Analysis of the amoebal-plasmoidal transition in physarum polycephalum.

This thesis describes work carried out to analyse a developmental process in a eukaryote, the amoebal-plasmodial transition in the Myxomycete Physarum polycephalum. Macroscopic, multinucleate plasmodia are normally formed by fusion of microscopic, uninucleate amoebae carrying different alleles of the mating type (mt) locus; haploid amoebae give rise to diploid plasmodia. Amoebae carrying the mth allele are capable of undergoing the amoebal-plasmodial transition with high efficiency within clones to give haploid plasmodia. Chapter 1 contains an introduction to the life cycle of P.polycephalum followed by a review of previous work on the amoebal-plasmodial transition in P.polycephalum and some other Myxomycetes. The work described in this thesis is briefly introduced and details of materials and methods are given. Chapter 2 describes studies undertaken to determine whether amoebal fusions occur within clonal cultures. These studies mainly concern mth strains; no clear conclusions emerge from genetical analyses but a cinematographic analysis indicates that plasmodium formation in mth strains occurs without amoebal fusion. Chapter 3 describes the isolation and genetical analysis of mutants defective in clonal plasmodium formation. The results of detailed analysis of 13 mutants allow three new genes to be identified. Two of these genes (npfB and npfC) are closely linked within the mating type locus. Strains carrying npfB- alleles mimic mt2 strains. A third gene (npfA) is unlinked to mt and a locus (apt-1) previously shown to be involved in plasmodium formation. A mutant allele of npfA appears to have no effect on crossed plasmodium formation, though it renders clonal plasmodium formation temperature-sensitive. Chapter 4 and 5 describe preliminary attempts to characterize physiologically and biochemically the mutant strains whose isolation and genetical analysis are described in Chapter 3. Chapter 6 contains a discussion of the work described in this thesis, with particular reference to the possible roles of the mating type locus.