Host-associated selection of sequence variants from a satellite RNA of cucumber mosaic virus

The sequence of lx-satRNA, a CMV-satRNA necrogenic for tomato that shows the unusual property of accumulating in cucumber and in squash to levels similar to those in tomato and in tobacco, was determined. When compared with the sequences of other necrogenic CMV-satRNAs that do not show this property, or with a sequence variant of lx-satRNA that retains it, the high accumulation of lx-satRNA in squash was correlated with the presence of two U/C transitions. The sequences of progeny lx-satRNA from passage experiments in tomato and in squash show that a sequence variant having a U at position 102 was selected in tomato and that the parent sequence with a C at 102 was consistently restored in squash when this U-variant was used as inoculum. The results from these passage experiments may be explained by the heterogeneous nature of RNA populations, built from a number of variants of a master sequence, and illustrate the validity of this concept for CMV-satRNAs. The data also show that a minor sequence change, such as the transition C/U at 102, may have a major effect on the fitness of sequence variants of CMV-satRNAs in different hosts, and this may be relevant to the evolution in nature of these small RNAs.