Isolation and Analysis of Amoebal–Plasmodial Transition Mutants in the MyxomycetePhysarum polycephalum

Abstract

SUMMARYPlasmodium formation in the MyxomycetePhysarum polycephalumnormally involves fusion of haploid amoebae, carrying different alleles at the mating type (mt) locus, to give diploid plasmodia. Strains carrying themthallele are capable of undergoing the amoebal–plasmodial transition with high efficiency within amoebal clones, resulting in the formation of haploid plasmodia. NMG mutagenesis ofmthamoebae, followed by an enrichment procedure, was used to isolate mutants in which such clonal plasmodium formation was either delayed or absent. Thirteen mutants of the second type were analysed. Three of these were temperature-sensitive for plasmodium formation. All thirteen mutants were able to form diploid crossed plasmodia when mixed with amt1strain. Three new genes were identified and designatednpfA,npfB andnpfC. A mutant allele ofnpfA renderedclonalplasmodium formation temperature-sensitive, but did not prevent crossing at the non-permissive temperature with derived strains carrying the same mutant allele. No recombination was detected betweennpfB ornpfC andmt, butnpfA was unlinked tomtand a locus (apt-1) shown in a previous study to be involved in plasmodium formation. The genesnpfB andnpfC were distinguished by complementation analysis. Strains of the genotypenpfB−;npfC+behaved in the same way as strains carrying themt2allele. The nature of the mutants and the role of the mating-type locus in the initiation of plasmodium formation are discussed.