, and were able to show that in fact they were originally viruses — specifically, members of the nudivirus family — that moved into the wasps’ genomes. Nudiviruses are known (though not well-studied) insect viruses, but apparently they’ve not been known to infect Hymenoptera.
As the commentary 4 on this paper observes:
Whereas viruses have been typically seen as either parasites or commensals, we must now recognize a potential for obligatory mutualism. … We would suggest that the more interesting lesson here for virologists and for evolutionary biologists may be that there is now reason to start thinking about virus-host relationships in much broader terms, so as to include not only mutualism, which may be a lot more common than previously contemplated, but also obligatory mutualism, as exemplified by the wasp-nudivirus story. 4
- Espagne, E. et al. Genome sequence of a polydnavirus: insights into symbiotic virus evolution. Science 306, 286-289 (2004[↩]
- Hi, Mom![↩]
- A. Bezier, M. Annaheim, J. Herbiniere, C. Wetterwald, G. Gyapay, S. Bernard-Samain, P. Wincker, I. Roditi, M. Heller, M. Belghazi, R. Pfister-Wilhem, G. Periquet, C. Dupuy, E. Huguet, A.-N. Volkoff, B. Lanzrein, J.-M. Drezen (2009). Polydnaviruses of Braconid Wasps Derive from an Ancestral Nudivirus Science, 323 (5916), 926-930 DOI: 10.1126/science.1166788[↩]
- Stoltz, D. B., and J. B. Whitfield. 2009. VIROLOGY: Making Nice with Viruses. Science 323:884-885[↩][↩]