The evolution of RNA viruses: A population genetics view

Abstract

RNA viruses are excellent experimental models for studying evolution under the theoretical framework of population genetics. For a proper justification of this thesis we have introduced some properties of RNA viruses that are relevant for studying evolution. On the other hand, population genetics is a reductionistic theory of evolution. It does not consider or make simplistic assumptions on the transformation laws within and between genotypic and phenotypic spaces. However, such laws are minimized in the case of RNA viruses because the phenotypic space maps onto the genotypic space in a much more linear way than on higher DNA-based organisms. Under experimental conditions, we have tested the role of deleterious and beneficial mutations in the degree of adaptation of vesicular stomatitis virus (VSV), a nonsegmented virus of negative strand. We also have studied how effective population size, initial genetic variability in populations, and environmental heterogeneity shapes the impact of mutations in the evolution of vesicular stomatitis virus. Finally, in an integrative attempt, we discuss pros and cons of the quasispecies theory compared with classic population genetics models for haploid organisms to explain the evolution of RNA viruses.

Figure 1

Schematic representation of the VSV genome, with its nontranscribed leader (l), and five consecutively transcribed mRNAs (gray blocks). Each letter refers to the proteins. From left to right: N, nucleocapsid; P, phosphoprotein; M, matrix; G, glycoprotein; L, viral replicase. Numbers below each block and above lines are lengths in nts of the genes and nontranscribed sequence, respectively.

Figure 2

Relative fitness to the ancestral populations of winners and losers in long-term competition experiments (G and H). Initial populations were neutral. Modified from ref. .

Figure 3

(A) Influence of effective population size on the fitness effect of beneficial mutations that are fixed. The model of the fitted line is described in ref. . (B) Effect of the effective population size on the rate of evolution. See text for more details. The dashed line corresponds to the fit of a liner model to the data. The solid line is the fit to a hyperbolic model. Both curves appear to be exponential because of the logarithmic scale of x axis.