Sponges are arguably the simplest animals and they lack a nervous system, but peculiar cells in their digestive chambers may be evolutionary precursors of neurons



Life



4 November 2021

An electron microscopy image showing a sponge neuroid cell (orange) with projections that may communicate with a digestive cell (green)

Jacob Musser, Giulia Mizzon, Constantin Pape, Nicole Schieber/EMBL

Sponges lack anything resembling brains, but they nevertheless may have played a key role in the early evolution of the nervous system. A new study finds that sponges contain cells that have some of the capabilities of neurons – and these may be the evolutionary precursors of true brain cells.

“The nervous system came about very early in animals and this transition is completely enigmatic so far,” says Detlev Arendt at the European Molecular Biology Laboratory in Heidelberg, Germany.

Most animals have brains, or at least neurons, the cells that are their building blocks. Neurons carry electrical signals along their length and can communicate with each other by releasing chemicals called neurotransmitters, often at specialised junctions known as synapses.

However, sponges are the exception. They are one of the oldest animal groups still extantpossibly the very oldest. And they don’t have a nervous system.

“Sponges don’t have anything that looks like neurons, synapses or brains,” says co-author Jacob Musser, also at the European Molecular Biology Laboratory. But his team has found they might have precursors of these things.

Musser, Arendt and their colleagues studied a freshwater sponge called Spongilla lacustris. They broke apart sponges and tracked individual cells to see which genes were active.

This revealed that the sponges were made up of 18 distinct cell types, each with a different pattern of gene activity. The team then stained the different cells to figure out where they were within the body.

One cell type stood out. The team calls them “neuroid” because they had long tendrils, resembling those of neurons. They were found in the sponge’s digestive chamber and made contact with many of the other cells within. Their gene activity pattern suggested they were secreting signalling chemicals, similar to those that neurons release at synapses to communicate with their neighbours.

Arendt emphasises that the sponges’ neuroid cells aren’t neurons. “We still think they don’t have a nervous system,” he says. But these cells may be coordinating the activities of the digestive cells. “We see a lot of vesicles in those neuroid cells that would indicate that they secrete something, which is a very strong indication for communication,” he says. “And we also know the kind of molecules they might produce.”

Sponge biologist Sally Leys at the University of Alberta in Canada describes the single-cell data as “superb”. But she is unconvinced by the team’s interpretation. “I think there’s no evidence shown in this paper that these are anything to do with a neuroid precursor at all.”

Leys argues that many of the genes the team focused on are widely used in complex organisms and aren’t specific to neurons. The sponges may be using them for different purposes. She wants to see functional studies demonstrating that the neuroid cells actually behave the way the genetics suggests they do.

Arendt points out that many of the genes and chemicals used by neurons are actually ancient: they predate the evolution of multicellular animals and can be found in our single-celled relatives. Later, the genes were duplicated and some versions became altered, ultimately leading to neurons that are highly specialised for fast communication.

That step happened early in animal evolution. The other candidate for the oldest animal group, the comb jellies, do have neurons – and they are organised into a net. “I would be fine with calling that a brain,” says Arendt. Arguments have raged over the past decade over whether sponges or comb jellies are the older group. Arendt is inclined to believe that sponges, being simpler, are older.

What should we make of the fact that the neuroid cells were in the sponges’ digestive system? Arendt and Musser say there is growing evidence that some neurons evolved from digestive cells. “There are neurons in a variety of different animals that share a lot of features with digestive cells and even come from some of the same embryonic precursors,” says Arendt. However, other neurons have more in common with cells that contract, such as muscle cells, and may represent a separate origin of neurons.

“A lot of researchers would have said several years ago neurons probably had a single origin,” says Musser. “That story seems to be changing.”

Journal reference: Science, DOI: 10.1126/science.abj2949

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