Complementation ofnpfmutations in diploid amoebae ofPhysarum polycephalum: the basis for a general method of complementation analysis at the amoebal stage

Abstract

SummaryHaploid amoebae ofPhysarum polycephalummay form plasmodia by crossing, a sexual process that involves cellular and nuclear fusions, or by selfing, an asexual process in which the development of a single amoeba into a plasmodium may involve neither cellular nor nuclear fusion. Mutant strains (npf) in which selfing is suppressed were previously assigned to several functional groups on the basis of their ability to cross with one another in certain combinations. In the present study hybrid, diploid amoebae were isolated from both crossing-compatible and incompatible mixtures ofnpfmutants. The diploid amoebae from mixtures of compatible strains readily formed plasmodia by selfing, but selfing was suppressed in the diploids from incompatible mixtures. Thus the crossing tests betweennpfmutants may be viewed as complementation tests: their results reflect the differing selfing abilities of the hybrid, diploid amoebae that formed in each mixture. Genetical and environmental factors affecting the efficiency of formation of diploid amoebae were studied, and the diploids were shown to be stable during repeated subcultures. Although diploid amoebae carrying complementingnpfmutations readily formed plasmodia by selfing at 26 δC, they could be cultured without plasmodium formation at 30 δC, a temperature that also inhibited selfing of the haploidnpf+strains. Ways are discussed of exploiting this combination of properties in a general procedure for isolating and testing diploids for dominance and complementation of amoebal mutations inP. polycephalum.