Analysis of cellular events during plasmodium development in Physarum polycephalum
In Physarum polycephalum, uninucleate, haploid amoebae develop into multinucleate syncytial plasmodia. Plasmodium development is controlled by the mating-type locus matA. Sexual development involves the fusion of pairs of amoebae carrying different alleles of matA; fusion between amoebae carrying the same allele of matA does not lead to plasmodium formation. Apogamic development is caused by mutations at this locus. Time-lapse cinematographic analysis of matA-heteroallelic and matA-homoallelic cultures indicated that amoebae were able to fuse at any age. In matA-heteroallelic cultures, amoebal fusion was followed by nuclear fusion, in interphase, to give a diploid zygote. The zygote underwent an extended period of growth before forming a binucleate plasmodium by mitosis unaccompanied by cytokinesis. During this cell cycle, the cells lost the ability to transform into flagellates and became irreversibly committed to development. Immunofluorescence microscopy showed that the change from amoebal to plasmodial microtubule organisation began during this cell cycle. In matA-homoallelic fusion cells, the cell cycle was not extended and there were no alterations in microtubule organisation. In apogamic strains, single haploid amoebae could develop into haploid plasmodia; developing amoebae entered an extended cell cycle ending in the formation of a binucleate plasmodium. As in sexual development, growth continued during this cell cycle, ability to undergo the amoeba-flagellate transformation was lost, the developing cell became committed to development and microtubule organisation began to alter. Development was analysed in two apogamic strains carrying additional mutations blocking plasmodium development. In both strains, development began with an extended cell cycle, leading to the formation of a binucleate plasmodium; development became abnormal shortly after this time. In one strain, the mutation had apparently affected the cytoskeleton or the cell membrane. In the other strain, nuclear structure appeared to be affected by the mutation.