Viruses may seem the scourge of the living world, hijacking organisms' genetic machineries for their own good and causing conditions from eczema to AIDS. But a family of wasps has turned these foes into friends. And as a paper in the 29 May issue of the Proceedings of the National Academy of Sciences shows, the collaboration has been going strong since the time of the dinosaurs.
Parasitic wasps have a gory lifestyle: Many lay their eggs in living caterpillars, which the larvae eat from the inside out. In the end, when the caterpillar is little more than a bag of wasp larvae, the parasites break through the skin, pupate, and emerge as full-fledged wasps. The caterpillars aren't defenseless, though--they have specialized blood cells that form a suffocating capsule around the intruders. In turn, wasps in the family Braconidae have enlisted so-called polydnaviruses to foil these defenses.
In the 1990s, says entomologist Jim Whitfield of the University of Illinois, Urbana-Champaign, researchers discovered that polydnaviruses live a slumbering life as part of the wasps' chromosomes. But in the ovaries of a mature female, they break free and attach themselves to the outside of the female's eggs. Carried with the egg into a caterpillar, the viruses produce proteins that stop its blood cells from encapsulating the wasp larvae. Some of these proteins are wasp venoms, suggesting that genes from the wasp have been transferred into the virus in a feat of natural genetic engineering.
In the new study, Whitfield shows that this symbiosis goes back a long way. Insects enclosed in New Jersey and Baltic amber show that the kinds of braconids that today carry polydnaviruses evolved between 93 million and 45 million years ago. To narrow down this time window, Whitfield studied sequences of three genes from 28 species of wasps--some virus-carriers and some not--and used differences among the species to build a family tree. He used the fossils, which had already been dated, to put dates on some of the branches. He found that the branch leading to the virus-taming wasps started 74 million years ago.
Other entomologists are pleased to finally know when the wedding took place in this intricate marriage of convenience. Nancy Moran, who studies the long-lived symbiosis between bacteria and aphids at the University of Arizona in Tucson, says: "This is another fascinating case [of] the long evolutionary history of insect-microbe interactions."