Exploring the effect of sex on an empirical fitness landscape

TL;DR

This study investigates how sex influences evolution on rugged fitness landscapes by analyzing empirical data from Aspergillus niger and simulating adaptation, revealing sex's disadvantage due to landscape ruggedness and recombination effects.

Contribution

It provides the first empirical fitness landscapes for Aspergillus niger and examines how sex impacts adaptation on these rugged landscapes through simulations.

Findings

Approximately 30% of single mutations are beneficial despite being deleterious in wild-type.

Presence of multiple local fitness maxima and minima in the landscapes.

Sex generally hinders adaptation on rugged landscapes due to disruption of local peaks.

Abstract

The nature of epistasis has important consequences for the evolutionary significance of sex and recombination. Recent efforts to find negative epistasis as source of negative linkage disequilibrium and associated long-term sex advantage have yielded little support. Sign epistasis, where the sign of the fitness effects of alleles varies across genetic backgrounds, is responsible for ruggedness of the fitness landscape with implications for the evolution of sex that have been largely unexplored. Here, we describe fitness landscapes for two sets of strains of the asexual fungus \emph{Aspergillus niger} involving all combinations of five mutations. We find that $\sim 30$% 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 escaping from local peaks. Sex facilitates escape from a local peak only for some parameter values on one landscape, indicating its dependence on the landscape's topography. We discuss possible reasons for the discrepancy between our results and the reports of faster adaptation of sexual populations.