Lack of evolutionary stasis during alternating replication of an arbovirusin insect and mammalian cells

The evolution of vesicular stomatitis virus (VSV) in a constant environment , consisting of either mammalian or insect cells, has been compared to the evolution of the same viral population in changing environments consisting in alternating passages in mammalian and insect cells. Fitness increases we re observed in all cases. An initial fitness loss of VSV passaged in insect cells was noted when fitness was measured in BHK-21 cells, but this effect could be attributed to a difference of temperature during VSV replication at 37 degrees C in BHK-21 cells. Sequencing of nucleotides 1-4717 at the 3' end of the VSV genome (N, P, M and G genes) showed that at passage 80 the number of mutations accumulated during alternated passages (seven mutations ) is similar or larger than that observed in populations evolving in a cons tant environment (two to four mutations). Our results indicate that insect and mammalian cells can constitute similar environments for viral replicati on. Thus, the slow rates of evolution observed in natural populations of ar boviruses are not necessarily due to the need for the virus to compromise b etween adaptation to both arthropod and vertebrate cell types. (C) 1999 Aca demic Press.