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Jan 08, 2014 04:03 PM EST

Elephant Shark's Entire Genome Sequenced, A First for Any Cartilaginous Fish

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For the first time, researchers have fully sequenced the genome of a cartilaginous fish and the study could potentially give brand new information on the evolution of bony vertebrates.

To be published Thursday, the study examined an elephant shark genome, which was relatively small. The genome contained just shy of a billion DNA base pairs, whereas a human genome contains about three billion.

The study also marks the first time a cartilaginous fish, which include sharks, rays and skates, had its entire genome sequenced, according to a press release.

Unlike other jawed vertebrates, the elephant shark does not convert tis cartilage into bone, due to a lack of secreted phosphoprotein genes. The elephant shark also does not produce certain immune system cells and protein receptors. This may suggest the adaptive immune system, which acutely targets and defends against specific diseases, may actually become stronger and more sophisticated over time.

"It's pretty well established in vertebrates that very little of this intron loss and creation occurs," San Francisco State University assistant professor of biology and study co-author Scott Roy said in the release. "It's unlikely to have that big of a change exactly and all at once."

Roy said one of the key discoveries was the elephant shark's extremely slow rate of evolution. He found in an analysis of the shark's genome's introns, which "interrupt" the gene sequencing and have to mesh together in order for the gene to be expressed. In his analysis, Roy found very few of these intron changes to have occurred in the elephant shark's genome sequence.

"Because vertebrate introns are very long, this may make it harder to create and delete them," Roy said. "In some tunicates, introns are about 40 nucleotides long, and this simply may make it easier for them to come and go over time."

Study lead author Byrappa Venkatesh, PhD, said parts of the elephant shark's chromosomes resembled those of a huma's. The slow evolution trait will be important in further studying the human genome.

"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," Venkatesh, research director at the Institute of Molecular and Cell Biology at the Agency for Science, Technology and Research, in Singapore, said in a press 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." 

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