Frank, S. A. 2003. Viral genetics: deadly partnerships [News and Views]. Nature 425:251-252.

Pairs of viral genomes work together to destroy their hosts more quickly. How this might occur remains unknown, but study of the phenomenon should provide insight into how genetic systems evolve.

Early in the history of life, different copies of replicating nucleic acids must have existed near each other. Some of these genomes probably parasitized their neighbours by becoming shorter, dropping essential information and using proteins encoded by the full-length molecules. The shorter parasitic genomes might have replicated faster and out-competed their fully endowed neighbours. Other pairs probably complemented each other to mutual benefit, favouring some method for the pair to disperse together. Viral systems provide our best window back through time, allowing us to glimpse how multi-copy genetic systems might have evolved. There are many known examples of shortened viral genomes exploiting functional partners, but writing in the Proceedings of the Royal Society, López-Ferber and colleagues now show that defective viral genomes are not always parasitic. They provide evidence that shortened genomes can work with full-length partners to mutual benefit.