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The lizard-like tuatara carries two distinct mitochondrial genomes

The lizard-like tuatara was already a stranger. Its superpowers include a century-long lifespan, resistance to many diseases, and a unique (for a reptile) cold tolerance. It now turns out that part of the animal’s genetic instruction book is as strange as its life history and can help explain its ability to withstand extreme temperatures.

Tuatara has two separate copies of the instruction manual for mitochondrial DNA, the researchers reported on Jan. 29 in Communications Biology.

“It’s the first evidence of a complete additional copy of the mitochondrial genome in a vertebrate,” says Chris Schneider, a Boston University herpetologist who did not participate in the study. Other vertebrates have only one copy of a mitochondrial genome. Mussels are the only other animal that has been found two.

Mitochondria are small energy factories and their genetic material, usually inherited from the mother, is essential for the cells to function. Recent studies show that mitochondrial DNA plays an important role in aging and in various human cancers, as well as in metabolic, muscular, and neurogenerative diseases (SN: 24/10/12). Studying the mitochondrial genome of other animals could provide clues about the internal workings of human disease, the researchers say.

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“The mitochondrial genome is much more important than people realize, given its association with aging and disease,” says Robert Macey, a genomic at the Peralta Genomics Institute in Oakland, California. "How it works in an animal that slowly ages in a cool environment could tell us something significant about how mitochondria work."

Efforts to decode the genetic composition of the tuatara began in 2012, with the launch of the Tuatara Genome Project led by Neil Gemmell, an evolutionary biologist at the University of Otago in Dunedin, New Zealand. After getting the blessing of the Maori to test the blood of the reptile (the tuatara are a taonga (special treasure) for the Maori), the team found that their genome was 50 percent larger than the human genome (SN: 8 / 5/20).

This discovery led to a deeper exploration of the mitochondrial part of the genome. Most techniques that decipher or sequence DNA cut it into small pieces, read it, and then reassemble the pieces. This provides a high resolution view of individual parts. Piloting a new technique that reads long segments of DNA, Macey's lab sequenced the mitochondrial genome of the tuatara at once, showing its overall structure. The technique, called Oxford Nanopore, "is undoubtedly the future of gene sequencing, that we can sequence entire molecules at once." By Macey.

Dan Mulcahy, a molecular biologist at the Smithsonian's Global Genome Initiative in Washington, D.C., and Macey were meditating on the data when Mulcahy recalls saying, "I think there may be two mt genomes."

The revelation came from comparing the cut puzzle pieces and the overall structure, and noticing that sections of the same part of mitochondrial DNA had striking differences in their gene sequences, such as how song notes can be arranged differently by two. different composers. The variation raised his eyebrows; mitochondrial DNA is usually only inherited from a mother's egg, so scientists expected to see a single copy of the mitochondrial genome, not two copies as they would see in nuclear DNA, which is inherited from both mother and father.

Together, the scientists meticulously put together two fully functional mitochondrial genomes. They found that genomes differed by a so-called 10.4 percent. In comparison, human and chimpanzee mitochondrial genomes differ by 8.9 percent. “The arrangement of tuatara genes is unlike any other vertebrate,” Mulcahy says.

When Laura Urban, a genomic scientist at the University of Otago, analyzed which gene sets differed between the two genomes, she noticed changes in those related to metabolism. An animal’s cellular metabolism is adjusted to help it cope with environmental extremes. Scientists say the double mitochondrial genome can give tuatara flexibility in the way its metabolisms respond to extreme temperatures.

“The tuatara has the most complicated mitochondrial genome I’ve ever seen,” Macey says. Finding the genetic basis of the animal’s metabolic feats could clarify the function of the mitochondrial genome, helping to find treatments for human metabolic diseases.

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