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 relevan t for our understanding of the evolution of sex. If epistasis is syner gistic, the main function of sex may be to facilitate selection agains t deleterious mutations. We use strains of the filamentous fungus Aspe rgillus niger with variable combinations of marker mutations that have been obtained by isolating segregants from a diploid between a wild-t ype strain and a related strain carrying a marker mutation on each of its eight chromosomes. The marker mutations include five auxotrophic a nd two resistance mutations. As a measure of fitness the mycelium grow th rate on supplemented medium has been used. The results suggest that the marker mutations have independent effects on fitness, and hence t hey do not support the deterministic mutation hypothesis of the evolut ion of sex. The apparent linear relationship between mutation number a nd log fitness is the result of interactions of opposite type (i.e., s ynergistic and antagonistic) that cancel each other's effect. However, due to an isolation bias caused by the fact that not all possible str ains with many mutations could be isolated, the results may be relativ ely biased towards an antagonistic relationship between mutation numbe r and log fitness.