Lurking in gutters, gardens and forests, just under-foot, lies something strange. Something almost alien. They lead independent lives as single cells foraging for, and feeding on, bacteria. But when food becomes scarce they work together, in their millions, forming a multicellular ‘organism’ of individuals. In this form they’re more mobile; moving about, looking for food or new environments, displaying something akin to a brainless form of intelligence.
They are everywhere, on almost every continent; but they didn’t arrive on a meteorite, like aliens bent on world domination. Instead they’ve been with us for nearly a billion years. They are the ‘slime moulds’. But wherever individuals cooperate for the greater good of their ilk, there are always cheaters; even among the slime moulds.
If you scoop up a handful of soil almost anywhere, the chances are there’s a slime mould in it. They come in two types. The individuals of one group can fuse together forming a weird blob; an enormous single cell filled with the thousands of nuclei, pulsating as it moves in search of food. One example of this group is the unpleasantly, but alarmingly accurately, named ‘dog vomit‘ slime mould…
The second group spends most of its time as individuals, but when food is scarce they form a community of individuals working together—a ‘slug’. Within the slug, some individuals are capable of specialising in their function.
Both groups form structures called sporangia, which contain spores of the slime mould, allowing them to be dispersed to new areas by the wind or through contact with animals as they pass by.
One of the most well studied slime moulds, an amoeba, is Dictyostelium discoideum. Most of its life cycle is spent prowling through a small patch of earth, following a bread-crumb trail of folic acid to its prey: bacteria. Once it’s had its fill, it divides into two and the new amoebas go their separate ways hunting for yet more food. Eventually, as their number grows, food becomes scarcer. They sound a chemical alarm; the signal to gather. As they meet, ‘glyco-proteins’ on their surface stick them together, forming a growing blob. The blob lengthens as it starts to move, making a ‘slug’ about 2-4 mm long.
The slug behaves more like an ‘it’ than a ‘they'; responding to its environment as it moves, diverting around obstacles, turning around if it can’t. It moves towards heat, light or increased humidity in its search for a suitable place. Eventually, once it’s satisfied with its new environment, it changes shape. The individual amoeba at the front, about 20% of the amoeba, sacrifice themselves, forming a stiff stalk of dead cells to hold aloft others; those at the rear who form a ‘fruiting body’ containing spores—much like a fungus. The sacrifice isn’t in vain, it ensures that at least their brethren survive to reproduce, and that most of their genes are passed on.
D.discoideum are happy to form slugs with other none non-related D.discoideum. But some of these are cheaters. Why sacrifice yourself? Why don’t we live to grow another day instead? Especially when you’re part of a slug with un-related D, discoideum. Surely you want your own closest relatives to survive?
And indeed cheaters do exist in slime moulds. But the problem with cheating is that you may be found out, or the trick used may have unintended effects elsewhere. It may have what biologists call a ‘fitness cost’. Fitness costs can be found in societies when duplicitous behaviour is noticed and punished by peers, or through the very mechanisms used to cheat, hampering them in some other way.
One mutation in D. discoideum increases the likelihood of the mutant amoeba going on to make spores rather than dying as the stem of the fruiting body. It does this by ensuring its closer to the rear of the slug when the fruiting body and stalk are formed. But the trick it uses to do this involves the loss of a gene the amoeba use to stick together, making them more slippery. The result is that, while they are less likely to be pulled towards the front of the slug, where the stalk forms, they are less likely to be picked up at all to form the slug in the first place.
Such fitness costs typically stop one group or another gaining the upper-hand over all, levelling the playing field to some extent. But there is not necessarily always a fitness cost.
Lorenzo Santorelli of the University of Oxford recently led research on D. discoideum and identified a strain with a gene called ChtB, that seems to let them get away with cheating scott free; at least in the lab. ChtB doesn’t appear to do anything directly to the mutant amoeba; instead it prevents its companions from forming pre-spores. It inhibits a gene called cotB, essential for forming the pre-spores, thereby forcing the ‘wild type’ amoeba to become stalk cells instead.
Oddly though, no fitness costs could be found associated with this mutation. Though the researchers were quick to point out that the conditions they would experience in the wild are very different from those in the laboratory, so it is possible that some mechanism would feedback on this cheating behaviour, just that we can’t identify it yet.
Slime moulds are weird, and they raise questions about where individual cells and organisms begin and end, evolution, memory, and—even without anything we would recognise as a brain—what we might count as ‘intelligence’. The more we find out about them, the more surprising they are. Studies on slime moulds have shown them to be capable of solving mazes, demonstrate a rudimentary ability to predict the future based on past events, or even help with civil engineering problem like road building.
Weird. There’s no other word for them.
Santorelli LA, Kuspa A, Shaulsky G, Queller DC, & Strassmann JE (2013). A new social gene in Dictyostelium discoideum, chtB. BMC evolutionary biology, 13 (1) PMID: 23298336
Top image: CC by Randomtruths,
Dictyostelium discoideum: CC by microbeworld
Dog vomit: CC wikipedia
Arcyria denudata: CC by Bill Bouton