Human beings are great big conglomerates of millions upon millions of cells. In this respect, we are very similar to other animals, as well as land plants, algae and filamentous fungi — we are multicellular.
Multicellularity is thought to have evolved independently several times throughout history. Traditional thinking was that the ancestors of land plants evolved multicellularity separately to the ancestors of animals, who evolved multicellularity separately to the brown algae forebears. It was thought to have evolved multiple times in fungi, red algae and slime molds. Even some bacteria — the blue-green cyanobacteria — figured out the secret to multicellularity somewhere along the line
Phylogenetic trees illustrates how different organisms on the tree of life relate to each other. The example on the left is best imagined as a tree lying on its side with the trunk at the far left and smaller, more recently formed branches toward the right. As you can see, the metazoans (animals) branched away from fungi and the amoebazoa (which includes slime molds) fairly close to the trunk of the tree. All of the branches within the metazoan block — separating worms and crustaceans and jellyfish — are more recent evolutionary divergences.
Because plants, animals, fungi and slime molds diverged so long ago in evolutionary terms, it isn’t easy to determine when key evolutionary events, like the evolution of multicellularity, happened in relation to other events. For example, did the ancestors of animals and slime molds diverge before or after multicellularity? Put another way, did a slime mold ancestor evolve multicellularity separately to an ancestor of metazoans (animals), or was it a common ancestor of both that figured it out?
Slime molds — the social amoebae
The aptly applied common name for slime molds is due to the slimy form they adopt during part of their life cycle. There are two types of slime molds — cellular slime molds and plasmodial slime molds. Both spend most of their life as single-celled amoeba that are able to become ‘social’ and aggregate into multicellular forms.
As I mentioned above, the traditional thinking has said that slime molds and animals evolved multicellularity separately. In part, this was because multicellularity in slime molds is vastly different to the multicellularity that you or I experience. Unlike us with our rigid multicellularity, slime molds are sometimes multicellular, sometimes unicellular. Slime mold multicellularity is conditional, not obligatory.
Cellular slime molds, such as the widely studied Dictyostelium discoideum, live mostly as free-roaming, single-celled soil amoebae that feed on bacteria and fungal spores. When food is abundant, slime mold amoebae prefer to go solo, as what are known as myxamoebae. But when food supplies are exhausted, the myxamoebae pool resources to go forth and multiply. Starvation triggers the cells to pump out the signalling molecule cyclic AMP, which acts as a chemoattractant that herds the separate cells into a tightly aggregated spherical mound.
Working now as one organism, the aggregate morphs into a slug that, like a real slug, has a head and a tail and is able to migrate along the ground towards more favourable conditions — towards light, heat and humidity. The 2–4 mm long slug — also called a pseudoplasmodium or (my favourite) a grex — secretes a protein– and cellulose–rich goo from its anterior (head) cells that leaves a slimy trail in its wake.
After a few hours, the slug stops migrating and the formation of a fruiting body — a process called culmination — begins. The resulting fruiting body is a tiny periscope-like structure just 1–2 millimeters tall, consisting of a droplet-shaped spore-filled head atop a rigid stalk. Once mature, the spore mass disperses and individual myxamoebae hatch from the mature spores.
Specialisation and organisation — further hallmarks of multicellularity
It is during culmination that two additional features of multicellularity become evident in slime molds — cellular specialisation and tissue organisation. In our own bodies, there are thousands of specialised cell types — from muscle cells, to nerve cells, to insulin-producing pancreatic islet cells. The specialisation allows for a division of labour throughout the body. For example, only our islet cells need produce insulin, and they produce enough insulin for all cells in our body to utilise the sugar in our diet; other cells are able to carry out other primary functions instead.
Coming back to our culminating slime mold, the cells at the anterior end of the slug develop into stalk cells, while cells at the posterior end of the slug develop into spores. The differentiation of these cells along two distinct developmental pathways takes place long before their visible identities as either spores that will be dispersed or stalk cells that will die are evident. Importantly, a functional slime mold fruiting body — and indeed a mobile slug — would not be possible without the specialisation of these cells, and their coordinated organisation within the multicellular organism.
What’s in a skin?
Formation of the fruiting body stalk is orchestrated by cells at the very top of the mature fruiting body, and it is these cells that are now shedding light on the question of how and when multicellularity evolved. That’s because the tip of the slime mold stalk has a feature that was thought to be a defining feature of metazoans — a polarised epithelium.
In animals, although there are myriad distinct cell types that comprise each functional body, there are just four basic tissue types: muscle tissue, nervous tissue, connective tissue and epithelial tissue. All cells within your own body can be classified into one of these four broad categories — your brain is obviously made up of nervous tissue, and your muscles of muscle tissue, but less obvious is your blood and fat cells, both of which are connective tissues.
Epithelial tissues line the surfaces of your organs and body cavities; they coat, protect and separate your body and all its constituent bits. Your skin is an epithelium, the lining of your gut is an epithelium, and there’s a stretchy epithelium lining the inside of your bladder. The simplest form that an epithelium can take is a tightly packed sheet one cell-layer thick, but layered, or stratified, epithelia can also form.
Each epithelium fulfills a unique function — secreting enzymes and mucus, forming a barrier resistant to physical and chemical trauma, and mediating communication between the tissue that lies beneath and the cavity that sits above. In the case of glands, epithelial tissues are responsible for pumping out hormones, sweat and tears.
Epithelial tissues are thought to be unique to animals. Yet in 2011, Daniel Dickinson, James Nelson and William Weis from Stanford University published a paper in Science describing an epithelium in the slime mold Dictyostelium.
At first glance, the slime mold epithelium at the tip of its fruiting body stalk looks very much like an animal epithelium — the cells are arranged into a single layer of cells wrapped into a tube, not dissimilar to the epithelium lining the fine capillaries that deliver oxygenated blood to your organs.
Whatever their location or ultimate function, all epithelial tissues in animals are polarised — they know their top from their bottom. After all, a mucus lining is only useful if the mucus is secreted onto the right surface of the epithelium. The slime mold epithelium also displays a top–bottom (or inside–outside) polarity. For example, the molecular scaffolding within the cells — the cytoskeleton — is arranged in a consistent, polarised configuration. The cellular organelles and the assortment of proteins embedded within the plasma membrane are also organised in a polar manner, indicating that the top of each cell is different to its bottom.
This polarisation of the slime mold epithelium is essential to its structure and function. Cellulose synthase enzymes embedded on the inner surface membranes secrete cellulose to form the rigid inner tube of the slime mold stalk, and actomysoin fibres that comprise part of the cytoskeleton help to restrict the diameter of the stalk by preventing over-expansion of the epithelium in the direction of its girth.
When the researchers looked to see whether the molecular polarity machinery in slime molds was the same as that in animals, they found both similarities and differences. In animals, the establishment of polarity relies on proteins that act as molecular connectors to cement adjacent cells together and provide spatial cues that direct polar growth and development. The protein complexes are made up of cadherins that extend out from the plasma membranes into the extracellular space between adjacent cells and attach to each other like velcro. Cadherins bind to other proteins in the plasma membrane, such as α- and β-catenin.
Dictyostelium has no cadherin equivalent (homolog), but does make proteins similar to both α- and β-catenin. This pared-back set of adhesion molecules, likely working with other unidentified molecules, appears to be enough to generate a polar epithelium in Dictyostelium, even though a more complex set of proteins is necessary in animals.
Implications for the evolution of multicellularity
As the authors explain in a Bioessays article that discusses their findings, the identification of an epithelium in Dictyostelium brings into question the independent origin of multicellularity in animals and social amoebae. It is possible that both lineages developed the catenin adhesion molecules separately — an example on convergent evolution, where a common goal is arrived at separately on two occasions.
But if some of the molecular machinery necessary for epithelium development is present in both animals and slime molds, then the development of an epithelium could pre-date the divergence of these two very distant relatives. If this is the case, if a common ancestor of both animals and slime molds was able to form an epithelium, then multicellularity was also a feature of this common ancestor. Multicellularity need not have independently arisen in animals and slime molds, as has previously been assumed.
The common ancestor of metazoans and slime molds would have live approximately 1.5 billion years ago. Divining the evolutionary events that took place this long ago is — to state the obvious — not an easy task. One thing that can help to elucidate past evolutionary events is to understand as best we can the living relatives on as many branches of the tree of life as possible. There isn’t much known about multicellularity in the frog-killing chytrid fungus, nor the colonial choanoflagellates — single-celled planktonic critters that are thought to be the closest relatives to the metazoan lineage. Model organisms, such as mice and fruit flies and Caenorhabditis elegans worms can tell us a whole lot about animal form and function, but delving into the more obscure branches of life can also tells us about who we are and where we come from. Of course, slime molds are pretty damn interesting in their own right, too.
- Dickinson, Nelson & Weis. 2011. A polarized epithelium organized by β- and α-catenin predates cadherin and metazoan origins. Science 331: 1336. doi:10.1126/science.1199633
- Dickinson, Nelson, & Weis. 2012. An epithelial tissue in Dictyostelium challenges the traditional origin of metazoan multicellularity. Bioessays 34: 833. doi:10.1002/bies.201100187