EVOLUTION OF A PERSISTENT APHTHOVIRUS IN CYTOLYTIC INFECTIONS - PARTIAL REVERSION OF PHENOTYPIC TRAITS ACCOMPANIED BY GENETIC DIVERSIFICATION

Foot-and-mouth disease virus (FMDV) shows a dual potential to be cytol ytic or to establish persistent infections in cell culture. FMDV R100, a virus rescued after 100 passages of carrier BHK-21 cells persistent ly infected with FMDV clone C-S8c1, showed multiple genetic and phenot ypic alterations relative to the parental clone C-S8c1. Several FMDV R 100 populations have been subjected to 100 serial cytolytic infections in BHK-21 cells, and the reversion of phenotypic and genetic alterati ons has been analyzed, An extreme temperature sensitivity of R100 reve rted totally or partially in some passage series but not in others, Th e small-plaque morphology reverted to normal size in all cases, The hy pervirulence for BHK-21 cells did not revert, and even showed an incre ase, upon cytolytic passage, Most of the mutations that had been fixed in the R100 genome during persistence did not revert in the course of cytolytic passages, but the extended polyribocytidylate tract of R100 (about 460 residues, versus 290 in C-S8c1) decreased dramatically in length, to the range of 220 to 260 residues in all passage series exam ined, In passages involving very large viral populations, a variant wi th two amino acid substitutions (L-144-->V and A-145-->P) next to the highly conserved Arg-Gly-Asp (RGD motif; positions 141 to 143) within the G-H loop of capsid protein VP1 became dominant, A clonal analysis allowed isolation of a mutant with the single replacement A-145-->P. V iral production and growth competition experiments showed the two vari ants to have a fitness very close to that of the parental virus, The r esults provide evidence that the repertoire of variants that could pot entially become dominant in viral quasispecies may be influenced by th e population size of the evolving virus, The net results of a series o f persistent-infection passages followed by a series of cytolytic pass ages was progressive genomic diversification despite reversion or stas is of phenotypic traits. Implications for the evolution of RNA viruses are discussed.