Lara Milwid Warburton: Amoebae

This post was originally going to be a two-line account about an amoeba trying to negotiate an obstacle, part of a collection of tales from the natural world. However, within minutes of my starting to write, it had become clear that the humble amoeba deserved considerably more attention than a mere two-line anecdote. So here you go:

WHAT IS AN AMOEBA?

An amoeba (plural amoebae) is a single-celled “shape-shifting” organism. It moves using temporary projections which it grows, called pseudopods (which literally means “false feet”). It eats by means of phagocytosis, meaning that projections from the cell wrap round the food, sealing it in a bubble called a vacuole, where it is digested. It reproduces via mitosis (cell division).

Phagocytosis

INTELLIGENT AMOEBAE

These little creatures are astonishingly intelligent for a single-celled animal. One study showed that an amoeba was more likely to turn left when its last turn was right, which suggests some sort of memory.

Further evidence of memory was shown in another study. Normally, amoebae slow down when it becomes cold. In the experiment, they started anticipating cold periods by slowing down in advance. When the experimenters stopped the cold snaps, the amoebae still slowed down in each experiment, expecting it to get cold. After a few rounds, they learned that the cold snaps had stopped coming, and they went back to their normal speed.

It seems that slime moulds (which some types of amoeba colonies form) can also find the shortest route through a maze. It is inordinately hard to find videos of this happening from beginning to end, but the following clip is amusing enough. In it, a slime mould goes looking round the maze for food. As it happens, it doesn’t find the food, but if it did, it would eliminate all useless or inefficient paths and just grow along the optimum route.

Slime mould looking for food in a maze

SLIME MOULDS

Certain types of amoebae form slime moulds. Here is a slime mould story:

THE AMOEBAE’S ODYSSEY

It was a dank, dark day on the forest floor. The amoeba colony had spent weeks frequenting the roots of a magnificent elder tree, doing the things that amoebae do: eating, oozing around, dividing, and deciding whether to turn left or right. But times were hard, and the bacteria, yeasts and protozoa on which the amoebae loved to feast were in short supply.

The hungry amoebae grew more and more miserable, until one day, one amoeba, whom we will call Bill, could stand it no longer.

“I’m hungry!” he wailed, in the wispy amoebic wail of cAMP (cyclic adenosine monophosphate), a chemical signal which his neighbours, whom we will call Alice and Rupert, heard immediately.

Alice and Rupert looked at the starving Bill, pityingly and empathetically. Then they looked at each other. And in unison, they opened let out their own wail: “I'm hungry!”

Their neighbours heard the cry too, and picked it up themselves, and within minutes, the wail was rising up from the entire colony (in waves, since the creatures had to alternate listening with crying, being unable to do both at once). “I'm hungry!” the heart-rending lament went up from a hundred thousand amoebae, frantic in their despair.

As if with a single mind, this profusion of individuals grew pseudopods (temporary legs, remember?) and started making their determined way towards little Bill, wailing all the way. As they reached Bill, they piled up on top of him and around him, more and more of them, until these microscopic creatures had unitedly formed a tower 2 millimetres tall. Unable to support itself any longer, the tower toppled over, and lay, an oblong blob, on the forest floor.

Spirals form in the colony as the amoebae migrate towards the cry

The amoebae, which had heretofore been identical to each other, now started to change, each taking on its own specialised function, and gradually, the oblong blob became a living, multicellular organism – a slug. And the slug moved. It set off on a journey. It left the tree roots, it ventured out across the forest floor, and it sought light.

The slug left a trail of slime in its wake. Some of its cells had taken on the role of sentinels – they patrolled through the slug’s body, looking for pathogens. When they found them, they phagocytised them (engulfed them, remember?), a self-sacrificial act indeed, for from time to time, the slug discarded these cells with the pathogens they contained, throwing them into the trail of slime and leaving them there to perish. There would always be more sentinels. Nobody is indispensable. Least of all you.

Finally, the slug reached a patch of forest floor where the dappled sunlight shone through the leaves high above. There, the slug came to a stop, and an extraordinary thing happened. Some of the cells – about one fifth of them – migrated to the top of the body and began to pile one on top of the other, making not so much a tower this time as a stalk. When the stalk was completed, the rest of the cells climbed up it, and when they reached the summit, they clumped together to form a ball called a fruiting body. In this fruiting body, they took on the form of spores, and from there, they were dispersed by a passing squirrel. They were transported by this unwittingly benevolent creature to their new station at the edge of a puddle. There, they transformed from spores back into amoebae and went on to build a comforting, welcoming home, where they lived a joyous and rewarding life, until the next time the food ran out.

AMOEBIC ALTRUISM

The altruism shown by the amoebae who volunteer themselves for stalk-building duty is very interesting. Hettie is one example of these amoebae. Poor Hettie will not become a spore and will not be whisked off to make a new home in greener pastures. She is destined to stay behind and die a slow, miserable death. But she doesn’t mind. Why? Because she is genetically identical to all the other amoebae in her colony. The only reason she might want to survive is to reproduce – but if her genetically identical colony-mates can escape to safer climes and they can reproduce, then Hettie’s genetic propagation is still ensured. So she sacrifices herself, in an apparently altruistic way, to a greater end.

That is not all there is to the matter of altruism, though. Because sometimes, the colony is not big enough to make a viable slug, and in that case, the “I’m hungry” call is sent out to other strains of amoeba too. Thus two or more colonies pool their resources to build a slug and the subsequent life-saving fruiting body.

What is the risk with this collaborative approach? Well, imagine that you are an amoeba in a neighbouring (and genetically different) colony, and you hear Bill’s “I’m hungry” cry. You and your colony-mates are also peckish, so you mobilise yourselves and go trundling off to your neighbour’s patch to join in the slug-building, food-seeking efforts.

Once stalk-building time arrives, you gather up some of your colony-mates into a huddle and tell them, “Listen. What say we sit back and let them build the stalk? Then, once the stalk is built, we’ll all just scamper up it and escape, and none of us has to get left behind and die.”

It makes sense, doesn’t it? And yet, for some reason, it doesn’t seem to happen. Researchers managed to engineer some amoebas (called Doris and Arthur) which systematically refused recruitment to stalk-cell duty. As far as this first step to self-preservation went, they were playing their cards right. But then, for reasons known only to Doris and Arthur, they could not, or would not, climb the stalk to join the evacuation vessel that is the fruiting body.

Of course, a single experiment is not conclusive, but it did suggest that perhaps the self-sacrificial stalk-making mechanism has become genetically entangled with the self-preservatory stalk-climbing mechanism, with the result that only amoebae with altruistic tendencies ultimately produce offspring.

SO, WHAT HAVE WE LEARNT?

Never underestimate an amoeba.