Slime Moulds Life Cycle

Slime moulds inhabit forest soil and consume bacteria and yeast, which they track by chemotaxis - chemical gradient sensing (of folic-acid in this case). Starvation, however, prompts the solitary cells to aggregate and develop as a true multicellular organism, producing a fruiting body comprised of cellular, cellulosic stalk supporting a bolus of spores. Thus, slime mould has evolved mechanisms that direct the differentiation of a homogeneous population of cells into distinct cell types, regulate the proportions between tissues and orchestrate the construction of an effective structure of the dispersal of spores. Many of the genes necessary for these processes in slime mould were inherited by Metazoa (animal with specialized cells) and fashioned through evolution for use within many different modes of development. Analysis of the proteins shows that slime mould diverged from the animal-fungal lineage after the plant-animal split, but it seems to have retained more of the diversity of the ancestral genome than have plants, animals or fungi, i.e., it possesses a level of complexity that is greater than the yeast, but much simpler than plants or animals. It represents one of the earliest branches from the last common ancestor of all eukaryotes (see Figure 01).

Figure 01 Multicelluarity
[view large image]

They begin their life from spore germination and multiplication.
The basic units of slime moulds are haploid amoebae, usually prowling around on the forest floor consuming bacteria.
Once the supply of food is exhausted, individual amoebae begin to move together. They form streams of cells called pseudoplasmodium moving slowly at about one millimeter/hour. It is now known that the process is initiated with the cAMP¹ signaling molecules released by the starving amoebae.
Eventually the streams come together and form an aggregation which is at the spot with high concentration of cAMP. The mass can number a hundred thousand cells and might reach a size of a few millimeters.
Then they stick together by secreting adhesion molecules and creating a slime sheath (cap) which covers all the cells in the mass. The polarity defined by the anterior (front), posterior (back) ends is established by oxygen gradient. The responsible gene for adhesion can be identified by blocking its expression. The resulting mass becomes a loose rubble that is incapable of further development.

Figure 02 Slime Mould Life Cycle [view large image]

	The mass of cells now begins to behave as one, gliding around on the ground as if it were a miniature garden slug leaving a track of slime behind (and hence the name, Figure 03). The movement of the slug is guided by the higher concentration of cAMP at the anterior region of the cap.
Figure 03 Multicelluar Development [view large image]

The migration continues toward higher concentration of ammonia (giving off by decaying organism), which means food for them.
The slug stops at a favourable spot. The anterio cells become a stalk while the posterior cells climb up and form a spherical head, the sorocarp, which develops into spores. The pattern in this fruiting body is based on a constant ratio of cells in the stalk and in the sorocarp (about 3/4). The cells have now been differentiated into a non-reproductive stalk and spores carrying the next generation. The stalk would soon wither away once its mission (of spore dispersal) has been accomplished. It is found that some strain of slime moulds manage to stay behind in the posterior area of the slug and thus become spores all the time; only their genes get copied into the next generation.
Finally, on germination of a spore, a pore appears in the cellulose coat through which the amoeba is liberated to begin a new cycle. This life cycle is asexual, there is no fusion of male and female sex cells (gametes). They exist entirely in haploid state. The slime moulds also run a sexual life cycle which is very different from the one just described.

¹cAMP is one of the morphogens (signaling molecules) identified in slime mould. The others include ammonia, DIF (signal to induce stalk formation), adenosine, and calcium.