Virulence evolution in a virus obeys a trade-off

The evolution of virulence was studied in a virus subjected to alternating episodes of vertical and horizontal transmission. Bacteriophage fl was used as the parasite because it establishes a debilitating but non-fatal infect ion that can be transmitted vertically (from a host to its progeny)z as wel l as horizontally (infection of new hosts). Horizontal transmission was req uired of all phage at specific intervals, but was prevented other wise. Eac h episode of horizontal transmission was followed by an internal of obligat e vertical transmission, followed by an interval of obligate horizontal tra nsmission etc. The duration of vertical transmission was eight times longer per episode in one treatment than in the other, thus varying the relative intensity of selection against virulence while maintaining selection for so me level of virus production Viral lines with the higher enforced rate of i nfectious transmission evolved higher virulence and higher rates of virus p roduction. These results support the trade-off model for the evolution of v irulence.