Elephant Shark Genome Sheds Light on Bone Formation and Immunity

An unusual shark with an elephant-like "trunk" may have shed some light onto bone formation and immunity. Scientists have sequenced the elephant shark's genome and have found why the skeleton of sharks is cartilaginous rather than bony.

In order to learn a bit more about the elephant shark and its cousins--sharks, rays, skates and chimaeras--the researchers decided to sequence the genome of the elephant shark. This species was chosen because of its compact genome, which is one-third the size of the human genome. Living off of the southern coast of Australia and New Zealand, the elephant shark uses its unusual snout to dig for crustaceans at the bottom of the ocean floor.

The researchers analyzed the shark genome and then compared it with human and other vertebrate genomes. This allowed them to discover a family of genes that was absent in the elephant shark, but present in all bony vertebrates. When the researchers deleted a member of this gene family in zebrafish, there was a corresponding reduction in bone formation. This, in particular, showed the gene family's significance in making bone.

"We now have the genetic blueprint for a species that is considered a critical outlier for understanding the evolution and diversity of bony vertebrates, including humans," said Wesley Warren, one of the researchers, in a news release. "Although cartilaginous vertebrates and bony vertebrates diverged about 450 million years ago, with the elephant shark genome in hand we can begin to identify key genetic adaptations in the evolutionary tree."

While examining the elephant shark genome, the researchers also found that the shark seems 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. Yet even with a rudimentary immune system, these sharks seem to possess robust immune responses and live long lives. This, in particular, is intriguing to scientists who hope to use the findings to potentially develop 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," said Byrappa Venkatesh, one of the researchers, in a news release. "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 findings are published in the journal Nature.