EVOLUTIONARY DYNAMICS OF CUCUMBER MOSAIC-VIRUS SATELLITE RNA DURING NATURAL EPIDEMICS IN ITALY

The evolutionary dynamics of 22 variants of cucumber mosaic virus sate llite RNA (CMV satRNA) isolated in Italy during virus epidemics from 1 988 to 1993 were investigated on the basis of their primary structure and biological properties. Most of the variants were amplified from to tal nucleic acid preparations extracted from field-infected plants, th us representing wild isolates of CMV satRNA. Eleven variants were asso ciated with subgroup II CMV strains, 10 with subgroup I and 1 with a m ixed infection by both strains. When inoculated onto tomato seedlings, the variants induced the phenotype (necrogenic or ameliorative) predi cted by their nucleotide sequence. Phylogenetic relationships between the satRNA variants were determined using the stationary Markov model, a stochastic model for evolution. For each satRNA, the Markov analysi s gave a good correlation between position in the phylogenetic tree an d biological properties. The variants with ameliorative and necrogenic phenotypes in tomato followed two different evolutionary dynamics in nature. Tfn-satRNA, a 390-nt-long molecule, followed a third type of e volutionary dynamic far apart from that of the shorter satRNA molecule s (i.e., those in the 334- to 340-nt-length class). Average values of the mean constant rate of nucleotide substitutions/site (K-subs/site) indicated that in nature the variants tend to keep their heterogeneity unchanged from one epidemic episode to the other, even if the outbrea ks occur in places very far from each other. This seems to be in agree ment with the proposed maintenance of a functional molecular structure as a constraint to CMV satRNA evolution. (C) 1997 Academic Press.