The widespread occurrence of sex and the associated process of genetic recombination represents one of the greatest unsolved problem in evolutionary biology. Asexual females can potentially produce twice as many daughters as sexual females, resulting in a ‘two-fold cost of sex’. The most plausible solutions are that sex facilitates adaptation to a changing environment and/or allows deleterious mutations to be eliminated more efficiently. We are addressing this problem with a mixture of theoretical and empirical work.

Our main findings are:

• A pluralist explanation of sex and recombination offers numerous advantages. In particular, mutational and environmental (e.g. host-parasite coevolution) explanations can interact synergistically to explain sex with far less restrictive parameter assumptions (West et al. 1999 J. Evol. Biol.; West 2002 Encyclopedia of Evolution).
• In the parasitoid wasp, N. vitripennis, synergistic epistasis between deleterious mutations is found when examining longevity, but not egg production (Rivero et al. 2003 Evolution).
• Crossing experiments can be used to test for synergistic epistasis, but not as initially imagined (West et al. 1998 Evolution).
• The intestinal nematode Strongyloides ratti facultatively has sex in response to stress, and not just genotype specific immune responses (West et al. 2001 J. Evol. Biol.).

Our ongoing work includes: (i) Estimating the deleterious mutation rate and recombination rates in N. vitripennis (photo above left); (ii) Examining the fitness consequences of parasites and mutations in Daphnia (photo above right). The Nasonia work is led by Tracy Reynolds, in collaboration with Peter Keightley. The Daphnia work is being led by Stuart Killick, in collaboration with Tom Little.

Selected Relevant Publications

• Killick, S.C., Carlsson, A.M., West, S.A. & Little, T.J. (2006) Testing the pluralist approach to sex: the influence of environment on synergistic interactions between mutation load and parasitism in Daphnia magna. Journal of Evolutionary Biology 19, 1603-1611.
  
• Sykes, E.M. & West, S.A. (2006) Evolution: Revenge of the clones! Current Biology 15, 547-548. {PDF}
  
• Rivero, A., Balloux, F. & West, S.A. (2003) Testing for epistasis between deleterious mutations in a parasitoid wasp. Evolution, 57, 1698-1703. {PDF}
  
• West, S. A. (2002) Sex. In Encyclopedia of Evolution (ed. By M. Pagel), pp. 1022-1030, Oxford University Press. {PDF}
• West, S.A., Gemmill, A.W., Graham, A., Viney, M.E. & Read, A.F. (2001) Immune stress and facultative sex in a parasitic nematode. Journal of Evolutionary Biology, 14, 333-337. {PDF}
• West, S.A. & Peters, A. (2000) Paying for sex is not easy. Nature, 407, 962. {PDF}
• West, S.A., Lively, C.M. & Read, A.F. (1999) A pluralist approach to sex and recombination. Journal of Evolutionary Biology, 12, 1003-1012. {PDF}  {PDF of paper & all commentaries}
• West, S.A., Peters, A.D. & Barton, N.H. (1998) Testing for epistasis between deleterious mutations. Genetics. 149, 435-444. {PDF}