Experimental evolution of plant RNA viruses

Abstract

Undoubtedly, viruses represent a major threat faced by human and veterinary medicines and by agronomy. The rapid evolution of viruses enables them to escape from natural immunities and from state-of-the-art antiviral treatments, with new viruses periodically emerging with deadly consequences. Viruses have also become powerful and are increasingly used tools in the field of experimental evolution. A growing body of evidence points that the evolution of viruses is mainly determined by key features such as their compacted genomes, enormous population sizes, and short generation times. In addition, RNA viruses also present large selection coefficients, antagonistic epistasis, and high mutation rates. Most of this knowledge comes from studies that have used either bacteriophages or animal viruses in cell cultures as experimental systems. However, plant viruses provide almost identical advantages for evolutionary studies and, in addition, offer an invaluable tool for studying the interplay between viruses and pluricellular hosts. Without seeking to be exhaustive, here we summarize some peculiarities of plant viruses and review recent experiments that have explored important questions on evolution, such as the role of deleterious mutation and neutrality, the effect of different transmission modes in the evolution of virulence, and the heterogeneous selective constraints imposed by multiple hosts.

Figure 1

Evolution of TEV fitness upon serial bottleneck transfers. Each symbol and line represents a different mutation accumulation lineage. Fitness was expressed as the ratio between the Malthusian parameter estimated for each clone and the value obtained to the ancestral TEV. (From de la Iglesia and Elena (2007)).

Figure 2

Fitness of a robust viroid, CChMVd, relative to a fit one, CSVd, in non-mutagenic and in mutagenic environmental conditions. Error bars represent standard errors. (From Codoñer et al., 2006).

Figure 3

(a) Evolution of virulence in horizontally and vertically transmitted BSMV populations. (From Stewart et al., 2005). (b) Tradeoff between the efficiency of horizontal and vertical transmissions for BSMV populations evolved upon a horizontal transmission scheme. (From Stewart et al., 2005). (c) Correlation between virulence and transmission rate in CMV. Different symbols represent different CMV genotypes. (From Escriu et al., 2003).