From 1986 to 1992, an epidemic of tomato necrosis caused by Cucumber mosaic virus (CMV) plus CMV satellite RNAs (satRNAs) occurred in eastern Spain. From 1989 onward, the frequency of tomato necrosis di-minshed, and it almost completely disappeared after 1991. Analyses of plants infected with CMV and with CMV satRNA and of the phenotype (necrogenic or nonnecrogenic for tomato) induced by some CMV satRNA variants, showed that the disappearance of tomato necrosis was due to changes in the genetic composition of the satRNA population (i.e., to its evolution toward decreased virulence). Analysis of components of the fitness of satRNA variants, necrogenic or nonnecrogenic for tomato, showed that necrogenic and nonnecrogenic variants did not differ in infectivity or in their accumulation level in tomato and that they represented the same fraction of encapsidated RNA. Other fitness components were positively correlated with the greater virulence of necrogenic variants, in that they were favored in mixed infections with nonnecrogenic variants and were more effectively passed into CMV progeny than were nonnecrogenic variants. On the other hand, necrogenic CMV satRNA variants caused a more pronounced depression in the accumulation of CMV than did nonnecro-genic variants, which could affect the efficiency of aphid transmission. Thus, the evolution of virulence in the CMV satRNA population can be explained by trade-offs between factors that determine virulence and factors that affect transmission, as predicted by theoretical models on the evolution of virulence in parasites.

Satellite RNAs (satRNAs) are associated with Cucumber mosaic virus (CMV) in tomato, most often causing severe epidemics of necrotic plants, and not associated with specific host symptoms. Laboratory studies on virus transmission by the aphid vector Aphis gossypii were performed to better understand the dynamics of field populations of CMV. The presence of satRNAs correlated with lower concentrations of virus in infected plants and with a decrease in the efficiency of transmission from satRNA-infected plants. Both the concentration of virus in CMV-infected tomato and the efficiency of transmission varied more extensively with nonnecrogenic satRNAs than with necrogenic satRNAs. A negative effect of satRNAs on virus accumulation can account, in part, for a decrease in the field transmission and recovery of CMV + satRNAs. Aphids behaved differently and probed less readily on plants infected with CMV + necrogenic satRNAs compared with plants containing non-necrogenic satRNAs. Aphid-mediated satRNA-free CMV infections were observed in test plants when aphids were fed on source plants containing CMV + nonnecrogenic satRNA; no comparable satRNA-free test plants occurred when aphids were fed on source plants containing necrogenic satRNAs. These results indicate that factors associated with transmission can be a determinant in the evolution of natural populations of CMV and its satRNA.

Genetic exchange by recombination, or reassortment of genomic segments, has been shown to be an important process in RNA virus evolution, resulting often in important phenotypic changes affecting host range and virulence. However, data from numerous systems indicate that reassortant or recombinant genotypes could be selected against in virus populations and suggest that there is coadaptation among viral genes. Little is known about the factors affecting the frequency of reassortants and recombinants along the virus life cycle. We have explored this issue by estimating the frequency of reassortant and recombinant genotypes in experimental populations of Cucumber mosaic virus derived from mixed infections with four different pairs of isolates that differed in about 12% of their nucleotide sequence. Genetic composition of progeny populations were analyzed at various steps of the virus life cycle during host colonization: infection of leaf cells, cell-to-cell movement within the inoculated leaf, encapsidation of progeny genomes, and systemic movement to upper noninoculated leaves. Results indicated that reassortant frequencies do not correspond to random expectations and that selection operates against reassortant genotypes. The intensity of selection, estimated through the use of log-linear models, increased as host colonization progressed. No recombinant was detected in any progeny. Hence, results showed the existence of constraints to genetic exchange linked to various steps of the virus life cycle, so that genotypes with heterologous gene combinations were less fit and disappeared from the population. These results contribute to explain the low frequency of recombinants and reassortants in natural populations of many viruses, in spite of high rates of genetic exchange. More generally, the present work supports the hypothesis of coadaptation of gene complexes within the viral genomes.

No abstract

The evolution over the past century of two tobamoviruses infecting populations of the immigrant plant Nicotiana glauca in New South Wales (NSW), Australia, has been studied. This plant species probably entered Australia in the 1870s. Isolates of the viruses were obtained from N. glauca specimens deposited in the NSW Herbarium between 1899 and 1972, and others were obtained from living plants in 1985 and 1993. It was found that the NSW N. glauca population was infected with tobacco mosaic tobamovirus (TMV) and tobacco mild green mosaic tobamovirus (TMGMV) before 1950 but only with TMGMV after that date. Half the pre-1950 infections were mixtures of the two viruses, and one was a recombinant. Remarkably, sequence analyses showed no increase in the genetic diversity among the TMGMV isolates over the period. However, for TMV, the genetic diversity of synonymous (but not of nonsynonymous) differences between isolates varied and was correlated with their time of isolation. TMV accumulated to smaller concentrations than TMGMV in N. glauca plants, and in mixed experimental infections, the accumulation of TMV, but not of TMGMV, was around 1/10 that in single infections. However, no evidence was found of isolate-specific interaction between the viruses. We conclude that although TMV may have colonized N. glauca in NSW earlier or faster than TMGMV, the latter virus caused a decrease of the TMV population below a threshold at which deleterious mutations were eliminated. This phenomenon, called Muller's ratchet, or a "mutational meltdown," probably caused the disappearance of TMV from the niche.