D. discoideum - Teamwork without a brain.

Introduction

I’m in the mood for a wholesome story, so today we’re going to talk about teamwork, specifically about a funky wee slime mould - Dictyostelium discoideum. Even though we call it a slime mould, d. discoideum is not actually a fungus, but an amoeba. What makes d. discoideum so interesting is that it functions both on its own as a single-celled organism, or they can come together to form a multicellular slug able to travel. 

These social amoebae aren't just interesting to read about, they are also used as a model for multicellular development and have been important to improve our understanding of a number of human conditions.

Dictyostelium discoideum is able to go through one of two life cycles, an asexual cycle involving the formation of a fruiting body, or a sexual cycle that involves the formation of a macrocyst. This article will focus on the asexual cycle. 

Coming together

D. discoideum is a bacterivore, meaning it eats bacteria. When they find themselves starving they can send out chemical signals which essentially tell the surrounding d. discoideum “hey guys, I think we’re out of food. Want to leave?”. They do this by secreting a molecule called cyclic adenosine monophosphate (or cAMP) which acts as a chemical messenger, passing the message on to the other d. discoideum cells. cAMP is a chemoattractant which means it can cause cells to move in response to its presence (chemotaxis). The d. discoideum sents out cAMP in pulses that move outward like waves as shown in figure 1. Once cAMP binds to the cAMP receptors belonging to surrounding d. Discoideum several different signalling pathways can be triggered, the production of more cAMP to keep the wave going, changes in gene expression, and of course, chemotaxis(1,2,3). Throughout the process of aggregation, the surrounding d. Discoideum moves towards the source of the cAMP. 

Figure 1, waves of cAMP.

Split into groups

The d. Discoideum cells will then be able to differentiate into prespore or prestalk cells, which will eventually go on to mature into spore and stalk cells respectively, that will go on to form the fruiting body. Communicating through chemical signals, the amoeba will pile together to form a mound. The prestalk cells move faster towards the source of the cAMP than the prespore cells, forming the tip on top of the mound(4). 

The Slug, Culmination, and what they become

This pile of amoeba, now with a tip of prestalk cells, will form the slug. The slug migrates towards light and warmth and may travel several centimetres in search of a better environment (5). Trust me, for a group of amoeba that is very impressive. Unfortunately for the amoeba, they can’t go on forever and must culminate before they run out of energy. The culminating stalk cells reach upwards, which will use up all of their energy, killing them. The spore cells will accumulate at the top of the fruiting body, as shown in figure 2, ending in mature spore cells held up by a dead stalk.

Figure 2, a simplified journey from the mound to fruiting body.

Teamwork makes the dream work

Once the fruiting body has been formed d. Discoideum can finally achieve its goal, asexual reproduction. The mature spores are released, and while the amoeba that created the fruiting body are left behind, it is so that the new generation that rises out of this event may go on to thrive in an area hopefully with more bacteria to eat. 

Cells depend on chemical signalling and transcriptional changes as they undergo germination in stages, first spores that were once dormant in the fruiting body are activated, and then the activated spores swell, increasing in size before the final stage, the emergence of new amoeba(6,7,8,9).

Read on

Dictyostelium discoideum is an amoeba with the ability to come together in times of starvation to function as a multicellular slug. It's also really interesting to read about. Seriously, there's so much more than I could fit in one article and if you want to read up on it yourself, here are some resources. 

1. Iglesias PA, Devreotes PN. Navigating through models of chemotaxis. Current Opinion in Cell Biology. 2008 Feb 1;20(1):35–40.

2. Song L, Nadkarni SM, Bödeker HU, Beta C, Bae A, Franck C, et al. Dictyostelium discoideum chemotaxis: Threshold for directed motion. European Journal of Cell Biology. 2006 Sep 27;85(9):981–9.

3. Hereld D, Devreotes PN. Cyclic AMP Receptors of Dictyostelium. In: Lennarz WJ, Lane MD, editors. Encyclopedia of Biological Chemistry [Internet]. New York: Elsevier; 2004 [cited 2022 Jun 1]. p. 488–93. Available from: https://www.sciencedirect.com/science/article/pii/B0124437109001356

4. Vasiev B, Weijer CJ. Modeling Chemotactic Cell Sorting during Dictyostelium discoideum Mound Formation. Biophysical Journal. 1999 Feb 1;76(2):595–605.

5. Jack CN, Buttery N, Adu-Oppong B, Powers M, Thompson CRL, Queller DC, et al. Migration in the social stage of Dictyostelium discoideum amoebae impacts competition. PeerJ. 2015 Oct 22;3:e1352.

6. Cotter DA, Raper KB. Properties of Germinating Spores of Dictyostelium discoideum. J Bacteriol. 1968 Nov;96(5):1680–9.

7. Cotter DA, Mahadeo DC, Cervi DN, Kishi Y, Gale K, Sands T, et al. Environmental regulation of pathways controlling sporulation, dormancy and germination utilizes bacterial-like signaling complexes in Dictyostelium discoideum. Protist. 2000 Aug;151(2):111–26.

8. Xu Q, Ibarra M, Mahadeo D, Shaw C, Huang E, Kuspa A, et al. Transcriptional Transitions during Dictyostelium Spore Germination. Eukaryot Cell. 2004 Oct;3(5):1101–10.

9. O’Day DH, Mathavarajah S, Myre MA, Huber RJ. Calmodulin-mediated events during the life cycle of the amoebozoan Dictyostelium discoideum. Biological Reviews. 2020;95(2):472–90.