The gluey tentacles of comb jellies may have revealed when nerve cells first evolved

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The rudimentary nervous system of the warty comb jelly may assist resolve when nerve cells arose.

Andrey Nekrasov/Alamy Stock Photo

The gluey tentacles of comb jellies may have revealed when nerve cells first evolved

TAMPA, FLORIDA—Swimming by means of the oceans, voraciously consuming plankton and different small creatures—and sometimes startling a swimmer—the gorgeous gelatinous plenty referred to as comb jellies gained’t be becoming a member of Mensa anytime quickly. But these fragile creatures have nerve cells—they usually supply insights in regards to the evolutionary origins of all nervous methods, together with our personal. Inspired by research of a glue-secreting cell distinctive to those plankton predators, researchers have now proposed that neurons emerged within the final frequent ancestor of at present’s animals—and that their progenitors had been secretory cells, whose main operate was to launch chemical substances into the setting.

Joseph Ryan, a computational evolutionary biologist the University of Florida Whitney Laboratory for Marine Bioscience in St. Augustine, steered that state of affairs final yr after tracing the event of nerve cells in embryos of comb jellies, among the many most historic animals. Earlier this week on the annual assembly of the Society for Integrative and Comparative Biology (SICB) right here, he marshaled proof from developmental research of different animals, all pointing to common origins for some neuron and secretory cells.

“What Ryan is proposing is novel and important,” says David Plachetzki, an evolutionary biologist on the University of New Hampshire in Durham. Among different mysteries, it might resolve a protracted debate about whether or not the nervous system evolved twice early in animal life.

Today, nerve cells are among the many most specialised cell varieties within the physique, capable of transmit electrical indicators, for instance. Some variations speak to one another, others relay info from the setting to the mind, and nonetheless extra ship directives to muscle tissue and different elements of the physique. They are additionally an nearly common characteristic of animals; solely sponges and placozoans, an obscure group of tiny creatures with the only of animal buildings, lack them.

When and the way the animal nervous system arose has remained murky, nevertheless. Ryan and Whitney lab postdoctoral fellow Leslie Babonis had been drawn into the controversy by their current evaluation of the developmental origin of the colloblast, a specialised cell distinctive to most comb jellies. Studding the tentacles of comb jellies, the cells secrete glue that grabs passing prey.

By tracing the event of particular person cells in comb jelly embryos and monitoring every cell’s gene exercise, Babonis found that colloblasts come up from the identical progenitor cells because the animal’s nerve cells. “That was not expected at all,” remembers Ryan, whose group printed these outcomes on 30 August 2018 in Molecular Biology and Evolution.

Since then, nevertheless, he’s realized of further research pointing to frequent origins for neurons and different secretory cells in embryonic improvement—and maybe in evolution. In his speak on the SICB assembly, he famous that one group confirmed greater than 25 years in the past that the stinging cells of jellyfish, one other specialised secretory cell sort, come up from the identical embryonic precursors because the animal’s nerve cells. He cited related proof for hydra and fruit flies. “It’s a really generalizable thing,” he says.

The discovering might settle a long-standing debate. In 2013, a analysis group analyzing the newly sequenced genome of a comb jelly referred to as the ocean gooseberry (Pleurobrachia bachei) found it was lacking a number of genes lively within the nervous methods of most animals: sure Hox genes, which management improvement, and the gene for the neurotransmitter serotonin. That discovery led the group to suggest that comb jellies evolved a nervous system independently from almost all other animals. But many puzzled how one thing so complicated might have evolved twice.

Finding a typical developmental supply for neurons in comb jellies, jellyfish, and lots of different branches of life suggests it didn’t, Ryan and others now say. The work reveals “the platform upon which the nervous system was built was there” within the final frequent ancestor of animals, says Timothy Jegla, a neurobiologist at Pennsylvania State University in University Park. “Relatively simple reprogramming [of] stem cells during development can lead to whole new cell types and tissues, and the nervous system is probably just another example of that.” Other researchers, nevertheless, say it’s nonetheless potential that nerve cells had a number of origins after the final frequent ancestor, every time arising from the identical stem cell lineage.

Next, Ryan, Babonis, and Whitney lab neurophysiologist Yuriy Bobkov hope to learn the way progenitor cells grow to be neurons by learning a easy sensory organ—the “warts” of the warty comb jelly, or sea walnut (Mnemiopsis leidyi). Recent work reveals that every wart incorporates about 500 nerve and muscle cells that react to mild, the odor of fish, and mechanical stimuli. Warts regenerate if reduce off, and by tracing gene exercise of their cells as they regrow and specialize, Ryan hopes his group can pin down the genes directing nerve cell formation—and maybe, he says, “peel back some of the complexity of the evolution of neurons.”

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