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

SUMMARY Plasmodium formation in the Myxomycete Physarum polycephalum normally involves fusion of haploid amoebae, carrying different alleles at the mating type ( mt ) locus, to give diploid plasmodia. Strains carrying the mt h allele are capable of undergoing the amoebal–plasmodial transition with high efficiency within amoebal clones, resulting in the formation of haploid plasmodia. NMG mutagenesis of mt h amoebae, 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 a mt 1 strain. Three new genes were identified and designated npf A, npf B and npf C. A mutant allele of npf A rendered clonal plasmodium 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 between npf B or npf C and mt , but npf A was unlinked to mt and a locus ( apt -1) shown in a previous study to be involved in plasmodium formation. The genes npf B and npf C were distinguished by complementation analysis. Strains of the genotype npf B − ; npf C + behaved in the same way as strains carrying the mt 2 allele. The nature of the mutants and the role of the mating-type locus in the initiation of plasmodium formation are discussed.