Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Fitness of RNA virus decreased by Muller's ratchet


WHY sex exists remains an unsolved problem in biology1–3. If mutations are on the average deleterious, a high mutation rate can account for the evolution of sex4. One form of this mutational hypothesis is Muller's ratchet5,6. If the mutation rate is high, mutation-free individuals become rare and they can be lost by genetic drift in small populations. In asexual populations, as Muller5 noted, the loss is irreversible and the load of deleterious mutations increases in a ratchet-like manner with the successive loss of the least-mutated individuals. Sex can be advantageous because it increases the fitness of sexual populations by re-creating mutation-free individuals from mutated individuals and stops (or slows) Muller's ratchet. Although Muller's ratchet is an appealing hypothesis, it has been investigated and documented experimentally in only one group of organisms—ciliated protozoa2. I initiated a study to examine the role of Muller's ratchet on the evolution of sex in RNA viruses and report here a significant decrease in fitness due to Muller's ratchet in 20 lineages of the RNA bacteriophage Φ6. These results show that deleterious mutations are generated at a sufficiently high rate to advance Muller's ratchet in an RNA virus and that beneficial, backward and compensatory mutations cannot stop the ratchet in the observed range of fitness decrease.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. Maynard Smith, J. The Evolution of Sex (Cambridge University Press, Cambridge, 1976).

    Google Scholar 

  2. Bell, G. Sex and Death in Protozoa. The History of an Obsession (Cambridge University Press, Cambridge, 1988).

    Google Scholar 

  3. Michod, R. & Levin, B. R. The Evolution of Sex. An Examination of Current Ideas (Sinauer, Sunderland, 1988).

    Google Scholar 

  4. Kondrashov, A. S. Nature 336, 435–440 (1988).

    Article  ADS  CAS  PubMed  Google Scholar 

  5. Muller, H. J. Mut. Res. 1, 1–9 (1964).

    Article  Google Scholar 

  6. Felsenstein, J. Genetics 78, 737–756 (1974).

    CAS  PubMed  PubMed Central  Google Scholar 

  7. Holland, J. et al. Science 215, 1577–1585.

  8. Chao, L. J. theor. Biol. 133, 99–112 (1988).

    Article  CAS  PubMed  Google Scholar 

  9. Semancik, J. S., Vidaver, A. K. & Van Etten, J. L. J. molec. Biol. 78, 617–625 (1973).

    Article  CAS  PubMed  Google Scholar 

  10. Horiuchi, K. in RNA Bacteriophages (ed. Zinder, N. D.) 29–50 (Cold Spring Harbor Laboratory, New York, 1975).

    Google Scholar 

  11. Mindich, L., Sinclair, J. F., Levine, D. & Cohen, J. Virology 75, 218–223 (1976).

    Article  CAS  PubMed  Google Scholar 

  12. Mindich, L., Cohen, J. & Weisburd, M. J. Bact. 126, 177–182 (1976).

    CAS  PubMed  PubMed Central  Google Scholar 

  13. Sinclair, J. F., Cohen, J. & Mindich, L. Virology 75, 198–208 (1976).

    Article  CAS  PubMed  Google Scholar 

  14. Maniatis, T., Fritsch, E. F. & Sambrook, J. Molecular Cloning. A Laboratory Manual (Cold Spring Harbor Laboratory, New York, 1982).

    Google Scholar 

  15. Sokal, R. R. & Rolf, F. J. Biometry (Freeman, San Francisco, 1981).

    Google Scholar 

  16. Falconer, D. S. Introduction to Quantitative Genetics, 2nd ed. (Longman, Harlow, 1981).

    Google Scholar 

  17. Wagner, G. P. & Gabriel, W. Evolution 44, 715–731 (1990).

    Article  PubMed  Google Scholar 

  18. Lenski, R. Science 248, 901 (1990).

    Article  Google Scholar 

  19. Becker, W. A. Manual of Quantitative Genetics, 3rd ed. (Washington State University Press, Pullman, 1975).

    Google Scholar 

Download references

Author information

Authors and Affiliations


Rights and permissions

Reprints and Permissions

About this article

Cite this article

Chao, L. Fitness of RNA virus decreased by Muller's ratchet. Nature 348, 454–455 (1990).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing