Friday, August 31, 2007

One Species' Entire Genome Discovered Inside Another's

From Science Daily

I confess I was a little mystified when I saw this headline. From my perspective as a phage biologist, this is not really news. Temperate phages, like lambda, do this all the time. But then wolbachia is such a neat beastie. It deserves a little press.

However, the article got me thinking about another host/parasite association: caterpillars and wasps (including Darwin's famous ichneumonidae). The female wasp, as is famously known, paralyses the caterpillar and lays her eggs inside the unfortunate creature. The eggs hatch and the resulting larvae devour the caterpillar from the inside out, while taking care to avoid the major organs so as to keep the caterpillar alive and the meat fresh.

Turns out this weird story gets even weirder. These wasps are infected with polydnaviruses. The full genome of the virus is integrated into the wasp's genome and the virus replicates only in specific cells in the female wasp's reproductive system. Here things get really interesting. The polydnaviruses are injected into the caterpillar along with the wasp eggs. The resulting infection does not lead to viral reproduction, but rather the viruses affect the caterpillar's immune system. Specifically they suppress the caterpillar's phagocytic hemocytes which, in the absence of viral infection, would encapsulate and kill the wasp eggs.

There are two known genera of polydnaviruses: the ichnoviruses and bracoviruses, infecting ichneumonid and braconid wasps respectively. There is little sequence homology between the two genera, suggesting that this virus "lifestyle" evolved independently in two wasp lineages. Rather than being parasitic to the wasp, the virus has evolved a symbiotic relationship and gang up to parasitise the caterpillars. How cool is that?

Photo by: David Wahl, American Entomological Institute