In the hierarchy of life on planet Earth there are three domains. These are, the Eukaryota, Bacteria and Archaea. The Eukaryota comprise all the life around you that can be seen with the naked eye1. They are characterised by each cell having a nucleus and other membrane-surrounded organelles (e.g. Mitochondria, Chloroplasts). Bacteria and Archaea do not have nuclei.
The domain Eukaryota nominally contains four kingdoms: Animalia (animals), Plantae (plants), Fungi and Protista1. Most people know that Animalia includes everything from the enormous blue whales which roam the oceans, to elephants, giraffes, sharks, emus, barracudas, wombats, cockatoos, echidnas, crayfish, mice, corals, budgerigars, prawns, grasshoppers, sponges, houseflies, spiders, slaters, ticks, leeches, earthworms, all the way to the tiny mites that live on your face.
The Plantae comprises all land plants such as angiosperms (flowering plants), conifers and other gymnosperms (e.g. Gingko), ferns, hornworts, liverworts and mosses. Some classifications also include green algae in the Plantae2.
The Fungi include yeasts and moulds, as well as the more familiar mushrooms and toadstools3.
During the week, I had cause to get into the scientific literature on another group among the Eukaryota; the slime moulds. Previously, my only interaction with them was seeing the occasional, flat, almost dinner plate sized, yellowish mass on the mulch in what passes for our garden. The slime mould that forms these masses is colloquially and appropriately called ‘dog’s vomit slime mould’ because it looks like an animal has had a liquid laugh on our garden. Slime moulds are one of the weirdest groups of organisms it seems possible to imagine, and are nominally grouped among the Protista.
Protista is simply a group of Eukaryotes that are not animals, plants or fungi. They are mostly single celled organisms, but some do live in colonies. They were grouped together mostly because they were single celled organisms. However, as better genetic information has become available, the relationships of the various groups have become clearer, but this understanding is still improving4.
Slime moulds develop fruiting bodies superficially similar to those of fungi. Indeed, in days gone by, they were classified with the Fungi, but are not now. Slime moulds actually eat bacteria, fungi and rotting organic matter.
There are generally two types of slime mould; a cellular type and a plasmodial type. The plasmodial type is enclosed within a single membrane without cell walls and effectively acts as one large cell. This ‘supercell’ is essentially a bag of cytoplasm containing thousands of individual nuclei. These can reach quite large sizes, up to a metre across and weighing as much as 20 kilograms, and they can move at a speed of up to one millimetre per second5. The dog’s vomit slime mould is of the plasmodium type.
The cellular slime moulds spend most of their lives as individual unicellular amoebae, but when food is a bit scarce and they are ready to reproduce, they release signal molecules into the environment which allows them to find each other and congregate to create swarms. While this sounds fairly mundane, what these amoebae can do when they aggregate in this way is bizarre. They join up into a tiny multicellular slug-like body, or grex, which can indeed move like a slug. To move this way there has to be a division of labour among the single cells, something rather odd for what was until a little while ago a collection of isolated single amoebae6,7.
As the slug migrates, it may encounter dangerous bacteria and toxins. Fortunately, the slug contains cells that act as sentinel cells. These sacrifice themselves by absorbing both bacteria and toxins and are eventually sloughed off the slug as it moves. Other cells then take over the role of being a sentinel7.
When the grex finds a suitable place, attracted by heat, light or humidity, it eventually grows into a fruiting body. Some of the amoebae become spores to begin the next generation, while other amoebae sacrifice themselves to become the stalk that lifts the spores up into the air. The cells forming this stalk die when their job is done8.
Studies of the genome of cellular slime moulds estimate that they evolved 600 million years ago. Preliminary studies suggest that the common ancestor of all living slime moulds is much older than that, maybe over a billion years ago. If that is so, they may well have colonised continents that were home only to films of bacteria9.
There is still much to be found out about slime moulds, and study of them will continue and may elucidate the origins of multicellularity, which has evolved many times in the past.