Why Sharks Have No Bones: Geneticists Sequence Half-Billion-Year-Old Shark Genome, Yielding Clues About Our Own Immune System

By Gabrielle Jonas on January 8, 2014 10:51 PM EST

Drawing of an Elephant Shark
Geneticists now know that the 1/2 billion-year-old Elephant Shark possesses only a rudimentary immune system, and that a fluky evolutionary event made it lose its skeleton in favor of cartilage. (Photo: State Library of New South Wal)

Geneticists Wednesday said they had sequenced the genome of the elephant shark — revealing much about the evolution of vertebrates. The shark, which uses its elongated snout to ferret out shellfish buried in the sand under the deep waters off of New Zealand and southern Australia, is almost unchanged since it first hit the scene on earth almost half a billion years ago. The sequencing has yielded exciting clues about the development of the immune system in all of us, and even answering the question of why sharks have no bones.

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The elephant shark's genome may help solve a mystery about the evolution of acquired immunity, which allows humans and other vertebrates to fight off new pathogens, according to lead author Dr. Byrappa Venkatesh, a geneticist at the Institute of Molecular and Cell Biology, in Singapore. Though elephant sharks possess killer T cells, which directly destroy body cells infected by viruses, they lack helper T cells, which assist in orchestrating a more comprehensive immune response to an infection — suggesting that acquired immunity evolved in a two-step rather than one-step process, as previously thought. Venkatesh's  article was published in Wednesday's issue of Nature.

Though an elephant shark's skeleton is made of cartilage, the fact that it also possesses bony fin spines and teeth have confounded evolutionary biologists. The genome sequence revealed that the happy accident of genetic material doubling transformed cartilage into bone in vertebrates. Meanwhile, sharks are missing a single gene family that regulates the process of turning cartilage into bone. When the researchers knocked out one of these same genes in a zebrafish, it severely reduced its bone-forming ability. Because the elephant shark is evolving much more slowly than any other vertebrate on the planet since it first appeared on earth 420 million years ago, its genome is a great model for inferring the state of the ancestral genomes. "Its value for comparative genomic studies is illustrated by our analysis of genetic events that led to the ossification of endoskeleton in bony vertebrates," Venkatesh wrote.

In addition, because the elephant shark is the verterbrate that has evolved the slowest of any one on earth, and was the first veterbrate to have a jaw, its genome will yield important secrets about ancient evolution. "The sequence will serve as an important baseline for comparing the genomes of many creatures," Venkatesh wrote. "We are going to use this as a reference for years to come."

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