An extracellular inducer of asexual plasmodium formation in Physarum polycephalum.

Abstract

Asexual conversion of amoebae to plasmodia was studied in the Colonia isolate of the myxomycete, Physarum polycephalum. When a culture of Colonia amoebae is grown on a bacterial lawn, a period of amoebic growth precedes the appearance of cells committed to the plasmodial state. The onset of plasmodium production appears to be related to amoebic nutrition since cultures supplied with fewer bacteria display earlier differentiation. For a period of time after differentiation is initiated, conversion of amoebae to plasmodia is rapid and proceeds as an exponential function of time. A filter-transmissible substance, apparently released by differentiating cells, is implicated in the control of this rapid conversion. The myxomycete Physarum polycephalum displays two strikingly different vegetative forms: microscopic, uninucleate, colorless amoebae and macroscopic, multinucleate, yellow plasmodia (1, 2). Amoebae of the Colonia isolate, which carry the allele mth at the mating type locus, readily undergo an asexual conversion to the plasmodial state. Because the change occurs without genetic alteration (3) and results in major, stable phenotypic alterations, the material provides a model system for studies on the control of cell differentiation. Mutants affecting the differentiation can be isolated (refs. 4 and 5; L. Davidow and C. E. Holt, manuscript in preparation; P. N. Adler, manuscript in preparation), and the present work provides a beginning for physiological and biochemical studies on the process of commitment to the plasmodial state.We report here the development of a technique which permits a quantitative analysis of the time course and extent of differentiation in a Colonia culture. With this technique, we have demonstrated that a differentiating culture of Colonia cells can induce early differentiation in a neighboring culture separated by filters which prevent direct cell contact between the two populations. The results favor the conclusion that differentiating cells elaborate a diffusible inducer of differentiation. MATERIALS AND METHODSMedia. Dilute plasmodial rich medium (dPRM) agar and liver infusion agar were made as described previously (4, 6). Agar containing dPRM adjusted to pH7 (dPRM7) rather than pH 4.6 was also used. Buffer-streptomycin agar was made by adding 0.25 g streptomycin sulfate (Sigma Chemical Co.) and 10 ml of 1 M citrate buffer (pH 5) to 1 liter of 1.5% agar. Final concentrations were 250,ug/ml and 0.01 M, respectively.Preparation of Amoebae. Plasmodia-free amoebae for use in starting kinetics experiments were prepared by growing the amoebae on agar plates at 300 and harvesting the amoebae prior to the onset of plasmodium formation. For experiments with live bacteria, the amoebae were grown on lawns of live Escherichia coli on liver infusion agar. For experiments with formalin-killed bacteria (7), the amoebae were subcultured serially on formalin-killed E. coli covered buffer-streptomycin plates to ensure elimination of live bacteria. Kinetic Experiments. At time zero, replica...

Activation of a -Tubulin Gene during Early Development of the Plasmodium in Physarum polycephalum

“…As a consequence, the total number of cells harvested at later stages of differentiation does not increase (data not shown). The number of amoebae decreases due both to transition into committed cells and to ingestion by binucleate cells (Youngman et al, 1977). The number of committed cells does not change substantially, but the percentage of committed cells still increases (see also Blindt et a/., 1986).…”

“…CL amoebae grow 0001-4550 0 1988 SGM and differentiate on agar plates with bacteria as a food source. In differentiating cultures of CL, amoebae grow and divide until they reach a certain cell density, at which time a presumptive extracellular inducing activity reaches a certain concentration (Youngman et al, 1977 ; Nader et al, 1984) and plasmodial development begins. Detailed cinematographic and cytological studies have revealed that uninucleate CL amoebae form multinucleate plasmodia via the following distinct intermediate stages :…”

A Cytophotometric Investigation of Haploid × Polyploid Matings in the Acellular Slime MoldDidymium Iridis

“…In a recent study we have demonstrated that haploid myxamoebae are not induced by polyploid myxamoebae to differentiate into plasmodia without ploidy change by a membrane transmissable factor (RITCH and THERRIEN 1987), even though soluble factors have been implicated in the induction of normal sexual competency in heterothallic Myxomycetes (Ross et at. 1973;SHIPLEY and HoLT 1982;ALBERT and THERRIEN 1985), and in the conversion of myxamoebae to plasmodia in the apomictic Colonia isolate of Physarum potycephatum (YouNGMAN et at. 1977).…”

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

“…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.…”