Fig. 1.

Schematic representation of viral quasispecies and fitness variations. (A) An infected individual contains multiple, replicating viral quasispecies, even within the same organ. Here two mutant spectra are depicted with their consensus sequences. Each horizontal line represents a genome, and each symbol on a line represents a type of mutation. Real viral quasispecies can contain thousands of continuously mutating genomes that conform huge mutant clouds. (B) The passage regime of the virus can affect virus population fitness. Small arrows represent repeated bottleneck passages which result in fitness decrease. The large arrow represents large population passages that generally result in fitness gain. Fluctuations in fitness shown at high and low fitness values (at the two small angles of the fitness triangle) have been observed experimentally and interpreted as the stochastic effect of mutations on fitness when fitness gain is limited by population size or when fitness is very low. [Based in (Escarmís et al., 2006, Novella, 2003, Novella et al., 1999); figure modified from (Domingo et al., 2006), with permission].