WASTEBOOK: Drooling Monkeys and the Evolution of Saliva

$817,000

New York

National Institutes of Health

Drool from monkeys, gorillas, orangutans, macaques, and humans was compared in an attempt to gain “insights into evolution of saliva.”

The study was funded, in part, from two National Institutes of Health (NIH) grants totaling $817,000 to the State University of New York at Buffalo (UB).

DNA samples of saliva from the primates were purchased and then analyzed using polymerase chain reaction (PCR).  PCR is a technique that copies and amplifies small segments of DNA to conduct molecular and genetic analyses.

The research specifically focused on the salivary mucin-7 (MUC7) gene, that “tells the body how to create a salivary protein of the same name.”  This protein, “which is long and thin, forms the backbone of a bottlebrush shaped molecule that helps to give spit its slimy, sticky consistency.”

The analysis found the instructions within the MUC7 gene “for building important components of the bottlebrush were repeated multiple times” in each of the five primate species studied.   Gorillas had the fewest copies of this information (4-5), while African green monkeys had the most (11-12). Humans fell somewhere in between, with 5-6.”

The researchers then simulated evolutionary changes in the composition of the saliva gene over 11 million years from a common ancestor.  They assumed “every 1 million year [sic], there is a random gain or loss of 0.5/1.0/1.5/2.0 copies for Orangutan and the common ancestor of Human, Chimpanzee and Gorilla separately. At 8 million years ago, the common ancestor of Human and Chimpanzee separated from Gorilla and they started the copy number gain and loss simulation separately. The same simulation continues to 5 million years ago that Human and Chimpanzee separated from each other, and start their copy number gain/loss process independently until present. We simulated this process 1,000 times for 4 different copy-number-change rates (0.5/1.0/1.5/2.0 copies per million year), and for each simulation, calculated the variation of final state of simulated copy numbers for Human, Chimpanzee, Gorilla and Orangutan. The observed copy number state in present is Human 5 or 6 copies, Chimpanzee 5 copies, Gorilla 4 or 5 copies, and Orangutan 6 or 7 copies.”

So what does that all mean?

“This diversity in humans and other primates is ‘fodder for rapid evolution,’” the scientists write in a study published in Scientific Reports.  It is “unusual for members of a single species to have varying numbers of tandem repeats,” which are “short strings of DNA found multiple times inside the gene.”

The researchers speculate that by “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.”

The authors of the study do caution that “gene predictions, especially for genes that have repeat content as in MUC7, may be error prone.”

As sticky as the subject may be, saliva does serve important functions. Other studies— including some conducted at UB— have examined the importance of saliva for human health. While the findings of those efforts may lead to exciting scientific breakthroughs, this particular study is nothing to drool over.

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