Physarum polycephalum mutants in the photocontrol of sporulation display altered patterns in the correlated expression of developmentally regulated genes.

Physarum polycephalum is a lower eukaryote belonging to the amoebozoa group of organisms that forms macroscopic, multinucleate plasmodial cells during its developmental cycle. Plasmodia can exit proliferative growth and differentiate by forming fruiting bodies containing mononucleate, haploid spores. This process, called sporulation, is controlled by starvation and visible light. To genetically dissect the regulatory control of the commitment to sporulation, we have isolated plasmodial mutants that are altered in the photocontrol of sporulation in a phenotypic screen of N-ethyl-N-nitrosourea (ENU) mutagenized cells. Several non-sporulating mutants were analyzed by measuring the light-induced change in the expression pattern of a set of 35 genes using GeXP multiplex reverse transcription-polymerase chain reaction with RNA isolated from individual plasmodial cells. Mutants showed altered patterns of differentially regulated genes in response to light stimulation. Some genes clearly displayed pairwise correlation in terms of their expression level as measured in individual plasmodial cells. The pattern of pairwise correlation differed in various mutants, suggesting that different upstream regulators were disabled in the different mutants. We propose that patterns of pairwise correlation in gene expression might be useful to infer the underlying gene regulatory network.

Futile attempts to differentiate provide molecular evidence for individual differences within a population of cells during cellular reprogramming.

The heterogeneity of cell populations and the influence of stochastic noise might be important issues for the molecular analysis of cellular reprogramming at the system level. Here, we show that in Physarum polycephalum, the expression patterns of marker genes correlate with the fate decision of individual multinucleate plasmodial cells that had been exposed to a differentiation-inducing photostimulus. For several hours after stimulation, the expression kinetics of PI-3-kinase, piwi, and pumilio orthologs and other marker genes were qualitatively similar in all stimulated cells but quantitatively different in those cells that subsequently maintained their proliferative potential and failed to differentiate accordingly. The results suggest that the population of nuclei in an individual plasmodium behaves synchronously in terms of gene regulation to an extent that the plasmodium provides a source for macroscopic amounts of homogeneous single-cell material for analysing the dynamic processes of cellular reprogramming. Based on the experimental findings, we predict that circuits with switch-like behaviour that control the cell fate decision of a multinucleate plasmodium operate through continuous changes in the concentration of cellular regulators because the nuclear population suspended in a large cytoplasmic volume damps stochastic noise.

A habitat colonisation model for spore-dispersed organisms: does it work with eumycetozoans?

Spore productivities and establishment probabilities of eumycetozoans were estimated and compared with quantitative data obtained from field surveys, using series of cultures of a given substrate. Spore numbers per spore case were found to increase from one to four in protostelids to up to 10(5)-10(6) in myxomycetes, whereas average spore size decreased slightly from 14.8 microm for protostelids to 10.3 microm in myxomycetes. Spore numbers of fructifications calculated from dimensions of spores and fruit bodies were in good agreement with direct counts carried out for six species of myxomycetes. A colonisation model is presented that estimates frequencies (as a percent of successfully colonized habitat islands), which is independent of a given density of spore rain and the sexual system of the species being considered. Whereas asexual species need a minimum spore rain of ca 0.7 spores per habitat island to reach a frequency of 50%, this figure is at least 2.4-fold higher for sexual species, depending from the incompatibility system assumed. Data from cultures indicate that the maximum potential spore rain is usually three orders of magnitude higher than the minimum figure required to create the observed frequencies. Eumycetozoans seem to follow the evolutionary trends predicted by the model. Species with sexual reproductive systems produce often more spores than asexual ones; many morphospecies have sexual and asexual strains; and back-conversion from sexual to asexual reproduction occurs occasionally.

Species richness and ecological characterization of myxomycetes and myxomycete-like organisms in the canopy of a temperate deciduous forest.

The ecological community of myxomycetes and myxomycete-like organisms (MMLO) in the canopy of living deciduous trees was studied in a riparian deciduous forest at Leipzig, Germany. A systematic survey carried out with a total of 146 moist chamber cultures resulted in 386 records of 37 taxa, with 32 myxomycetes, two myxobacteria, two protostelids and the fruit body forming ciliate Sorogena stoianovitchae, the latter recorded for the first time for Europe. With 94% of all cultures positive for MMLO, these organisms are present consistently in the investigated sections of white-rotten twigs attached to living trees at 10-30 m above the ground. Our sampling recovered a majority of the likely species, with 37 out of the 42-45 predicted according to a species-accumulation curve and two other estimators of species richness. Nonmetric multidimensional scaling revealed pH, water-holding capacity and stage of decay to explain most of the variation in species distribution. Arcyria cinerea and Perichaena depressa as the most common species occurred in 32% and 29% of all samples, respectively. Viewing the sampled twigs as habitat islands and a single spore as sufficient to establish a population, a simulation program assuming a random spore rain estimated an average of 0.4 and 0.35 spore hits per twig as necessary to explain the observed frequencies. This is matched by the potential productivity of the substrate. All fruit bodies from the cultured twigs would be able to create a spore rain of 86 (A. cinerea) or 40 (P. depressa) spore hits per twig when dispersed evenly over the plot. The terminal fall velocity of spores was measured, revealing that it took about 5 h for a spore to land (30 m) in still air and indicating high dispersal ability for canopy-inhabiting MMLO.