Chapter 28: Phylum Acrasiomycota: Acrasid Cellular Slime Molds

The defining feature of acrasid amoebae is their limax morphology, which consists of a single lobose pseudopodium enabling slug-like movement through cytoplasmic flow patterns that gave rise to their common name. A fundamental distinction separates acrasids from the related Dictyosteliomycota: their aggregation behavior occurs individually or in minimal clusters without cyclic adenosine monophosphate signaling, and their multicellular fruiting structures, called sorocarps, form without the characteristic migratory slug stage present in dictyostelids. Acrasid sorocarps are structurally simpler, lacking the cellulose-reinforced stalk tubes found in other slime mold groups, and crucially, all cells retain the capacity to germinate and revert to solitary amoebae, including cells that differentiate into stalk tissue. The chapter surveys morphological diversity across acrasid genera, noting that certain groups such as Acrasis and Guttulina produce flagellated cells, an uncommon trait within cellular slime molds, while specialized genera like Copromyxa and Copromyxella possess distinctive tubular mitochondrial cristae in contrast to the platelike cristae typical of most acrasids. The life cycle of Acrasis rosea exemplifies acrasid development, beginning with spore germination into pigmented amoebae containing unique cytoplasmic p-bodies and carotenoid compounds, progressing through encystation as microcysts under adverse environmental conditions, and culminating in multicellular sorocarp formation under favorable circumstances. Sexual reproduction remains undocumented in these organisms, though evidence hints at variation in cellular fusion mechanisms and nuclear dynamics. Classification encompasses four recognized families including Acrasidae, Guttulinopsidae, Copromyxidae, and Fonticulidae, though some genera exhibit ambiguous phylogenetic placement. Contemporary evolutionary research suggests potential connections to heterolobose amoebae including Naegleria, positioning acrasids within broader protistan evolution and highlighting their significance as transitional organisms that challenge traditional taxonomic boundaries.