Exploring the effect of sex on empirical fitness landscapes

The nature of epistasis, the interaction between different genes in their effect on fitness, has important consequences for the evolutionary significance of sex and recombination. In particular, sign epistasis, where the sign of the fitness effects of alleles varies across genetic backgrounds, is responsible for ruggedness of the fitness landscape with several unexplored implications for the evolution of sex. Here, we describe fitness landscapes for two sets of strains of the asexual fungus Aspergillus niger involving all combinations of five mutations. We find that about 30% of the single-mutation fitness effects are positive despite their negative effect in the wild-type strain, and that several local fitness maxima and minima are present. We then compare adaptation of sexual and asexual populations on these empirical fitness landscapes using simulations. The results show a general disadvantage of sex on these rugged landscapes, caused by the break down by recombination of genotypes on fitness peaks. A detailed quantitative analysis of this effect is presented for a two-locus model. The talk is based on joint work with Su-Chan Park and Arjan de Visser.

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