Genetic variability and evolution of the satellite RNA of cucumber mosaic virus during natural epidemics

The genetic structure of populations of cucumber mosaic virus (CMV) satellite RNA (satRNA) and its evolution were analyzed during the course of a CMV epidemic in tomatoes in eastern Spain. A total of 62 variants of CMV-satRNA from epidemic episodes in 1989, 1990, and 1991 were characterized by RNase protection assay (RPA); RPA patterns defined 60 haplotypes in the CMV-satRNA population. RPA of nine CMV-satRNAs of known sequences showed that numbers of nucleotide substitutions per site (dij) between different satRNAs can be estimated from RPA data. Thus, dij were estimated for any possible pair of field CMV-satRNA types, and nucleotide diversities within and between yearly subpopulations were calculated. Also, phylogenetic relationships among CMV-satRNAs were derived from RPA data (by parsimony) or from dij (by neighbor joining). From these analyses, a model for the evolution of CMV-satRNAs in field epidemics can be built. High genetic variability of CMV-satRNA results in very heterogeneous populations, even compared with those of other RNA genomes. The high diversity of the population is maintained through time by the continuous generation of variants by mutation, counterbalanced by negative selection; this results in a certain replacement of haplotypes from year to year. The sequential accumulation of mutations in CMV-satRNA leads to fast genetic divergence to reach what appears to be an upper permitted threshold.