Evolutionary history conditions the timing of transmission in vesicular stomatitis virus

It has been postulated that early transmitted viruses would evolve to be more virulent than late transmitted ones. The reason for this prediction is that early transmission selects for rapid viral replication and, consequently, rapid host death, whereas late transmission would select for slow-replicating viruses that permit longer survival to the host. To test this prediction, experimental lineages of vesicular stomatitis virus (VSV) had been adapted to three different transmission dynamics during more than 100 generations. Transmission dynamic differed in the stage of infection at which transmission took place: early, intermediate or late. Regardless the timing of transmission imposed during the competition experiments done for estimating fitness, lineages adapted to an intermediate time of transmission reached higher fitness than viruses adapted to either early or late transmission. Viruses adapted to early and late transmission schedules showed a trade-off in their performance at other transmission times, with higher fitness at their own transmission time than at any other. The basis of fitness differences, in terms of growth parameters, have also been explored. Fitness correlated with the rate of growth and with the moment of maximum population density but not with the maximum density itself. In addition, a positive correlation between virus performance at late transmission and stability outside the cellular host has been detected.