Sponge neuroid cell (orange) extends arms that enwrap the feeding apparatus of a sponge digestive cell (green) to create a link for targeted communication. The image was taken using electron microscopy.
Credit: Jacob Musser, Giulia Mizzon, Constantin Pape, Nicole Schieber/EMBL
Brain cells, all 86 billion of them, communicate with one another across microscopic gaps between them called synapses. Communication across this gap is via neurotransmitter substances such as acetyl choline. A similar process is involved at the neuromuscular junction between nerves and muscle cells. This has been well understood for some time but what was a mystery was how this system evolved.
Certainly it happened early in the evolutionary history of multicellularity because all but the most primitive multicellular organisms such as sponges have a nervous system of some sort.
This is the sort of mystery that Creationists latch onto to fool their dupes with the false dichotomy that if science hasn't explained it, the locally popular god must have created it, leaving their scientifically illiterate targets to assume that if science hasn't explained something then science can't explain it and never will.
Now though, scientists working for EMBL have discovered that this method of cell communication evolved in just those primitive organisms, sponges, for an entirely different function - an example of something Creationists hate - evolutionary repurposing, or exaptation, of an existing process or structure for a new function. This is, of course, how apparently irreducibly complex processes and systems such as flagella evolve.
In so doing, the scientists have also solved another mystery - why do sponges have the genes for synaptic control when they don't have a nervous system?
The EMBL news release explains:
Individual neurons in a brain communicate via synapses. These connections between cells lie at the heart of brain function and are regulated by a number of different genes. Sponges do not have these synapses, but their genome still encodes many of the synaptic genes. EMBL scientists asked the question why this might be the case. Their latest findings are published today [Nov 5, 2021] in the journal Science.In the abstract to their published paper, the authors say:
We know that these synaptic genes are involved in neuronal function in higher animals. Finding them in primitive species like sponges begs the question: if these animals don’t have brains, what is the role of these genes? As simple as that sounds, answering this question was beyond our technological abilities so far.[…]
Detlev Arendt, EMBL Group Leader
EMBL Developmental Biology Unit
Heidelberg, Germany.
To study the role of these synaptic genes in sponges, the Arendt lab established microfluidic and genomic technologies in the freshwater sponge Spongilla lacustris. Using these techniques, the scientists captured individual cells from several sponges inside microfluidic droplets and then profiled each cell’s genetic activity.
[…]
Sponges use their digestive chambers to filter out food fromWe showed that certain cells in the sponge digestive chambers activate the synaptic genes. So even in a primitive animal lacking synapses, the synaptic genes are active in specific parts of its body.the water and interact with environmental microbes. To understand what the cells expressing synaptic genes do, the Arendt group joined forces with six EMBL teams as well as collaborators in Europe and worldwide. Working with EMBL’s Electron Microscopy Core Facility, Yannick Schwab’s team and Thomas Schneider’s group operating synchrotron beamlines at EMBL Hamburg the researchers developed a new correlative imaging approach. “By combining electron microscopy with X-ray imaging on a synchrotron beamline we were able to visualise the stunning behaviour of these cells,” Dr Schwab explained.
Our results point to the cells regulating feeding and controlling the microbial environment as possible evolutionary precursors for the first animal brains. Truly food for thought!
Jacob Musser, lead author
EMBL Developmental Biology Unit
Heidelberg, Germany.
The scientists captured three-dimensional snapshots of cells crawling throughout the digestive chamber to clear out bacterial invaders and sending out long arms that enwrap the feeding apparatus of specific digestive cells. This behaviour creates an interface for targeted cell–cell communication, as it also happens across synapses between neuronal cells in our brains.
Sponges and evolutionary originsSo, what started out as part of a system for regulating their feeding, and a hygiene system for defending them against opportunist bacteria in a sponge's feeding chamber, gave rise to the genes which were later available for developing simple, then increasingly complex brains and the nervous control of muscles. Probably not the sort of news that will be welcomed in the Creationist news outlets where endarkenment rather than enlightenment is the main objective.
Sponges represent our distant animal relatives. They do not have a nervous system but do have a simple body for filter feeding. Surveying the cell types in the freshwater sponge Spongilla lacustris, Musser et al. found that many genes important in synaptic communication are expressed in cells of the small digestive chambers. They found secretory machinery characteristic of the presynapse in small multipolar cells contacting all other cells and also the receptive apparatus of the postsynapse in the choanocytes that generate water flow and digest microbial food. These results suggest that the first directed communication in animals may have evolved to regulate feeding, serving as a starting point on the long path toward nervous system evolution. —BAP
Abstract
The evolutionary origin of metazoan cell types such as neurons and muscles is not known. Using whole-body single-cell RNA sequencing in a sponge, an animal without nervous system and musculature, we identified 18 distinct cell types. These include nitric oxide–sensitive contractile pinacocytes, amoeboid phagocytes, and secretory neuroid cells that reside in close contact with digestive choanocytes that express scaffolding and receptor proteins. Visualizing neuroid cells by correlative x-ray and electron microscopy revealed secretory vesicles and cellular projections enwrapping choanocyte microvilli and cilia. Our data show a communication system that is organized around sponge digestive chambers, using conserved modules that became incorporated into the pre- and postsynapse in the nervous systems of other animals.
Musser, Jacob M.; Schippers, Klaske J.; Nickel, Michael; Mizzon, Giulia; Kohn, Andrea B.; Pape, Constantin; Ronchi, Paolo; Papadopoulos, Nikolaos; Tarashansky, Alexander J.; Hammel, Jörg U.; Wolf, Florian; Liang, Cong; Hernández-Plaza, Ana; Cantalapiedra, Carlos P.; Achim, Kaia; Schieber, Nicole L.; Pan, Leslie; Ruperti, Fabian; Francis, Warren R.; Vargas, Sergio; Kling, Svenja; Renkert, Maike; Polikarpov, Maxim; Bourenkov, Gleb; Feuda, Roberto; Gaspar, Imre; Burkhardt, Pawel; Wang, Bo; Bork, Peer; Beck, Martin; Schneider, Thomas R.; Kreshuk, Anna; Wörheide, Gert; Huerta-Cepas, Jaime; Schwab, Yannick; Moroz, Leonid L.; Arendt, Detlev.
Profiling cellular diversity in sponges informs animal cell type and nervous system evolution.
Science 5 Nov 2021; Vol 374, Issue 6568, 717-723; DOI: 10.1126/science.abj2949
© 2021 The authors, published by The American Association for the Advancement of Science
Reprinted by kind permission under license #5184140453563
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