Dictyostelia

Dictyostelia are common amoebae, mostly known from forest soil and litter. They respond individually to adverse conditions by forming simple amoeboid cysts (microcysts) or en masse by aggregation. Aggregates can include >100,000 amoebae and culminate in either thick-walled highly resilient macrocysts (zygocysts), the sexual cycle of Dictyostelia, or multicellular fruiting bodies (sorocarps). In contrast to Myxomycetes, Dictyostelia form pseudoplasmodia, and sorocarp formation in most species includes cellular differentiation and cell death (~20% of the aggregate). Traditional classification recognized three genera based on sorocarp morphology. However, these morphologies are now known to have evolved multiple times and thus correspond to morphotypes rather than phylogenetic taxa. Acytostelid morphotypes (traditional Acytostelium spp.) have tiny delicate sorocarps with acellular stalks (no cell death). Polysphondylid morphotypes (traditional Polysphondylium spp.) have cellular stalks bearing regularly spaced whorls of side branches. Dictyostelid morphotypes (traditional Dictyostelium spp.) also have cellular stalks but with diverse morphologies ranging from solitary sorocarps with a single sporehead to sorocarps with mul- tiple sporeheads on irregularly spaced side branches and/or various arrangements of clustered sorocarps. There are ~150 described species, which molecular phylogeny places into eight distinct divisions, and the taxonomy of group is now being formally revised accordingly. The best-known species is Dictyostelium discoideum, an important model organism widely used to study, e.g., cell signal- ing, cellular differentiation, and social behavior. Other taxa are also under devel- opment as models, including full genome data from all major divisions. Given its age (~600 myr), molecular depth, and small number of described species, it is expected that substantial dictyostelid diversity remains to be discovered.