Gooey trails help slime moulds to navigate
Slime memory Brainless slime moulds leave a gooey trail behind them to remember where they've been, new research has found.
The findings suggest early life forms developed such externalised spatial "memory" to navigate, long before the evolution of brains.
"It's the first time that a spatial memory system has been shown in a unicellular organism," says Reid, who did the research as part of his PhD, supervised by Dr Madeleine Beekman.
Single-celled slime moulds are in a separate group from animals, plants and fungi and are known as "protists".
"They're weird little creatures," says Reid.
The organisms can sense light and chemicals and move around in response to these using a pulsing action of actin and myosin filaments within the cell
While they move away from light, because it suggests a drying environment, they move towards food by following the strength of chemical signals.
"Most unicellular organisms can detect how a gradient of chemicals is changing and they can move along that gradient if they want to get to the source," says Reid.
But, how do slime moulds navigate and make sure they don't waste time moving over ground they've already covered?
Reid and colleagues had noticed slime moulds left a gooey trail behind them and wondered whether this might be being used as a memory prop by the brainless organism to stop it from backtracking.
To test this idea Reid and colleagues first set up a Y-shaped maze where the slime mould had to choose which arm of the Y to go down.
When they coated one arm with slime, the organism almost always chose the other arm.
In a second experiment, they put a dry plastic U-shaped barrier in an agar dish with a slime mould placed at the top of the U, and a food source placed below the U.
The researchers then watched while the slime mould made its way towards the food source, stopped when it got to the barrier, and then found its way out of the U and around the side to get to the food, without going over the slime trail it had left behind.
"It hit the barrier and found its way straight out again and around and straight to the food source," says Reid. "It wasn't really fazed by it at all."
By contrast, he says, robots not programmed to recognise and avoid areas they've been before would either stop at the barrier or keep looping around without escaping the trap.
But to be really sure the slime trail was being used as a memory prop, the researchers had to 'knock out' the prop in some way and then see if it affected the organism's navigation.
To do this, they covered the whole surface of the agar plate with slime and then tested the slime mould again.
"When we did that we found the slime mould was much worse at solving the [U-shaped barrier] problem," says Reid.
He says the findings support the idea that the slime mould is using extracellular slime to navigate.
"Before any kind of brain evolved, single-cells moving around the environment would remember where they'd been by leaving chemicals in the environment," says Reid.
"It's something that happened before human memory could have evolved."