Genetic Structure of Natural Populations of the Plant RNA Virus Tobacco Mild Green Mosaic Virus

Abstract

The statistical analysis of population subdivision in five populations of tobacco mild green mosaic virus (TMGMV) naturally infecting Nicotiana glauca has shown that the populations can be considered as samples of an almost panmictic one. In spite of the high mutation rate of this RNA virus, the genetic divergence between pairs of haplotypes, both within and between subpopulations, is very small. The observed amount of variability cannot be exclusively explained in terms of the neutral theory of molecular evolution. Under such a model, an unrealistic estimate of the effective population size equal to 86 viral particles is obtained. Positive Darwinian selection can be invoked to account for the variability observed. No direct experimental evidence of selection is presented, but, by means of both (a) high selection coefficients for driver mutations and (b) reasonable neutral and driver mutation rates for RNA viruses, the amount of variability observed can be accounted for in terms of a model of periodic, positive, Darwinian selection.