Sponges barely qualify as animals. Like the ones by your sink, most lay docilely on the ocean floor without moving. They lack muscles, lungs or gills, a gut, and obvious nerve cells, and eat whatever bits of nutrients float their way. Yet a new study that characterized their cells has revealed one unexpectedly like a nerve cell: It and the cells with which it interacts have some of the same working genes as nerve cells. And the two cell types may work together to coordinate feeding, like our complex brain does for us. Still, some sponge biologists question whether the highlighted nervelike cell is what is claimed.
If the work holds up, “this will have tremendous significance for understanding the origin of the animal nervous system,” says Scott Nichols, an evolutionary developmental biologist at the University of Denver who was not involved with the work.
Nerve cells have special properties that let them communicate at lightning speeds. Each has a central body and long processes that reach out to other cells, creating junctions called synapses between each neuron in the network. When stimulated, a nerve cell sends an electrical impulse down its processes and releases chemicals called neurotransmitters that travel across the gap to the next neuron. The neurotransmitters activate proteins in the receiving cell and can start another electrical signal. To understand how such a complex cell-to-cell communication system originally evolved, researchers have increasingly turned to animals like sponges that arose early in life’s evolution for clues. Though they lack nerve cells, sponges have some of the same specialized genes for synapses that brainier creatures possess, studies have shown.
Yet determining how sponges use those genes has been no easy task given their strange architecture. A sponge consists of a network of interconnected canals, which include small digestive chambers lined with specialized digestive cells with whiplike projections. The whips beat in sync to pull water across the cell’s netlike collar to capture microscopic particles, even floating DNA, for food. “Their distinct body plan has made it difficult to compare them to other animals,” says Detlev Arendt, an evolutionary biologist at the European Molecular Biology Laboratory (EMBL).
To learn more, he, EMBL evolutionary biologist Jacob Musser, and their colleagues employed a technology called single-cell sequencing to assess the messenger RNA (mRNA) in each cell, which offers a representation of any active genes. Even though most cells in an organism have the same set of genes, each cell type activates a unique subset. By identifying groups of active genes, scientists can classify cell types and understand how they evolved.
Musser and his colleagues separated whole Spongilla lacustris sponges into individual cells, sorting each into a tiny droplet and labeling the mRNA to be sequenced. They developed a technique to fluorescently tag active genes so they could see exactly where each was being used. Their method “could be transformative for understanding sponge cell biology,” says Sally Leys, a sponge biologist at the University of Alberta, Edmonton, who was not involved with the work.
In Science today, Musser, Arendt, and their team of international collaborators report distinguishing 18 kinds of sponge cells, including a few resembling specialized cells in humans and more complex animals. One type has gene activity resembling a more complex animal’s muscle—those cells help sponges expand and contract.
The team also zeroed in on cells that landed them in the middle of a sponge controversy. Originally called neuroid cells, and also referred to as central cells, these cells have only been reported by a few sponge research groups over the decades—they are seemingly few in number and crawl around the middle of a sponge’s digestive chamber.
Musser, Arendt, and their colleagues report finding some of these neuroid cells and documenting that they have active genes in common with the side of a synapse that sends a signal to another nerve cell. And next to neuroid cells are digestive cells whose active genes include those typically expressed in the receiving cell of a synapse, the scientists showed. “This was an exciting finding because it suggested to us that the first animal nervous systems may have evolved to regulate feeding or monitor microbes in the environment,” Musser says.
To see whether the two cell types were indeed interacting, he and his colleagues developed a new way to visualize a whole sponge using x-rays produced in a machine called a synchrotron. When they then zoomed in on digestive chambers to get a 3D view of neuroid cells and their surroundings using electron microscopy (see video, below), they discovered that neuroid cells have many chemical-filled bubbles and form arms that reach out and wrap around the digestive cell’s feeding collar, perhaps transmitting chemicals that could change the behavior of the digestive cells.
That’s reminiscent of communicating nerve cells and suggests neuroid and digestive cells may be ancestral to cells that form the two sides of the synapse. “With neurons it always takes two cells to tango, with one cell sending the signal and one cell receiving it,” Arendt says. The team suspects the earliest such cellular pairing may have occurred on the outer feeding surface of an even simpler animal ancestor. During later evolution—beyond sponges—these duos narrowed their point of contact to the small gap of modern synapses and adopted electrical signaling.
If the group is right and sponges do coordinate their cells’ behavior with signals akin to neurons, “it will certainly shatter our preconceptions that sponges lack neuron-related cell types,” Nichols says. But he cautions that the evidence so far is suggestive, not definitive.
Leys and Bernard Degnan, a marine biologist at the University of Queensland, St. Lucia, also want more proof—they are among the sponge biologists who have looked for neuroid cells and not seen them in their animals, so they are not sure what Musser and colleagues have actually homed in on.
Nonetheless, even if the nervous system evolution story doesn’t hold up, Degnan calls the study a tour de force for identifying key cell types, which will be very helpful to understanding and appreciating sponges in general. Leslie Babonis, an integrative biologist at Cornell University, agrees. “Any snorkeler would be blown away to learn that the slimy mat they just floated by is actually an animal composed of at least 18 unique cell types!” she says.