Dictyostelium Discoideum - Life Cycle and Reproduction

Life Cycle and Reproduction

The life cycle of D. discoideum begins as spores are released from a mature sorocarp (fruiting body). Myxamoebae hatch from the spores under warm and moist conditions. During their vegetative stage, the myxamoebae divide by mitosis as they feed on bacteria. The bacteria secrete folic acid, attracting the myxamoebae. When the supply of bacteria is depleted, the myxamoebae enter the aggregation stage.

During aggregation, starvation initiates the creation of a biochemical machinery which includes glycoproteins and adenylyl cyclase. The glycoproteins allow for cell-cell adhesion and adenylyl cyclase creates cyclic AMP. Cyclic AMP is secreted by the amoebas to attract neighboring cells to a central location. As they move towards the signal, they bump into each other and stick together by the use of glycoprotein adhesion molecules.

The migration stage begins once the amoebas have formed a tight aggregate and the elongated mound of cells tip over to lie flat on the ground. The amoebas work together as a motile pseudoplasmodium, also known as a slug. The slug is approximately 2–4 mm long and is capable of movement by producing a cellulose sheath in its anterior cells through which the slug moves. Part of this sheath is left behind as a slimy trail as it moves towards attractants such as light, heat, and humidity in a forward-only direction. Cyclic AMP and a substance called differentiation-inducing factor (DIF), help to form different cell types. The slug becomes differentiated into prestalk and prespore cells that move to the anterior and posterior ends, respectively. Once the slug has found a suitable environment, the anterior end of the slug will form the stalk of the fruiting body and the posterior end will form the spores of the fruiting body. Anterior-like cells, which have only been recently discovered, are also dispersed throughout the posterior region of the slug. These anterior-like cells form the very bottom of the fruiting body and the caps of the spores. After the slug settles into one spot, the posterior end spreads out with the anterior end raised in the air, forming what is called the "Mexican hat," and the culmination stage begins.

The prestalk cells and prespore cells switch positions in the culmination stage in order to form the mature fruiting body. The anterior end of the Mexican hat forms a cellulose tube, which allows the more posterior cells to move up the outside of the tube to the top, and the prestalk cells move down. This rearrangement forms the stalk of the fruiting body made up of the cells from the anterior end of the slug, and the cells from the posterior end of the slug are on the top and now form the spores of the fruiting body. At the end of this 8–10 hours process, the mature fruiting body is fully formed. This fruiting body is 1–2 mm tall and is now able to start the entire cycle over again by releasing the mature spores that become myxamoebae.

Generally reproducing asexually, D. discoideum are still capable of sexual reproduction if certain conditions are met. If two amoebae of different mating types are present in a dark and wet environment, they can fuse during aggregation to form a giant cell. The giant cell will then engulf the other cells in the aggregate and encase the whole aggregate in a thick, cellulose wall to protect it. This is known as a macrocyst. Inside the macrocyst, the giant cell divides first through meiosis, then through mitosis to produce many haploid amoebae that will be released to feed as normal amoebae would. While sexual reproduction is possible, it is very rare to see successful germination of a D. discoideum macrocyst under laboratory conditions.