Two Distinct Mitochondrial Genomes Found in Lizard-like Tuatara

Tuatara have two distinct copies of the genetic instruction manual for making mitochondria. It’s the first evidence of a full additional copy of the mitochondrial genome in a vertebrate. Other vertebrates have only one copy of a mitochondrial genome. Mussels are the only other animal ever found to have two.

Efforts to decode the tuatara’s genetic makeup began in 2012, with the launch of the Tuatara Genome Project. After getting the blessing of the Maori people to sample the reptile’s blood (tuatara are a taonga (special treasure) to the Maori), the team found its genome to be 50 percent larger than the human genome. This discovery led to deeper exploration of the mitochondrial part of the genome. Most techniques that decipher, or sequence, DNA chop it into small pieces, “read” it, then reassemble the pieces. That provides a high-resolution look at individual puzzle pieces. Piloting a new technique that reads long DNA segments, the lab sequenced the tuatara’s mitochondrial genome in one fell swoop, showing its overall structure. The revelation came from comparing both the chopped puzzle pieces and the overall structure, and noticing that sections from the same part of the mitochondrial DNA had striking differences in their gene sequences — like the way the notes of song might be arranged differently by two different composers.

The variation raised eyebrows; mitochondrial DNA is usually inherited only from a mother’s egg, so the scientists expected to see a single copy of the mitochondrial genome, not two copies like they would see in nuclear DNA, which is inherited from both mother and father. Together, the scientists painstakingly assembled two fully functional mitochondrial genomes. They found the genomes differed by an eye-popping 10.4 percent. In comparison, human and chimpanzee mitochondrial genomes differ by 8.9 percent. The tuatara’s arrangement of genes is unlike any other vertebrate. When scientists analyzed which sets of genes differed between the two genomes, they noticed changes in ones related to metabolism. An animal’s cell metabolism adjusts to help it cope with environmental extremes. The double mitochondrial genome might give tuatara flexibility in how their metabolisms respond to temperature extremes, the scientists say.

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