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) sa tellite RNA (satRNA) and its evolution were analyzed during the course of a CMV epidemic in tomatoes in eastern Spain. A total of 62 variant s of CMV-satRNA from epidemic episodes in 1989, 1990, and 1991 were ch aracterized by RNase protection assay (RPA); RPA patterns defined 60 h aplotypes in the CMV-satRNA population. RPA of nine CMV-satRNAs of kno wn sequences showed that numbers of nucleotide substitutions per site (d(ij)) between different satRNAs can be estimated from RPA data. Thus , d(ij) 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 we re derived from RPA data (by parsimony) or from d(ij) (by neighbor joi ning). From these analyses, a model for the evolution of CMV-satRNAs i n field epidemics can be built. High genetic variability of CMV-satRNA results in very heterogeneous populations, even compared with those o f other RNA genomes. The high diversity of the population is maintaine d through time by the continuous generation of variants by mutation, c ounterbalanced by negative selection; this results in a certain replac ement 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.