Top Story: January 13, 2014

The Slow-Evolving Elephant Shark

elephant, shark, genome, cartilage, cartilaginous, bones, immunity, rays

By Molly Michelson

Findings from a publication last week demonstrate that elephant sharks are resistant to change. Heck, who isn’t? But these small, funny-looking fish provide an excellent example in the natural world. Their genome shows that this species has changed very little during their 420 million years on the planet.

At only one billion base pairs, the elephant shark’s genome is the smallest among its shark and ray brethren, and thus, it became the first cartilaginous fish to have its genome sequenced.

And despite the size of the shark’s small genome, researchers got a lot of results from the sequencing! The team found that the elephant shark is the slowest evolving vertebrate sequenced to date, making it more of a living fossil than even the ancient coelacanth.

Considering both the age of elephant sharks and their cartilaginous nature, according to Scientific American, the researchers,

…hoped to gain insight into the evolution of bony skeletons, because elephant sharks (like other sharks, mantas and rays) have skeletons made of cartilage, a trait that is now rare among vertebrates but was the norm when elephant sharks first evolved.

Indeed, by analyzing the elephant shark genome and comparing it with other genomes, the scientists discovered a family of genes that is absent in the elephant shark but present in all bony vertebrates such as chickens, cows, mice, and humans. When the researchers deleted a member of this gene family in zebrafish, they observed a reduction in bone formation, highlighting the gene family’s significance in making bone. The team says the findings have important implications for understanding bone diseases such as osteoporosis and for developing more effective therapies to treat these conditions.

But wait, there’s more! In a surprise finding, the team found that the elephant shark appears to lack special types of immune cells that are essential to mounting a defense against viral and bacterial infections and for preventing autoimmune diseases such as diabetes and rheumatoid arthritis. Despite possessing relatively rudimentary immune systems, however, Elephant sharks exhibit robust immune responses and live long lives. The new discovery opens up the possibility of developing new strategies to shape the immune response in humans.

“The slow-evolving genome of the elephant shark is probably the best proxy for the ancestor of all jawed vertebrates that became extinct a long time ago,” says lead author Byrappa Venkatesh, of the Agency for Science, Technology, and Research in Singapore. “It is a cornerstone for improving our understanding of the development and physiology of human and other vertebrates as illustrated by our analysis of the skeletal system and immune system genes.”

The research was published last week in Nature.

Image: Byrappa Venkatesh

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