A sun-tailed monkey. Credit: Peggy Motsch
Key points:
- Scientists believe that tail loss in primates occurred about 25 million years ago.
- Researchers compared the DNA of tail-less apes and humans to DNA from tailed monkeys to identify a genetic code insertion that may explain tail loss.
- The difference in tails was linked to the random insertion of the AluY transposon in an intron within the TBXT gene that influenced alternative splicing to result in a variety of tail lengths.
Scientists believe that tail loss in primates including gorillas, chimpanzees, and humans occurred about 25 million years ago, when they evolved away from Old World monkeys. A new study, published in Nature, explains how evolution removed humans’ tails and points to a genetic change in ancient ancestors.
Researchers compared the DNA of tail-less apes and humans to DNA from tailed monkeys. They found a change to the genetic code—within the TBXT gene—that was shared by apes and humans, but missing in monkeys. More specifically, the difference in tails was not linked to mutations in the TBXT gene, but instead resulted from the insertion of a DNA transposon into the regulatory code.
The AluY transposon insertion randomly occurred in an intron within the TBXT gene that influenced alternative splicing to result in a variety of tail lengths. Researchers used this information to engineer a series of mice with TBXT insertions, which resulted in a variety of tail effects including mice born without tails.
The research team also identified an AluY insertion that remained in the same location within the TBXT gene in humans and apes and resulted in the production of two forms of TBXT RNA. They theorized that one RNA form would directly contribute to tail loss.
“This finding is remarkable because most human introns carry copies of repetitive, jumping DNA without any effect on gene expression,” explained senior author Jef Boeke of NYU Grossman School of Medicine. “This particular AluY insertion did something as obvious as determine tail length.”
Genes frequently influence more than one function, meaning changes that bring an advantage in one area may be detrimental elsewhere. Although tail loss was highly beneficial, the associated gene insertion may have other consequences. In their mouse experiments, the research team identified a small uptick in neural tube defects in mice that had insertions in the TBXT gene.
“Future experiments will test the theory that, in an ancient evolutionary trade-off, the loss of a tail in humans contributed to the neural tube birth defects, like those involved in spinal bifida,” said co-senior author Itai Yanai of NYU.