The animal kingdom is practically dripping in mucus.
Amphibians, snails and slugs are among the most famous masters of slime, but even the loneliest microorganism can ooze a viscous slime from time to time.
In our own species, mucus is produced in the mouth, nose, throat, lungs, intestine, cervix, and urinary tract, all for a variety of purposes.
However, the origin of the world’s slime is a mystery.
Despite the many similarities between mucus, many forms have evolved in parallel rather than in a branching, treelike fashion.
Across the glands and across mammals, a small study has found that many mucus genes do not in fact share a common ancestor.
This is unusual because most genes with similar functions originate from a shared ancestral gene that is passed down from generation to generation because it confers survival benefits.
Even in our own species, the genes encoding mucus proteins belong to several families. One secretes gelforming mucus proteins, while another produces mucus proteins attached to a cell’s membrane. There are also “orphan” genes that code for mucus production that don’t fit anywhere else.
Each of these separate lineages probably evolved independently, and now researchers think they’ve figured out where they came from.
By comparing the genes that code for mucus, known as mucin genes, among 49 mammalian species, the team found that nonmucin proteins can evolve into smooth mucin proteins when short repeated chains of amino acids (building blocks of proteins).
Among all mucin genes studied, these random repeats were counted 15 different times.
In other words, some mammalian genes that code for nonmucin proteins lean toward viscosity over time. According to the authors of the current study, proteins that are rich in the organic acid proline are more likely to be sticky across generations.
“I don’t think it was previously known that protein function could evolve in this way, from a protein gaining repeated sequences,” says evolutionary biologist Omer Gokcumen of the University at Buffalo.
“A protein that’s not a mucin becomes a mucin just by gaining repeats. That’s an important way evolution makes slime. It’s an evolutionary trick, and now we document that this happens over and over again.”
The authors made their discovery when studying human saliva. During the experiments, they noticed that a particular mucin gene in humans had similarities to another seen in mice.
When they tried to find a common ancestor, however, they failed.
The mucin gene in mice appeared to have evolved independently, although part of the gene shared a structure seen in the genes responsible for human tears, which are not considered mucus.
“We think that somehow this lacrimal gene ends up being reused,” explains Gokcumen.
“It gets the repeats that give it mucin function, and it’s now abundantly expressed in mouse and rat saliva.”
If Gokcumen and his colleagues are correct, their results offer scientists a new genetic evolutionary mechanism: the formation of a new gene function without the usual process of a gene duplication event.
This parallel series of mutations in unrelated genes that result in the same function is an example of convergent evolution (where selective pressure shapes the same function from unrelated biological origins, such as bat and bird) that happens at the genetic level.
“If these mucins keep evolving from nonmucins over and over again in different species at different times, it suggests that there is some kind of adaptive pressure that makes it beneficial,” says evolutionary geneticist Petar Pajic of the University at Buffalo.
“And then at the other end of the spectrum, maybe if that mechanism goes off the rails, happens too much or in the wrong tissue, maybe it can lead to diseases like certain cancers or mucosal diseases.”
While the study of mucus may not seem like the most magical of scientific endeavors, it’s hardly a quest to smell.
The study was published in Advances in Science.