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Mucus has evolved at least 15 times in mammals

Mucus-producing mucin proteins have repeatedly and independently arisen in mammals, possibly through co-option of existing proteins in slime factories.


26 Aug 2022

Slime is filled with mucus, produced by many independently evolved proteins in mammals.

Sandra Mailer/Shutterstock

Mucus-producing proteins appear to have evolved in at least 15 independent instances in mammals, possibly by incorporating existing mucus-producing proteins.

From the sticky saliva of a dog to the slippery coating of a slug, mucus is found almost everywhere in the animal kingdom. “Almost all animals, even yeast and bacteria, have mucus,” he says. Omer Gokcumen at the University at Buffalo in the State of New York. “It is a type of substance essential for life.”

Mammals produce mucus through toilet brush-shaped proteins called mucins, which make body fluids sticky and slippery. Most animals have numerous mucins whose viscous products combine to create the correct thickness and smoothness in different areas of the body.

Gokcumen first investigated mucins after making an unexpected discovery in mice. He noted that the primary mucin in human saliva, called MUC7, is absent in rodents. Instead, mice saliva is thickened with a mucin called MUC10 that humans lack. When he investigated, he and his team found that the two mucins were evolutionarily unrelated, a break from the usual trend in which animals share proteins from a common ancestral gene.

Then the team found another surprise. MUC10, the mouse saliva protein, remarkably resembled the protein that lubricates human tears, called PROL1. Unlike mouse mucin, PROL1 lacked specific amino acid repeats, the sugar-coated building blocks of a protein.

“We had these two different mucins with two different evolutionary origins. We’re like, that’s really cool, and we want to know if this is really happening somewhere else, or is it just one of those weird, meticulous, evolutionary once-in-a-lifetime stories? Gokumen says.

Through a genetic analysis of 49 different mammals, from pangolins to rhinos, the team was able to identify 15 distinct mucins that were not present in other species, which Gokcumen calls “orphan mucins.” Finding one new mucin would have been surprising, he says, but finding more than a dozen was a shock.

“[These mucins] they don’t even exist in other species. They are only specific for cows, only specific for ferrets, only specific for humans,” says Gokcumen. “The reason why [mucins] they are strange because they don’t come from a single genetic ancestor, but instead seem to be evolving independently in different lineages in different ways,” he says.

The team suspects that the new mucins are co-opted from existing proteins. By duplicating sections of specific amino acids, the proteins elongate and transform into a slime-producing mucin.

Most species with unique mucins have only one, but others stood out: ferrets have a total of five unique mucins just to themselves.

Gokcumen anticipates that many unique mucins remain to be discovered. Next, he hopes to investigate how many times the goo has turned into slugs and snails.

magazine reference: Progress of science, DOI: 10.1126/sciadv.abm8757

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