Natural selection drives extremely rapid evolution in antiviral RNAi genes

Viruses are famous for evolving quickly, but the organisms they infect can't be expected to sit idly by.

There is now new evidence that animals in fact do an impressive job of keeping up in the ongoing evolutionary arms race between viruses and their hosts.

Studying a special class of genes thought to have evolved in part as a defense system against viruses, researchers have found evidence that these genes are indeed among the fastest-evolving in the genome of the fruit fly, Drosophila. The work is reported by University of Edinburgh researchers Dr. Darren Obbard and Dr. Tom Little and colleagues and appears in the March 21st issue of Current Biology.

Viruses hijack the cells of other organisms, using them as factories to copy themselves. Animals and plants have a number of different locks and codes that help to keep the viruses out, but viruses are capable of evolving new ways of breaking the codes extremely quickly. The new research shows that genes controlling a part of the immune system that fights viruses actually evolve much faster than almost all other genes, evidence of the host's evolutionary race to keep the viruses at bay.

In their research, the scientists found evidence that some genes that participate in so-called RNAi mechanisms evolve much faster than the vast majority of other genes in the fly's genome. This is relevant for the virus-host arms race because RNAi pathways--which exist in both plants and animals--participate in molecular defenses against viruses by homing in on viral genetic material and directing its enzymatic destruction. Viruses can evolve quickly to out-maneuver RNAi mechanisms, but hosts would be expected to rapidly evolve countermeasures of their own to resist new viral strategies. The new research provides evidence for such rapid evolution of some RNAi genes: The researchers found that some RNAi components evolve far faster than 97% of all other fruit fly genes. This rapid evolution illustrates the vital role of RNAi in antiviral defense, and it shows that these genes can be central players in the evolution of host organisms in response to the ever-changing strategies of viral attacks.