A new study in the journal Scientific Reports shows that adaptation isn’t just about creating new tools for survival, it’s also about tweaking the ones we have.
Genomic location, organization and subexonic copy number variation of the MUC7 gene in the human reference genome: the gray bar indicates the chromosomal band where MUC7 resides; the zoomed-in version of the gene shows the two coding exons (thick blue bars), the UTR region at the 3’ and 5’ ends (thin blue bars) and intronic regions (blue line in-between exons). Start and end of the transcript are indicated by the numbers at each end. The different functional domains within the MUC7 protein are designated by different colors at the bottom of the panel. Image credit: D. Xu et al.
The study authors, from the United States and Greece, examined a gene called the salivary mucin-7 gene (MUC7) that tells the body how to create a salivary protein of the same name.
The MUC7 protein forms the backbone of a bottlebrush-shaped molecule that helps to give spit its slimy, sticky consistency.
The study found that within the MUC7 gene, instructions for building important components of the bottlebrush were repeated multiple times in each of the primate species studied: gorillas had the fewest copies of this information (4-5), African green monkeys had the most (11-12), chimpanzees had 5 copies, orangutan 6-7 copies, and humans had 5-6 copies.
Through an in-depth analysis of MUC7’s evolutionary history, the authors concluded that having numerous copies of the repeated instructions likely conferred an evolutionary advantage to primates — possibly by enhancing important traits of saliva such as its lubricity and, perhaps even more importantly, its ability to bind to microbes (a capability that may help curb disease).
“Evolution can favor the expansion of tried-and-true genetic tools, in addition to the development of totally new ones,” said lead co-author Dr. Omer Gokcumen, from the State University of New York at Buffalo.
“You don’t always have to invent a new tool. Sometimes, you just need to amplify the tool you already have.”
In the case of MUC7, repeating key genetic instructions over and over resulted in longer, denser proteins, which are likely better at performing two protective tasks: lubricating the mouth and latching onto microbes, an action that’s thought to expedite the removal of disease-causing pathogens from the oral cavity.
The genetic instructions that are repeated within the MUC7 gene are what scientists call tandem repeats – short strings of DNA found multiple times inside the gene.
This work shows that as primates evolved, the DNA in their MUC7 tandem repeats sometimes changed in places. But the genetic material stayed the same in one key way: pieces of DNA that told the body how to make the amino acids serine and threonine, two vital building blocks of the bottlebrush backbone, persisted in all primates.
The directions for creating serine and threonine were found in the same location in tandem repeats across humans, gorillas, orangutans, macaques and African green monkeys.
“The likelihood of this happening at random is small, which hints that those genetic sequences provided an evolutionary advantage to their hosts,” Dr. Gokcumen said.
“This hypothesis is bolstered by the crucial role that serine and threonine play in the MUC7 protein’s function.”
Within MUC7, the two compounds act as anchoring points for sugar molecules, which protrude from the protein backbone like the bristles of a brush. It’s these bristles that carry out the important task of binding to microbes.
The research elucidates how tandem repeats may serve as modular building blocks for rapid evolutionary adaptation.
“Tandem repeats may be a major way that many different genes in the genome quickly adapt to their environments,” said lead author Duo Xu, also from the State University of New York at Buffalo.
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D. Xu et al. 2016. Recent evolution of the salivary mucin MUC7. Sci. Rep. 6, 31791; doi: 10.1038/srep31791
