Conservation Genetics
05/29/14 1:24 PM

Elephant shark sheds light into bone creation

The Elephant shark is a living fossil.
The Elephant shark is a living fossil.

The elephant shark is a 'living fossil' fish who earned his name from the bottom of his face, that resembles the end of an elephant's trunk.  It has turned to be the most slowest evolving genome of all known vertebrates, whose DNA has barely changed over hundreds of millions of years, with a skeleton made up in cartilage rather than bone and a very simple immune system. An international team of scientist from 12 institutions analyzed it's genome and their foundlings shed light into bone creation carrying potential implications for human bone disease treatment like osteoporosis, and also opens new paths on immune research.

The research, published in the journal Nature, compares elephant shark with human, chicken and other vertebrate genome, finding out that they are unable to replace cartilage with bone because of their lack of only one gene that all vertebrates have. This means that all gene family members involved in bone formation are present in the elephant sharkDNA, except the one that encodes the protein which is required for the collagen to become calcified turning into bone. The absence of this link -- Is the secretory calcium-binding phosphoprotein or SCPP-- in the bone creation chain leads to a cartilaginous fish.

Scientists deleted this gene family in a zebrafish, and they observed a consequently reduction in bone formation, proving it's role in osseous making process. They think that this could be a starting point for a better understanding of human bone diseases such as osteoporosis.  "We speculate that one or more SCPP-related genes, probably Sparc, Sparcl1 or both, mediate the mineralization of skeleton in these vertebrates." says the essay.

But this is not the only crucial gene missing. Unexpectedly researchers couldn't find a component of the immune system: almost none T-helper cells, that are basic in humans because they recognize certain molecules and activates the body immune response to fight against bacterial and viral infections and to prevent autoimmune disease like diabetes.  Despite of this, elephant shark have strong immunity responses and live long and healthy lives. "The structure of the immune system of the elephant shark is very different from mammals," said Thomas Boehm, co-author and director at the Max Planck Institute of Immunobiology and Epigenetics in Freiburg, Germany. "It is obvious thatsharks 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," stated Boehm.

This suggests that helper and regulatory functions of T cells became more elaborate in the first bony vertebrates after they split out from cartilaginous creatures and "provides fresh insight into the mechanism of bone formation and the origin of adaptive immunity" process.

Look into Earth's evolution
Also known as the Australian ghost shark, this estrange fish grows to about 120 centimetres in length, and inhabits temperature waters of the southern coast of Australia and New Zealand, at depths of 200 to 500 meters. It is not a realshark at all. It is, strictly speaking, a chimera, a cousin of sharks, skates and rays who took a separate evolutionary path around 420 million years ago. They are all cartilaginous fishes and the world's oldest-living jawed vertebrates, that diverged from the bony vertebrates 450 million years ago, and give us a look into the Earth's evolution.

This animal was selected among the cartilaginous fishes because their DNA is extremely compact, being less than a third that of humans. Researchers estimated the molecular evolutionary rate of the  elephant shark and compared it with other fishes concluding that their genetic structure has remained robustly unchanged though million years. "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," Byrappa Venkatesh, research director of the Agency for Science, Technology and Research's Institute of Molecular and Cell Biology, said in a statement.

"With the elephant shark genome in hand, we can begin to identify key genetic adaptations in the evolutionary tree, " added senior author Wesley Warren, PhD, research associate professor of genetics at The Genome Institute at Washington University School of Medicine.

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