Sponge Genes Mean the Origin of Neurons and Other Cells

In fact, many multifunctional cells in sponges express modules of genes that are commonly associated with specialized cells in more complex animals such as vertebrates. For example, sponges of neuroid cells not only express some of the presynaptic machinery of neurons, but also express immune genes. (It is possible that if neuroid cells monitor the microbial content in the digestive chambers for sponges, these immune genes will help play that role.) Sponges also have cells called pinacocytes that simultaneously contract like cells in muscle to squeeze the animal and remove dirt or unwanted debris. ; Pinacocytes have some sensory machinery that responds to nitric oxide, a vasodilator.

“Nitric oxide is what relaxes our smooth muscle in our blood vessels, so when our arteries dilate, that’s the nitric oxide that causes relaxation,” Musser said. “And we’ve actually shown through paper experiments that nitric oxide also regulates contractions in this sponge.” Like glutamate, nitric oxide may be part of an early signaling mechanism to coordinate primitive sponge behaviors, he suggests.

“Our data are very consistent with this idea that many important parts of machinery existed early in animal evolution,” Musser said. “And a lot of early animal evolution is about to start dividing it into different cells. But it’s likely that these early cell types have so many uses, and they have to do a lot of things.” The oldest animal cells, like their close protozoan relatives, likely needed Swiss Army knife cellulars.As multicellular animals evolved, their cells may have different role, a division of work that may have led to more specialized cell types.But different generations of animals may divide things differently and to different degrees.

If mixing and matching genetic modules was an important theme in early animal evolution, then comparing the arrangement and expression of modules in different species can tell us about their history — and about the possible limits how much fun it is to shuffle. A researcher looking for answers is Arnau Sebé-Pedrós, who studied cell type evolution at the Center for Genomic Regulation in Barcelona and who published the first atlases of cell types on sponges, placozoans and comb jellies in 2018.

Sebé-Pedrós hypothesized that the spatial configuration of genes along chromosomes could be revealed because genes together could share regulatory machinery. “I was really surprised at the level of conservation of gene orders in animal genomes,” he said. He suspects that the need to co-regulate sets of function-related genes keeps them in the same chromosomal neighborhood.

Scientists are still in the early days of learning how cell types evolve and relate to each other. But just as important in explaining the mud origins of animal evolution, sponge cell atlases also make a major contribution by revealing the possibilities of animal cell biology. “It is important not only to understand the origin of animals,” says Sebé-Pedrós, “but also to understand things that may be very different from anything we know about other animals.”

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Original story reprinted with permission from Quanta Magazine, an editorial independent publication of Simons Foundation whose mission is to improve the public’s understanding of science by covering research developments and trends in mathematics and the physical and life sciences.

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