Elephant Shark DNA Decoded
January 8, 2014

Decoded Shark DNA Sheds Light On Evolution Of Bone Formation And Immunity

Ranjini Raghunath for redOrbit.com - Your Universe Online

An international team of scientists has deciphered the genetic blueprint of the elephant shark - the oldest living animal in the cartilaginous fish family and the first to have its DNA decoded.

Among fish, cartilaginous fish such as sharks and rays stand out because their skeleton is not made of bones. Their body is supported instead by cartilage, a flexible tissue that resembles muscle.

Analysis of the elephant shark’s DNA answers questions such as which genes influence bone formation, why elephant sharks have a primitive immune system, and why they evolve very slowly. The findings could also help researchers understand and treat bone diseases better as well as shed light on how the immune system evolved in humans and other higher animals.

The elephant shark lives off the coast of Australia and New Zealand and is named for its trunk-like snout which it uses to dig for food on the seafloor. Its genome is about one-third the size of the human genome, and the smallest among all jawed fishes.

Comparing its DNA with that of humans and other animals reveals some interesting facts about the shark’s evolution and body structure. For one, the shark’s genome lacks a family of genes that code for a class of proteins called phosphoproteins that are present in bony animals. When the researchers knocked out this gene family in zebrafish, they found that it affected the fishes’ ability to form bones. The absence of these genes is probably why sharks have cartilage instead of bone in their body, the researchers suspect.

Elephant sharks also lack genes for special immune cells called T-helper cells. These cells are thought to be vital for defending the body against parasites and prevent diseases such as diabetes and rheumatoid arthritis. Killing these T-cells is also one of the first steps the HIV virus takes in breaking down the human body’s defenses.

Yet even without these cells, these sharks have long lives and are somehow able to protect themselves against diseases.

“It is obvious that sharks can efficiently deal with all kinds of infections without this particular cell type. This indicates that nature can come up with different solutions to the same problem,” said Thomas Boehm, co-author and biologist at Max Planck Institute, Germany.

Further research could unlock secrets about how such a primitive immune system evolved into a more advanced form in humans and other animals, the researchers believe.

Surprisingly, the elephant shark’s DNA also appeared to be more closely related to human DNA than other fish such as zebrafish and pufferfish, which have bony skeletons. It’s genome has also remained largely unchanged over hundreds of millions of years, particularly in specific DNA segments called introns, the researchers found.

“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, lead author and scientist at the A*STAR Institute, 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 study was published online in the journal Nature.