Genetic bottlenecks

Changes in population size may have important effects on genetic variation and on the survival potential of viral species. Genetic bottlenecks are evolutionary events that reduce genetic variation of a population in a stochastic manner and result in founding populations that can lead to genetic drift. In nature, genetic bottlenecks may occur at different points during the life cycles of plant RNA viruses. For example, transmission events, both horizontal and vertical, and systemic infections represent events in the virus life cycles that may impose a bottleneck. Recently, genetic bottlenecks have been shown experimentally in plant virus populations during systemic movement within the plant and horizontal transmission from plant to plant by aphid vectors. The most important implication of genetic bottlenecks is the reduction in population size and intensification of genetic drift, which can reshape the RNA virus population and may lead to the emergence of new virus strains. Another effect of genetic bottlenecks is to reduce the size of the effective populations below the threshold needed to ensure the transmission of the fittest variants. Consequently, the viral population may become progressively dominated by less fit mutants, a process known as Muller's ratchet, and will succumb by a mutational meltdown.