Cellular events during sexual development from amoeba to plasmodium in the slime mould Physarum polycephalum

Abstract

Time-lapse cinematography and immunofluorescence microscopy were used to study cellular events during amoebal fusions and sexual plasmodium development in Physarum polycephulum. Amoeba1 fusions occurred frequently in mixtures of strains heteroallelic or homoallelic for the mating-type locus mafA, but plasmodia developed only in the matA-heteroallelic cultures. These observations confirmed that mafA controls development of fusion cells rather than cell fusion. Analysis of cell pedigrees showed that, in both types of culture, amoebae fused at any stage of the cell cycle except mitosis. In mafA-heteroallelic fusion cells, nuclear fusion occurred in interphase about 2 h after cell fusion; interphase nuclear fusion did not occur in mafA-homoallelic fusion cells. The diploid zygote, formed by nuclear fusion in mafA-heteroallelic fusion cells, entered an extended period of cell growth which ended in the formation of a binucleate plasmodium by mitosis without cytokinesis. In contrast, no extension to the cell cycle was observed in mafA-homoallelic fusion cells and mitosis was always accompanied by cytokinesis. In rnatA-homoallelic cultures, many of the binucleate fusion cells split apart without mitosis, regenerating pairs of uninucleate amoebae; in the remaining fusion cells, the nuclei entered mitosis synchronously and spindle fusion sometimes occurred, giving rise to a variety of products. Immunofluorescence microscopy showed that rnatA-heteroallelic fusion cells possessed two amoebal microtubule organizing centres, and that most zygotes possessed only one; amoebal microtubule organization was lost gradually over several cell cycles. In mafAhomoallelic cultures, all the cells retained amoebal microtubule organization.

A developmental mutation (npfL1) resulting in cell death in Physarum polycephalum

“…The stage of plasmodium formation at which normal development ceased was variable. In npjL+ strains, there was similar variability in the timing of developmental changes in tubulin isotype expression, microtubular organization and cell behaviour, suggesting that variability was a normal part of plasmodium development (Blindt, 1987 ;Bailey et al, 1987Bailey et al, , 1990Solnica-Krezel et al, 1990. Our film of RA376 showed that there was also variability in the events at the end of the extended cell cycle; some developing cells divided rather than becoming binucleate and later became binucleate without passing through another long cell cycle.…”

“…When two amoebae carrying different matA alleles fused, nuclear fusion followed, in interphase, about two hours after amoebal fusion (Bailey et al, 1990). Since ability to undergo interphase nuclear fusion occurred only in matA-heteroallelic fusion cells, fusion may have been switched on by the presence of two unlike matA alleles in the same cell and was presumably switched off by the time of binucleate plasmodium formation.…”

“…A few of the multipolar spindles (Class V) seen in RA614 were tripolar with the DNA equally distributed between the poles; this type of spindle is characteristic of diploid amoebae (Lemoine et al, 1984;Bailey et al, 1990) which ocur at a low frequency even in vegetative cultures of amoebae. Most multipolar spindles seen in RA614, however, occurred in developing cells and had more than three spindle poles with the DNA unequally divided between the poles (Fig.…”

“…In the apogamic strain CL, binucleate plasmodium formation is preceded by an extended cell cycle of similar length to that observed in sexual development (Anderson et al, 1976;Bailey et al, 1987). During the extended cell cycle in both sexual and apogamic development, uninucleate cells become committed to plasmodium formation (Youngman et al, 1977;Burland et al, 1981) and many of the changes in cellular organization that accompany development are initiated (Blindt, 1987;Bailey et al, 1990;Solnica-Krezel et al, 1990.…”

“…1986). Time-lapse cinematography of sexual plasmodium formation has shown that nuclear fusion occurs shortly after amoebal fusion and the resulting diploid zygote enters an extended cell cycle at the end of which it becomes binucleate by mitosis without cytokinesis (Bailey et al, 1990). In apogamic strains, a haploid amoeba develops into a haploid plasmodium; such strains result from gad mutations which map at matA (gad = greater asexual differentiation; .…”