Toothed whale echolocation organs evolved from jaw muscles

Genetic analysis finds evidence suggesting that acoustic fat bodies in the heads of toothed whales were once the muscles and bone marrow of the jaw.

Dolphins and whales use sound to communicate, navigate and hunt.

New research suggests that the collections of fatty tissue that enable toothed whales to do so may have evolved from their skull muscles and bone marrow.

Scientists at Hokkaido University determined DNA sequences of genes which were expressed in acoustic fat bodies -- collections of fat around the head that toothed whales use for echolocation.

They measured gene expression in the harbor porpoise (Phocoena phocoena) and Pacific white-sided dolphin (Lagenorhynchus obliquidens). Their findings were published in the journal Gene.

The evolution of acoustic fat bodies in the head -- the melon in the whale forehead, extramandibular fat bodies (EMFB) alongside the jawbone, and intramandibular fat bodies (IMFB) within the jawbone -- was essential for sound use such as echolocation.

However, little is known about the genetic origins of those fatty tissues.

"Toothed whales have undergone significant degenerations and adaptations to their aquatic lifestyle," said Hayate Takeuchi, a PhD student at Hokkaido University's Hayakawa Lab and first author of the study.

One adaptation was the partial loss of their sense of smell and taste, along with the gain of echolocation to enable them to navigate in the underwater environment.

The researchers found that genes which are normally associated with muscle function and development were active in the melon and EMFBs.

There was also evidence of an evolutionary connection between the extramandibular fat and the masseter muscle, which in humans connects the lower jawbone to the cheekbones and is a key muscle involved in chewing.

"This study has revealed that the evolutionary tradeoff of masticatory muscles for the EMFB -- between auditory and feeding ecology -- was crucial in the aquatic adaptation of toothed whales," said Assistant Professor Takashi Hayakawa of the Faculty of Environmental Earth Science, who led the study.

"It was part of the evolutionary shift away from chewing to simply swallowing food, which meant the chewing muscles were no longer needed."

Analysis of gene expression in the intramandibular fat detected activity of genes related to immune functions, such as the activation of some elements of the immune response and regulation of T cell formation.

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Searching for ancient bears in an Alaskan cave led to an important human discovery

The first people to live in the Americas migrated from Siberia across the Bering land bridge more than 20,000 years ago. Some made their way as far south as Tierra del Fuego, at the tip of South America. Others settled in areas much closer to their place of origin where their descendants still thrive today.

In "A paleogenome from a Holocene individual supports genetic continuity in Southeast Alaska," published Friday in the journal iScience, University at Buffalo evolutionary biologist Charlotte Lindqvist and collaborators show, using ancient genetic data analyses, that some modern Alaska Natives still live almost exactly where their ancestors did some 3,000 years ago.

Lindqvist, PhD, associate professor of biological sciences in the UB College of Arts and Sciences, is senior author of the paper. In the course of her extensive studies in Alaska, she explored mammal remains that had been found in a cave in the state's southeast coast. One bone was initially identified as coming from a bear. However, genetic analysis showed it to be the remains of a human female.

"We realized that modern Indigenous peoples in Alaska, should they have remained in the region since the earliest migrations, could be related to this prehistoric individual," says Alber Aqil, a UB PhD student in biological sciences and the first author of the paper. This discovery led to efforts to solve this mystery, which DNA analyses are well suited to address when archeological remains are as sparse as these were.

Learning from an ancestor

The earliest peoples had already started moving south along the Pacific Northwest Coast before an inland route between ice sheets became viable. Some, including the female individual from the cave, made their home in the area that surrounds the Gulf of Alaska. That area is now home to the Tlingit Nation and three other groups: Haida, Tsimshian, and Nisga'a.

As Aqil and colleagues analyzed the genome from this 3,000-year-old individual -- "research that was not possible just 20 years ago," Lindqvist noted -- they determined that she is most closely related to Alaska Natives living in the area today. This fact showed it was necessary to carefully document as clearly as possible any genetic connections of the ancient female to present-day Native Americans.

In such endeavors, it is important to collaborate closely with people living in lands where archeological remains are found. Therefore, cooperation between Alaska Native peoples and the scientific community has been a significant component of the cave explorations that have taken place in the region. The Wrangell Cooperative Association named the ancient individual analyzed in this study as "Tatóok yík yées sháawat" (Young lady in cave).

Genetic continuity in Southeast Alaska persists for thousands of years

Indeed, Aqil and Lindqvist's research demonstrated that Tatóok yík yées sháawat is in fact closest related to present-day Tlingit peoples and those of nearby tribes along the coast. Their research therefore strengthens the idea that genetic continuity in Southeast Alaska has continued for thousands of years.

Human migration into North America, although it began some 24,000 years ago, came in waves -- one of which, about 6,000 years ago -- included the Paleo-Inuit, formerly known as Paleo-Eskimos. Importantly for understanding Indigenous peoples' migrations from Asia, Tatóok yík yées sháawat'sDNA did not reveal ancestry from the second wave of settlers, the Paleo-Inuit. Indeed, the analyses performed by Aqil and Lindqvist helped shed light on the continuing discussion of migration routes, mixtures among people from these different waves, as well as modern territorial patterns of inland and coastal people of the Pacific Northwest in the pre-colonial era.

Oral history links an ancient woman to people living in Southeast Alaska today

The oral origin narratives of the Tlingit people include the story of the most recent eruption of Mount Edgecumbe, which would place them exactly in the region by 4,500 years ago. Tatóok yík yées sháawat, their relative, therefore informs not just modern-day anthropological researchers but also the Tlingit people themselves.

Out of respect for the right of the Tlingit people to control and protect their cultural heritage and their genetic resources, data from the study of Tatóok yík yées sháawat will be available only after review of its use by the Wrangell Cooperative Association Tribal Council.

"It's very exciting to contribute to our knowledge of the prehistory of Southeast Alaska," said Aqil.

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