An international team of researchers has extracted and analyzed ancient DNA from fragmentary remains of 12 individuals of the Steller’s sea cow (Hydrodamalis gigas), an extinct sirenian and one of the largest Quaternary mammals, recovered from beaches of Bering Island. They’ve identified convergent evolution between Steller’s sea cow and cetaceans but not extant sirenians, suggesting a role of several genes in adaptation to cold aquatic environments. They’ve also found that Steller’s sea cow diversity was already declining some 900 to 800 years before their discovery, implying that environmental changes also contributed to their extinction.
The diversity of the Steller’s sea cow (Hydrodamalis gigas) was already declining some 900 to 800 years before its discovery by Georg Wilhelm Steller. Image credit: R. Ellis.
The Steller’s sea cow is an extinct species of marine mammal from the order Sirenia.
The species inhabited the coastal areas of the North Pacific Ocean — including the Bering Sea — during the Pleistocene and Holocene epoch.
It was described in 1741 by the German biologist Georg Wilhelm Steller, who participated in the Vitus Bering’s Great Northern Expedition.
It became extinct 27 years later, presumably as a result of human-driven habitat change and overexploitation.
Adult Steller’s sea cows reached a length of 10 m (33 feet), weighed over 10 tons, and stored up to 10 cm (3.9 inches) of blubber in some areas of the body. This made them ideal resources for human hunters, who exploited them for meat, fat, and skin.
Their skin was thick, hairless, and roughly textured, an attribute that led Steller to describe it as resembling ‘more the bark of an old oak tree, than the skin of an animal.’ The rough, bark-like skin may have been an adaptation to their shallow water habitat, perhaps preventing abrasion on ice or rocks.
For centuries, naturalists relied solely on phenotypes to describe species, lacking opportunity to explore the genetic underpinnings of those traits.
“The archeological record is not very good, but from the demographic analysis of the genomes it looks like their population had been declining for at least a half million years or so,” said Professor Beth Shapiro, a researcher in the Department of Ecology and Evolutionary Biology and the Howard Hughes Medical Institute at the University of California, Santa Cruz.
In their research, Professor Shapiro and colleagues extracted ancient DNA from fragmentary remains of 12 Steller’s sea cow individuals recovered from beaches of Bering Island, which they radiocarbon dated to 2,205 to 1,155 years before the present.
They then sequenced and analyzed the genomes of two of the best preserved individuals.
They found that two lipoxygenase genes associated with skin development were inactivated in Steller’s sea cows.
Remarkably, the same genes are inactivated by mutations in humans who are affected by a skin disease, called ichthyosis, characterized by thick, rough skin.
“We can only speculate about what drove this adaptation in Steller’s sea cows,” said Dr. Molly Cassatt-Johnstone, a researcher in the Department of Ecology and Evolutionary Biology at the University of California, Santa Cruz.
“The thick skin could very well have helped them maintain their core temperature in cold water, and it could also have been defensive or protective.”
Scanning the genomes of other marine mammals, the scientists found that the lipoxygenase genes inactivated in Steller’s sea cows are also inactivated in whales and other cetaceans, but are active in seals, sea lions, sea otters, polar bears, dugongs, and manatees.
Cetaceans shed their outer layer of skin rapidly, which prevents the buildup of thick skin that would otherwise occur in the absence of the lipoxygenase enzymes.
The authors also found similarities between Steller’s sea cows and cetaceans in certain genes involved in energy metabolism, suggesting that both lineages evolved similar adaptations to cold-water habitats.
The development of the same adaptations in unrelated species is known as convergent evolution.
“In this case, Steller’s sea cows and whales have independently evolved mutations in the same genes, multiple times in different lineages,” Dr. Cassatt-Johnstone said.
Steller’s sea cows once inhabited shallow coastal waters around the Pacific Rim from California to Japan, but by the time of Steller’s voyage they were restricted to the waters around the Commander Islands, which include Bering Island.
The genetic evidence of a long-term decline suggests that environmental changes played a role, but the extinction of the last remaining population was caused by humans.
“It was a long, slow decline, and then we killed off the last population,” Professor Shapiro said.
“Genomes from the past can help us understand species today.”
“Understanding how the sea cow adapted to cold-water environments can provide insights into these endangered warm-water species, which we are studying now.”
The team’s results were published in the journal Science Advances.
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Diana Le Duc et al. 2022. Genomic basis for skin phenotype and cold adaptation in the extinct Steller’s sea cow. Science Advances 8 (5); doi: 10.1126/sciadv.abl6496
