Polysphondylium pallidum

In damp weather, a Polysphondylium pallidum myxamoeba can move around at a speed of about 1 millimetre (0.04 in) per hour, leaving a chemical trail behind it. When it finds the trail left by another myxamoeba, it follows it, superimposing its own trace signal, and more and more individuals collect together in this way. Under favourable conditions, the myxamoebae agglutinate and stick together to form a "pseudoplasmodium" in which they remain separate individuals but behave as if the whole mass was a single organism. The pseudoplasmodium can move around and in due course develops into a fruiting body called a sporangium, about a third of the cells forming a stalk and the remaining cells forming a ball at the top where they develop into spores. The spores have a smooth wall containing cellulose, a material not found among fungi which have cell walls strengthened by chitin. As the ball dries out, the spores are dispersed by the wind. The myxamoebae that form the supporting stem die, having sacrificed themselves for the greater good.^[5][7]

Slime moulds are of interest to developmental biologists because they represent a link between single-celled organisms and multi-celled organisms.^[4] In an experiment to study the conditions necessary for agglutination, Polysphondylium pallidum was cultured on plates of hay-infusion agar. A thin surface layer of Escherichia coli was added, created by allowing a drop of suspension to spread out over the surface of the non-nutrient agar to form a layer of even thickness. The myxamoebae were inoculated centrally on the plate. The culture was then incubated under different conditions of light and darkness and it was found that agglutination was greatly increased by exposure to light. Even one minute of illumination soon after incubation had started was sufficient to trigger multiple centres of agglutination. One minute of exposure at a later stage was less effective.^[8] Although light was involved in agglutination, the introduction of some light exposed myxamoebae did not cause dark-only cells to clump. Other possible stimuli were tried but mostly had negative responses; heat was ineffective; CO₂ had little effect but reduced clumping in light treated cells; potassium hydroxide had little effect; charcoal increased the number of clumps in light treated cells and caused a slight increase in number of dark cells that agglutinated; mineral oil has similar effects to charcoal in light treated cells but a more marked effect in the dark cells where the resulting aggregations nearly reached that found in light treated cultures. The conclusions drawn from these experiments were that some form of suppressor may accumulate in the vicinity of the myxamoebae preventing them from agglutinating. Light exposure encouraged agglutination before much suppressor was present. Charcoal and mineral oil increased agglutination by absorbing the suppressor.^[8]