Acrasidae

Acrasid slime molds
Acrasis rosea
Scientific classification
Domain:	Eukaryota
Clade:	Discoba
Superphylum:	Discicristata
Phylum:	Percolozoa
Class:	Heterolobosea
Order:	Acrasida
Family:	Acrasidae van Tieghem 1880 ex Hartog 1906
Genera
Acrasis van Tieghem 1880 Pocheina Loeblich & Tappan 1961
Synonyms
Acrasiaceae Poche 1913 em. Olive 1970 Guttulinaceae Zopf 1885 ex Berlese 1888 nom. rej. Guttulininae Doflein & Reichenow 1952 Pocheinaceae Loeblich & Tappan 1961 nom. cons.

Acrasis rosea amoebae and spores under microscope

The family Acrasidae (ICZN, or Acrasiomycota, ICBN) is a family of slime molds which belongs to the excavate group Percolozoa. The name element acrasio- comes from the Greek akrasia, meaning "acting against one's judgement". This group consists of cellular slime molds.

The terms "Acrasiomycota" or "Acrasiomycetes" have been used when the group was classified as a fungus ("-mycota"). In some classifications, Dictyostelium was placed in Acrasiomycetes, an artificial group of cellular slime molds, which was characterized by the aggregation of individual amoebae into a multicellular fruiting body, making it an important factor that related the acrasids to the dictyostelids.

Each cell keeps its individuality even when it forms a stalk and fruiting body to reproduce. Slime molds were originally thought to be in a monophyletic group Mycetozoa, with little distinction between Acrasis and Dictyostelids, however scientists uncovered that they were distinct groups, and eventually that Acrasis was incredibly distant on the tree of life. Instead, it is found in Heterolobosia with Naegleria, away from other myxamoeba.

Ecology

Acrasis is found in terrestrial habitats on dead or decaying bark or dead tissue still attached to plants. They are often cultured using yeast which makes up most of their diet, but they are known to participate in cannibalism in their solitary mobile stage of life. They may also be found on living tree bark.

Evolutionary History

Historically it was thought that Acrasis was a sister group to Dictyostilids, other slime mold amoebas that belong to Amoebazoa, due to how they both aggregate in order to form a fruiting body. However, in their amoeboid form it was realized they were fundamentally different and molecular phylogenetic studies placed Acrasis in Heterolobosea with the brain-eating amoebae Naegleria fowleri. One particular morphological difference between Acrasis and Dictyostilids is that the stalks of the fruiting body in Acrasis are trunk like and do not contain a cellulose sheath.

Heterolobosea belong within Discoba which belongs within Excavata. Out of all Discobids, Acrasis has the most compact mitochondrial genome that requires additional transport activity due to the number of genes lost. tRNA genes, which are commonly found in most mitochondria sequences, are scarce in Acrasis and require transportation in for the translation of the remaining mitochondrial genes. The reason for this gene deficiency is because of lateral genetic transfer from the mitochondria to the nucleus. What is uncommon is how recently this phenomenon occurred in the Acrasis lineage, as sequence comparisons indicate lateral gene transfer after Acrasis split with Naglaeria. This has rarely been found in any other living species of Eukarya including Naglaeria, as most transfers between the mitochondria and nucleus happened in early endosymbiosis of mitochondrial ancestor into the ancestor of all Eukarya.

Reproduction

When resources such as water or food become limiting, the amoeba will release pheromones such as acrasin to aggregate amoebal cells in preparation for movement as a large (thousands of cells) grex or pseudopod. When in the grex, the amoeboids reproduce, resulting in fruit-like structures called spores, which develop into unicellular molds of the same species.

Its reproductive cycle can be broken up to three distinct life stages where the Acrasis cell experiences morphological and intracellular changes [2].

Vegetative/Solitary Stage

After Acrasis spores are released, they germinate into free living limax amoebae, where they use a single pseudopodium to move forward, reaching to become up to 32 micrometers long. During this stage they may experience conditions of starvation or dehydration where they differentiate into a microcyst that has an extracellular cell wall. This microcyst can then differentiate back into the limax amoebae form. Alternatively, if conditions are favorable, a stimulus can signal the amoebae to aggregate together.

Pseudoplasmodial Stage

Upon stimulation they begin to aggregate into the “slug” that will eventually begin to form a mound with others of the same species. Each cell keeps its individuality and only minor intracellular alterations are seen. One of the alterations seen in the cells between the vegetative and pseudoplasmodial stage is the decrease in number and volume of food vacuoles. 

Differientiated Stage

Within the mound one amoeba differentiates into a stalk cell that the others rest atop of, creating a structure called the sorogen. After the stalk grows from repeated cell differentiation into basal stalk cells, select cells form distal spore cells and the sporocarp structure from which they are released. This forms the fruiting body that overall has great plasticity through the ability to branch. Throughout this process from the solitary stage to the formation of the fruiting body, each cell maintains its individuality. Following the formation of the fruiting body, spores are released, and the cycle begins anew.

Further reading

• C.J. Alexopolous, Charles W. Mims, M. Blackwell et al., Introductory Mycology, 4th ed. (John Wiley and Sons, Hoboken NJ, 2004) ISBN 0-471-52229-5