Slime Molds

Slime molds were once placed in the kingdom Fungi, although some authorities now place them in the kingdom Protoctista (Protista). Most fungi are composed of threadlike filaments of eukaryotic cells called hyphae, collectively referred to as a mycelium. Slime molds are composed of an acellular mass of naked protoplasm with no cell walls in its vegetative state. They are typically found in damp, shady areas with abundant organic matter, although they may move to bright areas to "fruit."

The vegetative stage is essentially a multinucleate mass of protoplasm (called a plasmodium) composed of numerous diploid nuclei. Depending on the species, the plasmodium may be only a few millimeters in diameter to large pizza-sized slime molds over 12 inches (30 cm) across. The plasmodium moves like a giant amoeba, flowing over the surface as it ingests organic matter, such as dead leaves and wood. It may be be brightly colored, such as the yellow Physarum polycephalum commonly studied in general botany laboratories. Some tropical slime molds are bioluminescent and glow in the dark. Slime molds are important heterotrophs in the decomposition of dead organic matter in temperate and tropical forests.

The transformation of the plasmodium into spore-bearing "fruiting bodies" or fructifications can occur within hours. Depending on the species, these fructifications can be in the form of sporangia, aethalia or plasmodiocarps. Sporangia may be stalked and have unique shapes (e.g. Stemonitis and Dictydium). Aethalia are large, thickened, cushion-shaped structures containing numerous spores (e.g. Fuligo). Plasmodiocarps often retain the branching habit of the original plasmodium (e.g. Hemitrichia). Plasmodiocarps are the most primitive type of fructification, while sporangia are the most advanced.

The life cycle of a slime mold begins with a haploid spore which is produced inside the fructification by meiosis. The germinated spore can transform into either an amoeba-like myxamoeba cell or a flagellated swarm cell. Both myxamoeba and swarm cells divide into large populations. This stage composed of amoeboid and flagellated cells is why some authorities place the slime molds in the kingdom Protoctista. Two myxamoeba cells or two swarm cells may join together in a cellular fusion (plasmogamy) followed by nuclear fusion (karyogamy). The combining of their cell contents (protoplasts) represents a very primitive form of sexual reproduction. It also provides a source of genetic variability; however, swarm cells always fuse with swarm cells and myxamoeba cells always fuse with myxamoeba cells. The fusion of two haploid cells results in a diploid zygote which transforms into a developing plasmodium. As the diploid nuclei divide, the plasmodium grows larger and larger, slowly moving (creeping) along the forest floor and feeding like a giant amoeba. The plasmodium of some slime fungi (Fuligo) can grow to the size of a large pizza.