Dictyostelid

The dictyostelids are a group of cellular slime moulds. When food (normally bacteria) is readily available they take the form of individual amoebae, which feed and divide normally. However, when the food supply is exhausted, they aggregate to form a multicellular assembly, called a pseudoplasmodium or slug (not to be confused with the gastropod mollusc called a slug). The slug has a definite anterior and posterior, responds to light and temperature gradients, and has the ability to migrate. Under the correct circumstances the slug matures forming a fruiting body with a stalk supporting one or more balls of spores. These spores are inactive cells protected by resistant cell walls, and become new amoebae once food is available.

In Acytostelium, the fruiting body is supported by a stalk composed of cellulose, but in other dictyostelids the stalk is composed of cells, sometimes taking up the majority of the original amoebae. With a few exceptions, these cells die during stalk formation, and there is a definite correspondence between parts of the slug and parts of the fruiting body. Aggregation of amoebae generally takes place in converging streams. The amoebae move using filose pseudopods, and are attracted to chemicals produced by other amoebae. In Dictyostelium, aggregation is signalled by cAMP, but others use different chemicals. In the species Dictyostelium purpureum, the grouping is by kinship, not just by proximity.

Dictyostelium has been used as a model organism in molecular biology and genetics, and is studied as an example of cell communication, differentiation, and programmed cell death. It is also an interesting example of the evolution of cooperation and cheating (see e.g. Strassman et al., 2000, Dao et al., 2000, Brännström & Dieckmann 2005). A large body of research data concerning D.discoideum is available on-line at DictyBase.

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Mechanism of aggregation in Dictyostelium

  The mechanism behind the aggregation of the amoebae relies on Cyclic adenosine monophosphate (cAMP) as a signal molecule. One cell, the founder of the colony, begins to secrete cAMP in response to stress. Others detect this signal, and respond in two ways:

• The amoeba moves towards the signal.
• The amoeba secretes more cAMP to boost the signal.

The effect of this is to relay the signal throughout the nearby population of amoebae and cause inward movement to the area of highest cAMP concentration.

Within an individual cell, the mechanism is as follows:

1. cAMP reception at the cell membrane activates a G-protein
2. G protein stimulates Adenylate cyclase
3. cAMP diffuses out of cell into medium
4. Internal cAMP inactivates the external cAMP receptor.
5. A different g-protein stimulates Phospholipase C
6. IP₃ induces calcium ion release
7. Calcium ions act on the cytoskeleton to induce the extension of pseudopodia.

Because the internal cAMP concentration inactivates the receptor for external cAMP, an individual cell shows oscillatory behaviour. This behaviour produces beautiful spirals seen in converging colonies and is reminiscent of the Belousov-Zhabotinsky reaction and cyclic cellular automata.

Genome

The entire genome of Dictyostelium discoideum was published in Nature in 2005 by geneticist Ludwig Eichinger and coworkers. It contains approximately 12,500 genes on 6 chromosomes. For comparison, the human genome has 20,000-25,000 genes on 46 chromosomes. There is a high level of the nucleotides adenosine and thymidine (~77%) leading to a codon usage that favors more adenosines and thymidines in the third position. Tandem repeats of trinucleotides are abundant in Dictyostelium, which in humans cause Trinucleotide repeat disorders.

Classification

First discovered in a North Carolina forest in 1935, Dictyostelium was at first classified under 'lower fungi.' and in subsequent years into the kingdoms Proctoctista, Fungi and Tubulomitochondrae. By the 90s most scientists accepted the current classification.
The genome of Dictyostelids is more similar to those of animals and plants than those of fungi, and many of the genes regulating development of the foetus are likely derived from a very similar organism.

References

1. The genome of the social amoeba Dictyostelium discoideum (2005) Nature 435, 43-57
2. Dictyostelium (2007)
3. Low Society (2004)
Strassman JE, Zhu Y, and Queller DC. (2000) Altruism and social cheating in the social amoeba Dictyostelium discoideum. Nature.

Dao DN, Kessin RH, and Ennis HL (2000). Developmental cheating and the evolutionary biology of Dictyostelium and Myxococcus. Microbiology

Brännsröm Å and Dieckmann U (2005). Evolutionary dynamics of altruism and cheating among social amoebas. Proceedings of the Royal Society of London B.