directional epistasis

Azevedo RBR, Lohaus R, Srinivasan S, Dang KK & Burch CL Sexual reproduction selects for robustness and negative epistasis in artificial gene networks. Nature 440:87-90.

• log(W_k) = − αk^β

• many types of interactions are possible, including directional epistasis, in which the average effect of spontaneous mutations changes in the presence of other mutations in the genome
• directional epistasis can be either negative (synergistic) or positive (antagonistic), depending on whether the average effect of mutations becomes more or less harmful, respectively, as the number of other mutations in the genome increases
• directional epistasis holds particular interest for evolutionary biologists because it is expected to determine the outcome of multiple evolutionary processes, notably the evolution of sex and recombination

• empirical studies on a variety of organisms have reported every conceivable form of directional epistasis: negative, positive and no significant directional epistasis
• these mixed results have not helped to clarify either the mechanistic or evolutionary causes of directional epistasis