Test of interaction between genetic markers that affect fitness in Aspergillus niger

In this paper we study whether and how a number of arbitrarily chosen marker mutations interact in their effect on fitness, which is relevant for our understanding of the evolution of sex. If epistasis is synergistic, the main function of sex may be to facilitate selection against deleterious mutations. We use strains of the filamentous fungus Aspergillus niger with variable combinations of marker mutations that have been obtained by isolating segregants from a diploid between a wild-type strain and a related strain carrying a marker mutation on each of its eight chromosomes. The marker mutations include five auxotrophic and two resistance mutations. As a measure of fitness the mycelium growth rate on supplemented medium has been used. The results suggest that the marker mutations have independent effects on fitness, and hence they do not support the deterministic mutation hypothesis of the evolution of sex. The apparent linear relationship between mutation number and log fitness is the result of interactions of opposite type (i.e., synergistic and antagonistic) that cancel each other's effect. However, due to an isolation bias caused by the fact that not all possible strains with many mutations could be isolated, the results may be relatively biased towards an antagonistic relationship between mutation number and log fitness.