How to maintain a high virulence: evolution of a killer in hosts of various susceptibilities.

Pathogens should evolve to avirulence. However, while baculoviruses can be transmitted through direct contact, their main route of infection goes through the death and liquefaction of their caterpillarhosts and highly virulentstrains still seem to be advantaged through infection cycles. Furthermore, one of them, Autographa californicamultiple nucleopolyhedrovirus, is so generalist that it can infect more than 100 different hosts. To understand and characterize the evolutionary potential of this virus and how it is maintained while killing some of its hosts in less than a week, we performed an experimental evolutionstarting from an almost natural isolate of AcMNPV, known for its generalist infection capacity. We made it evolve on 4 hosts of different susceptibilities for 10 cycles and followed hosts survival each day. We finally evaluated whether the generalist capacity was maintained after evolving on one specific host species and tested an epidemiological model through simulations to understand how. Finally, on very highly susceptible hosts, transmission- virulencetrade-offs seem to disappear and the virus can maximize transmission and virulence. When less adapted to its host, the pathogen9s virulencehas not been modified along cycles but the yield was increased, apparently through an increased transmission probability and an increased latent period between exposition and infection.