Basic concepts in RNA virus evolution

A hallmark of RNA genomes is the error-prone nature of their replication and retro-transcription. The major biochemical basis of the limited replication fidelity is the absence of proofreading/repair and postreplicative error correction mechanisms that normally operate during replication of cellular DNA. In spite of this unique feature of RNA replicons, the dynamics of viral populations seems to follow the same basic principles that classical population genetics has established for higher organisms. Here we review recent evidence of the profound effects that genetic bottlenecks have in enhancing the deleterious effects of Muller's ratchet during RNA virus evolution. The validity of the Red Queen hypothesis and of the competitive exclusion principle for RNA viruses are viewed as the expected result of the highly variable and adaptable nature of viral quasispecies. Viral fitness, or ability to replicate infectious progeny, can vary a million-fold within short time intervals. Paradoxically, functional and structural studies suggest extreme limitations to virus variation. Adaptability of RNA viruses appears to be based on the occupation of very narrow portions of sequence space at any given time.
Work in Madrid supported by grants DCICYT PB94-0034-C02-01, FIS 95/0034-1, and Fundación Ramón Areces. Work in Valencia supported by grant DCICYT PB94-0034-C02-02. and in La Jolla by grant AI-14627 from the National Institute of Allergy and Infectious Diseases. N.S. was supported by a felloship from Comunidad Autónoma de Madrid, and S.F.E. and J.Q. by fellowships from Ministerio de Educación y Ciencia (Spain).