Humans, whales, elephants, and naked mole-rats all share a somewhat rare trait for mammals: their bodies are covered with little to no hair. The common ancestors of each of these species are considerably hairier which must mean that hairlessness evolved multiple times independently. To identify genomic regions that appear to have evolved at a faster or slower evolutionary rate along hairless lineages, researchers from the University of Pittsburgh and the University of Utah scanned the genomes of multiple mammalian species. They identified a number of protein-coding genes as well as noncoding regions that might explain how hairlessness evolved in mammals.
Hair is a defining mammalian characteristic with a variety of functions, from sensory perception to heat retention to skin protection.
Although the mammalian ancestor is believed to have had hair, and in fact the development of hair is a key evolutionary innovation along the mammalian lineage, numerous mammals subsequently lost much of their hair.
Many marine mammals, including whales, dolphins, porpoises, manatees, dugongs, and walruses, have sparse hair coverage likely related to hydrodynamic adaptations to allow those species to thrive in a marine environment.
Large terrestrial mammals such as elephants, rhinoceroses, and hippopotamuses also have little hair, likely to enable heat dissipation diminished by the species’ large sizes.
Notably, humans are also relatively hairless, a characteristic that, while stark, has long been of mysterious origin.
“We have taken the creative approach of using biological diversity to learn about our own genetics,” said senior author Dr. Nathan Clark, a human geneticist at the University of Pittsburgh.
“This is helping us to pinpoint regions of our genome that contribute to something important to us.”
To detangle the mystery of mammalian hair loss, Dr. Clark and his colleagues searched for genes in hairless animals that evolved at faster rates compared to their counterparts in hairy animals.
“As animals are under evolutionary pressure to lose hair, the genes encoding hair become less important,” Dr. Clark said.
“That’s why they speed up the rate of genetic changes that are permitted by natural selection.”
“Some genetic changes might be responsible for loss of hair. Others could be collateral damage after hair stops growing.”
To perform the search, the authors developed computational methods that could compare hundreds of regions of the genome at once.
They surveyed 19,149 genes and 343,598 regulatory regions that were conserved across the dozens of mammalian species analyzed.
In the process, they took steps to discount genetic regions responsible for evolving other species-specific traits, such as adapting to aquatic life.
“The fact that the unbiased screen identified known hair genes demonstrated that the approach worked,” Dr. Clark said.
“It also suggests that the genes identified in the screen that are less well-defined could be just as important for having hair — or not having it.”
The team is now using the same approach to define genetic regions involved in preventing cancer, extending lifespan, and understanding other health conditions.
“This is a way to determine global genetic mechanisms underlying different characteristics,” Dr. Clark said.
The findings were published in the journal eLife.
Amanda Kowalczyk et al. Complementary evolution of coding and noncoding sequence underlies mammalian hairlessness. eLife, published online November 7, 2022; doi: 10.7554/eLife.76911
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