The all-female Korean Haenyeo divers show genetic adaptions to cold water diving

"The Haenyeo are amazing, and their incredible ability is written in their genes," says geneticist Melissa Ilardo of the University of Utah. "The fact that women are diving through their pregnancy, which is a really tough thing to do, has actually influenced an entire island's people."

The Haenyeo, or "women of the sea," dive year-round in social collectives to harvest food for their communities. They begin training at around age ten and continue for their whole lives. Inspired by the Haenyeo's remarkable diving abilities, the researchers wanted to know whether they have distinguishable physiological traits that help them cope with the strain of diving, and if so, whether these traits are due to genetic adaptation or training.

To find out, the team compared the physiological traits and genomes of 30 Haenyeo divers to 30 non-Haenyeo people from Jeju, as well as 31 people from mainland Korea. To match the age of the divers, the average age of all participants was 65. The researchers compared the participants' heart rate and blood pressure at rest and during "simulated dives" where the participants held their breath while submerging their faces in cold water.

"If you hold your breath and put your face in a bowl full of cold water, your body responds as if you're diving," says Ilardo. "A lot of the same processes happen in your body that would happen if you were to jump in the ocean, but it's done in a way that's safe for people with no diving experience."

The team's genomic analysis showed that Jeju residents -- both Haenyeo and non-Haenyeo -- were distinct from individuals from mainland Korea, suggesting that all Jeju residents are descended from the same ancestral population.

"We can essentially think of everyone from Jeju as either 'diving Haenyeo' or 'non-diving Haenyeo,' because their genetics are the same," says Ilardo.

The genomic analysis also revealed two gene variants in the Haenyeo that may help them cope with the pressures of diving, making the Haenyeo the second known population of traditional breath-hold divers that has evolved for diving. One gene is associated with cold tolerance, which could make the divers less vulnerable to hypothermia. The other gene is associated with decreased diastolic blood pressure (i.e., blood pressure in between heart contractions). The variant was found in 33% of participants from Jeju but only 7% of mainland participants.

"This association may reflect natural selection to mitigate the complications of diastolic hypertension experienced by female divers while diving through pregnancy," says Ilardo. "Since Bajau women also dive while they're pregnant, we wonder whether pregnancy is actually driving a lot of the genetic changes in these diving populations."

During the simulated dives, all of the participants showed decreased heart rates, but the Haenyeo's heart rates dropped significantly more than those of either control group. On average, the divers' heart rates decreased by 18.8 beats per minute (bpm) compared to a decrease of 12.6 bpm in the Jeju non-divers. A lowered heart rate during diving is beneficial because it saves energy and conserves oxygen. Since their genomic analysis indicated that Haenyeo and non-diving Jeju are genetically members of the same population, the researchers concluded that this feature is likely due to the divers' training.

"Because the Haenyeo have been diving for a very long time, their heart rate has been trained to drop more," says Ilardo. "This was something we could actually visually see -- we had one diver whose heart rate dropped by over 40 beats per minute in less than 15 seconds."

The researchers say that these findings highlight the potential of studying traditional diving populations to better understand human genetic and physiological adaptation.

"We're really excited to learn more about how these genetic changes may be affecting the health of the broader population of Jeju," says Ilardo. "If we can more deeply characterize how those changes affect physiology, it could inspire the development of therapeutics to treat different conditions, such as hypertensive disorders of pregnancy and stroke."

This research was supported by the Office of Naval Research, the National Institutes of Health, and the National Science Foundation.