Mysterious slime mold behaves like a multicellular collective when feeding

(ORDO NEWS) — A rare and mysterious slime mold may hold clues to the emergence of multicellularity.

The scientists found that the normally single-celled species Fonticula alba coalesces into an invasive entity that rips apart a bacterial colony at a certain stage in its life cycle, feasting as one organism. This collective has animal-like properties and behaves like an aggressive cancer.

This discovery could help us understand how multicellular organisms evolved from simple single-celled organisms that were the basis of all life on Earth.

Fonticula alba is a rare and peculiar type of slime mold. It was isolated in 1960 from a sample found in dog feces in Kansas; subsequently, its life cycle and behavior were studied in the laboratory where it was cultivated.

Like many other slime molds (which aren’t actually fungi, but members of the protist kingdom, sort of an overarching group for anything that can’t be neatly classified as an animal, plant, or fungus), F. alba spends most of its life cycle as a single-celled organism, feeding on bacteria as part of the decomposition cycle.

When it’s time to reproduce, these single cells combine into a multicellular form to grow volcano-like fruiting bodies that release spores to grow… more F. alba.

It also doesn’t look like other slime molds. F. alba is more closely related to fungi than other slime molds and is placed in the same clade as fungi.

A group of researchers led by biologist Christopher Thoret at the University of Geneva in Switzerland wanted to learn more about the little-studied life cycle of this organism, so they set about cultivating it in the lab.

Although slime mold can feed on various types of bacteria, a common fecal bacterium called Klebsiella pneumoniae was identified as the optimal co-culture for F. alba as early as 1979, so the researchers used it.

They grew colonies of K. pneumoniae and introduced slime mold at various stages of the bacterium’s life cycle. The researchers found something very unusual and unexpected: towards the end of the life of the bacteria, when K. pneumoniae ran out of food, F. alba combined into a multicellular state – not to reproduce, but to feed on bacteria.

As the slime mold gathered together and moved into the bacterial colony, it developed transitional cell columns to form tentacle-like filaments; they were similar to the hyphae we know from fungi, which perform a number of functions, including searching for sources of nutrients. These strands cooperatively searched for and invaded the bacterial agar plate in search of new food sources.

Mucus mold cells come together to form these filaments, with a single “leader” cell at the tip and “follower” cells at the back. Somehow, these cells communicate with each other, while the leader cell transmits information to the follower cells.

When the researchers destroyed the leader cell in the antennae with a laser, the follower cells fell into disarray and could no longer seek out new food sources. The same disorder was not observed when one of the follower cells was damaged. This suggested that cells do indeed play a role in this multicellular state.

According to the researchers, this suggests a previously undiscovered origin of fungal hyphae.

“We propose a hypothesis according to which hyphae could have a direct aggregative origin,” the researchers write in their paper.

“The last common ancestor of fungi and F. alba may have assembled amoeboid cells into head-to-tail invasive assemblages.”

Cancer cells also use leader-follower dynamics for invasion, which suggests that different types of cells can exhibit similar behavior for different reasons. This means that F. alba could be a powerful model organism for understanding the emergence of multicellularity as a general concept, the team said.

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