For a multitude of reasons, great white sharks should be considered nature's ocean-dwelling superheroes — they're big and strong, live long lives, can heal their wounds remarkably fast, and it's even likely that they rarely get cancer. But how is it possible that these ancient giants have so many superhero-like traits? Scientists have now taken a major step toward answering that question by decoding the entire genome of the great white shark.
An international team of researchers led by scientists at the Save Our Seas Foundation Shark Research Center and the Guy Harvey Research Institute at Nova Southeastern University in Florida sequenced the genome of the great white shark (Carcharodon carcharias) and compared it with the genomes of several other vertebrate species. The team discovered a wealth of unusual genetic characteristics that might explain why white sharks are the superheroes (or supervillains, if you're a plump sea lion) of the sea. Their study was published online on Monday (Feb. 18) in the journal Proceedings of the National Academy of Sciences.
Sequencing the great white shark genome was no small task — the genome consisted of 4.63 billion base pairs (the nitrogen-containing molecules that make up the "rungs" of the DNA ladder), which is about 1.5 times the size of the human genome. "It's quite an impressive effort," said Dovi Kacev, a marine molecular ecologist and researcher at the National Marine Fisheries Services Southwest Fisheries Science Center in California, who was not involved with the study. [Image Gallery: Great White Sharks]
Nearly 60 percent of the white shark genome consisted of repeated genetic sequences, which is similar to what's seen in the human genome. What's special about the white shark genome was that many of those repeated regions are codes for a special group of genes known as LINEs.
"These [LINEs] make copies of themselves and then reinsert randomly in various locations in the genome, and in the process they make double-stranded breaks in the DNA that need to be repaired," said Michael Stanhope, an evolutionary biologist at Cornell University in New York. Stanhope co-led the study with Nicholas Marra and Mahmood Shivji, conservation biologists at Nova Southeastern University.
Those frequent breaks in the DNA make the genome unstable, which typically leads to a higher risk of problem-causing genetic mutations that can eventually lead to cancer. But white sharks seem to have evolved a way to avoid such genomic instability.
The researchers found that the white shark genome contained a lot of genes responsible for maintaining genetic stability — things like DNA-repair genes and tumor-suppressing genes. And when the researchers compared the white shark's stability genes with analogous genes in other vertebrates, they found small changes in the gene sequence that suggest a specific pattern of evolutionary adaptation for these genes in white sharks.
"Think of it as fine-tuning the role of these genes in maintaining genome stability in the white shark," Stanhope said.
People have speculated that sharks have a much lower rate of cancer than other animals, but "there's not a lot of real data to say that with certainty," Kacev said. Nonetheless, the abundant presence of specially adapted stability genes could explain the potential cancer resistance.
"If you want to prevent cancer, you need to maintain the stability of your genome," Stanhope said, which means avoiding genetic mutations. An accumulation of excess mutations leads to cancer, but the shark genome seems specifically designed to prevent that. "These are things we would have to test in the lab, though, to really know," he said.
Stanhope also cautioned that while white sharks might have a genetic adaptation to prevent them from getting cancer, that does not mean that consuming shark products could prevent a human from getting cancer, despite what proponents of "alternative medicine" may claim.
Genetics may also explain another one of the white shark's superpowers: the ability to heal quickly. The team discovered several white shark genes that are known to play important roles in vertebrate wound-healing processes. And, similar to the stability genes, the wound-healing genes in the white shark were under the same kind of positive evolutionary selection pressures, meaning there's a tendency for the number of these beneficial traits to increase.
"We also found an enrichment of genes for both wound healing and genome stability genes in the white shark compared to other vertebrates," Stanhope said. In other words, the white shark genome has a higher proportion of these types of genes compared with what's seen in other vertebrate genomes.
While the team discovered a number of genes that may be responsible for many of the great white's super-shark abilities, there was one characteristic that wasn't clearly accounted for by the genome: the shark's sensitive sniffer.
Sharks are known for having a keen sense of smell, so the researchers expected to find numerous olfactory receptor (OR) genes, which are the genes responsible for the effective schnozes of other vertebrates. But the white shark genome contained exceptionally few of these genes. What the researchers found instead was another category of genes, called vomeronasal genes. These genes can also be involved in the perception of smell, but they aren't normally as abundant as OR genes, Stanhope said. In the white shark genome, however, there are more than dozen vomeronasal genes, so the researchers suggested that maybe those genes drive smell perception in the white shark.
"We're still at a point here where we're scratching the surface, but having this genome sequence gives us a starting point for addressing these questions in more detail," Kacev said. This research will undoubtedly help answer questions about other species, too, he added. "Understanding the genome of the white shark is not only important for the white shark, but it's a scaffold, or jumping-off point to understand related species."
Great white sharks are one of the world's most well-recognized marine creatures, but their populations are struggling as people continue to hunt them out of fear and greed. "People have these images of [white sharks] that are depicted in movies and TV shows as being these killing machines," Kacev said. Yet, the reality is that humans kill far more sharks than sharks kill humans.
"Humans kill many, many millions of sharks every year across the world for silly, fake medicine reasons; for shark fin soup and all sorts of reasons that are just tragic, really," Stanhope said. The researchers hope their study helps more people realize how special these ancient vertebrates are.
"I hope that people recognize the remarkable, biological adaptations of these animals and as a consequence, have a greater appreciation for them," Stanhope said.
Originally published on Live Science.
'''Naukowcy z Nova Southeastern University, Cornell University College of Veterinary Medicine oraz Monterey Bay Aquarium zsekwncjonowali kompletny genom żałracza białego składający się z 4,63 miliarda par zasad, a więc półtora raza większy od genomu ludzkiego. DNA rekina porównywano następnie z genomem innych kręgowców, m.in rekina wielorybiego i człowieka. Dzięki badaniu potwierdzono, że długowieczność tego gatunku zależna jest od stabilności genomu i wielu genów służących do naprawiania DNA.'''

Około 60% genomu żarłacza błękitnego stanowiły powtarzające się fragmenty DNA, z czego znaczna część to retrotranspozony typu LINEs (Long interspersed nuclear element) tworzące przerwy w niciach DNA wymagające naprawy. Duża częstotliwość takich przerw może powodować większą niestabilność genomu i przyczyniać się do nowotworzenia, jednak presja selekcyjna badanych rekinów doprowadziła do tego, że ich odporność na raka jest znacznie większa. U żarłacza białego występuje wiele genów służących do naprawiania DNA i uaktywniających odpowiedź na uszkodzenia, co stabilizuje genom. Ponieważ ryby te są znane także z dużej czułości zapachowej, spodziewano się też odkrycia licznych genów receptorów węchowych. W genomie było ich jednak niewiele, za to odkryto inną grupę kilkunastu genów (''vomeronasal genes''), które mogłyby odpowiadać za rozpoznawanie zapachu.

Żarłacze białe przedstawiane są w dziełach kultury masowej jako bardzo groźne dla człowieka zwierzęta. Jak stwierdził jeden z autorów badań, Michael Stanhope, w rzeczywistości to ludzie zabijają je znacznie częściej z powodu przekonań dotyczących skuteczności medycyny alternatywnej. Zakłada przy tym, że lepsze poznanie biologii tych kręgowców może zmienić ich obraz w świadomości ludzi.