The Greenland shark is the longest-living vertebrate in the world. The fish, which live in the deep areas of the North Atlantic and the Arctic Ocean, can live to be around 400 years old, and possibly even much longer. An analysis of the genome suggests that improved DNA repair could play an important role in the extreme longevity.
Such findings can help to better understand general mechanisms of longevity, the researchers hope. Nature has given living creatures very different maximum ages. Among pine trees, for example, there is a nearly 5,000-year-old specimen called Methuselah. The longest-living land mammals are humans: the age record is held by the Frenchwoman Jeanne Louise Calment, who died on August 4, 1997 at the age of 122 years and 164 days.
Gigantic genome
The study by the team led by bioinformatician Steve Hoffmann from the Leibniz Institute on Aging – Fritz Lipmann Institute (FLI) in Jena has not yet been published in a specialist journal, so it has not yet been independently reviewed. According to the researchers, deciphering the genome was a challenge simply because of its size: with almost 6.5 billion base pairs, the genetic code of the Greenland shark is twice as long as that of humans and the most extensive of all currently known shark genomes.
In general, only a few animals with even larger genomes are known to date, such as axolotls and lungfish, it was said. As with these species, the enormous size of the Greenland shark genome is primarily due to repeating elements, known as transposable elements or jumping genes. They make up more than 70 percent of the Greenland shark genome, the study states.
Actually harmful – but useful here?
This is surprising because a high proportion of such elements is actually considered harmful to the organism – but in the case of the Greenland shark, this does not seem to be the case. On the contrary, it is suspected that the activity of transposable elements could have contributed to the extreme longevity. For example, genes involved in the repair of DNA damage may use the machinery of the elements.
“In each of our cells, DNA is damaged thousands of times every day and specialized molecular mechanisms are constantly repairing it,” explained co-author Alessandro Cellerino from the FLI. Comparative genome studies have shown that long-lived mammalian species can repair their DNA exceptionally efficiently.
Good DNA repair = long life?
The results for the Greenland shark are further evidence that DNA repair could be a general mechanism underlying the evolution of exceptional longevity, the researchers concluded. In addition, the data provide a basis for estimating the genomic diversity and thus the population size of the endangered species.
A research group led by Julius Nielsen from the University of Copenhagen reported in the journal Science in 2016 that Greenland sharks can live to be 400 years old. These sharks therefore only reach sexual maturity after about 150 years. The animals can grow to be more than five meters long, but grow very slowly.
A study presented last year found that the genetic makeup of sharks changes much more slowly than that of other vertebrates. The rate of change in epaulette sharks in particular is only about one twentieth of the rate in humans, as researchers led by Manfred Schartl from the University of Würzburg reported in the journal “Nature Communications”. This is the lowest mutation rate known to date in vertebrates.
In shark species in cold waters with even lower metabolic rates – such as the Greenland shark (Somniosus microcephalus) – even lower mutation rates could be expected, the scientists explain.
Everything has advantages and disadvantages
A low rate of change has advantages and disadvantages, as the research team explains: The fact that changes in the genome creep in so rarely is a possible explanation for the exceptionally low risk of cancer in sharks. However, for the same reason, they may adapt more slowly to environmental changes than other animals.
Changes in genetic material are the basis for evolution: some provide a survival advantage for the affected animals and are therefore more likely to survive because these specimens have a greater chance of producing more offspring. But cancer is also based on spontaneous small changes in the DNA that lead to malfunctions in the affected cell and uncontrolled cell proliferation.
Unchanged for half a billion years
Sharks are very old in terms of evolutionary history. They have populated the world’s oceans for around 400 to 500 million years; their basic appearance has hardly changed in this enormous period of time. Sharks reach sexual maturity late, have a slow metabolism, live to a very old age and have few offspring. Overfishing, loss of habitat and climate change are causing a decline in the populations of many shark species.
© dpa-infocom, dpa:240913-930-231125/1
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