TAMPA, FLORIDA—Swimming through the oceans, voraciously consuming plankton and other small creatures—and occasionally startling a swimmer—the beautiful gelatinous masses known as comb jellies won’t be joining Mensa anytime soon. But these fragile creatures have nerve cells—and they offer insights about the evolutionary origins of all nervous systems, including our own. Inspired by studies of a glue-secreting cell unique to these plankton predators, researchers have now proposed that neurons emerged in the last common ancestor of today’s animals—and that their progenitors were secretory cells, whose primary function was to release chemicals into the environment.
Joseph Ryan, a computational evolutionary biologist the University of Florida Whitney Laboratory for Marine Bioscience in St. Augustine, suggested that scenario last year after tracing the development of nerve cells in embryos of comb jellies, among the most ancient animals. Earlier this week at the annual meeting of the Society for Integrative and Comparative Biology (SICB) here, he marshaled evidence from developmental studies of other 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 at the University of New Hampshire in Durham. Among other mysteries, it could resolve a long debate about whether the nervous system evolved twice early in animal life.
Today, nerve cells are among the most specialized cell types in the body, able to transmit electrical signals, for example. Some versions talk to each other, others relay information from the environment to the brain, and still more send directives to muscles and other parts of the body. They are also an almost universal feature of animals; only sponges and placozoans, an obscure group of tiny creatures with the simplest of animal structures, lack them.
When and how the animal nervous system arose has remained murky, however. Ryan and Whitney lab postdoctoral fellow Leslie Babonis were drawn into the debate by their recent analysis of the developmental origin of the colloblast, a specialized cell unique to most comb jellies. Studding the tentacles of comb jellies, the cells secrete glue that grabs passing prey.
By tracing the development of individual cells in comb jelly embryos and monitoring each cell’s gene activity, Babonis discovered that colloblasts arise from the same progenitor cells as the animal’s nerve cells. “That was not expected at all,” recalls Ryan, whose team published those results on 30 August 2018 in Molecular Biology and Evolution.
Since then, however, he’s learned of additional studies pointing to common origins for neurons and other secretory cells in embryonic development—and perhaps in evolution. In his talk at the SICB meeting, he noted that one team showed more than 25 years ago that the stinging cells of jellyfish, another specialized secretory cell type, arise from the same embryonic precursors as the animal’s nerve cells. He cited similar evidence for hydra and fruit flies. “It’s a really generalizable thing,” he says.
The finding could settle a long-standing debate. In 2013, a research team analyzing the newly sequenced genome of a comb jelly known as the sea gooseberry (Pleurobrachia bachei) discovered it was missing multiple genes active in the nervous systems of most animals: certain Hox genes, which control development, and the gene for the neurotransmitter serotonin. That discovery led the team to propose that comb jellies evolved a nervous system independently from almost all other animals. But many wondered how something so complex could have evolved twice.
Finding a common developmental source for neurons in comb jellies, jellyfish, and many other branches of life suggests it didn’t, Ryan and others now say. The work shows “the platform upon which the nervous system was built was there” in the last common 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, however, say it’s still possible that nerve cells had multiple origins after the last common ancestor, each time arising from the same stem cell lineage.
Next, Ryan, Babonis, and Whitney lab neurophysiologist Yuriy Bobkov hope to learn how progenitor cells develop into neurons by studying a simple sensory organ—the “warts” of the warty comb jelly, or sea walnut (Mnemiopsis leidyi). Recent work shows that each wart contains about 500 nerve and muscle cells that react to light, the smell of fish, and mechanical stimuli. Warts regenerate if cut off, and by tracing gene activity of their cells as they regrow and specialize, Ryan hopes his team can pin down the genes directing nerve cell formation—and perhaps, he says, “peel back some of the complexity of the evolution of neurons.”