Amoebae	Aggregation
Slug
Fruiting Body	Dictyostelium Discoideum Life Cycle

Dictyostelium discoideum is a cellular slime mould whose natural habitat is soil and leaf litter where it predates bacteria by phagocytosis, grows and divides by mitosis. You can hear more about it in radio interviews broadcast on 29th August, 2009 (ABC Science Show interview by Robyn Williams) and the 25th June 2000 (as a ca. 4 Mb mp3 file or a ca. 9 Mb wav file on "Einstein-A-Go-Go", the weekly Sunday science broadcast from the Melbourne community radio station 3RRR).

Dictyostelium is one of a handful of nonmammalian model organisms recognized by the NIH for their importance in biomedical research. The intracellular signalling pathways that are disturbed in diseases of the central nervous system arose in the common ancestor of plants, slime moulds, fungi and animals with the result that they can be profitably studied in simple, readily manipulated organisms like Dictyostelium.

The Dictyostelium discoideum life cycle begins with differentiation of starving amoebae to a form where they become capable of synthesizing, secreting and being attracted by extracellular cAMP. The resulting aggregation process forms a multicellular migratory organism, the "slug", which migrates through a cellulose/protein extracellular matrix, the "slime sheath", that collapses behind to form a trail. Slugs are phototactic, thermotactic and weakly chemotactic. After a variable period of migration the slug stops and forms a fruiting body consisting, to a first approximation, of a droplet of spores supported by a tapered stalk and basal disc.

The behaviour and morphogenetic movements of the slug are controlled by the slug's tip (its "brain") via what are believed to be extracellular tip activation and inhibition signals. The tip activation signal is probably carried by 3-dimensional scroll waves of cAMP emanating from the tip. The tip inhibition signal has been proposed to be carried by a small non-volatile, diffusible molecule (Slug Turning Factor, STF), and/or ammonia, and/or adenosine. Phototactic and thermotactic behaviour seem to be controlled by modulation of the tip activation/inhibition system.