Large Bottleneck Size in Cauliflower Mosaic Virus Populations during Host Plant Colonization

Abstract

The effective size of populations (Ne) determines whether selection or genetic drift is the predominant force shaping their genetic structure and evolution. Despite their high mutation rate and rapid evolution, this parameter is poorly documented experimentally in viruses, particularly plant viruses. All available studies, however, have demonstrated the existence of huge within-host demographic fluctuations, drastically reducing Ne upon systemic invasion of different organs and tissues. Notably, extreme bottlenecks have been detected at the stage of systemic leaf colonization in all plant viral species investigated so far, sustaining the general idea that some unknown obstacle(s) imposes a barrier on the development of all plant viruses. This idea has important implications, as it appoints genetic drift as a constant major force in plant virus evolution. By co-inoculating several genetic variants of Cauliflower mosaic virus into a large number of replicate host plants, and by monitoring their relative frequency within the viral population over the course of the host systemic infection, only minute stochastic variations were detected. This allowed the estimation of the CaMV Ne during colonization of successive leaves at several hundreds of viral genomes, a value about 100-fold higher than that reported for any other plant virus investigated so far, and indicated the very limited role played by genetic drift during plant systemic infection by this virus. These results suggest that the barriers that generate bottlenecks in some plant virus species might well not exist, or can be surmounted by other viruses, implying that severe bottlenecks during host colonization do not necessarily apply to all plant-infecting viruses. Whether selection or stochastic genetic drift is the major force driving the evolution of a virus depends largely on the size of the viral population, with the former being predominant in large populations and the latter taking over when population sizes are transiently or durably reduced. This question has been intensively debated in both plant and animal viruses, as demographic fluctuations throughout viral life cycles are poorly understood. In plant viruses, an extremely small population size—down to a few founder genome units colonizing each leaf—has been formally estimated in two instances, and all other virus species investigated so far have consistently been shown to undergo extreme demographic bottlenecks during systemic invasion of their host. This situation conveys the general idea that all viruses are confronted with “universal barriers” in plants, imposing repeated transient decreases in their population size, thus making genetic drift a major constant driver of their evolution. Here, using the example of Cauliflower mosaic virus, we mitigate this general idea by showing that at least one virus species can overcome such putative limiting barriers and massively invade leaves with hundreds to thousands of founding genome units.