Montana State researcher featured in Nature for work on rare reptile genome

August 14, 2020

BOZEMAN -- A Montana State University researcher contributed to a novel project with scientists from around the country and world that sheds light on one of Earth's most important reptile species.

Chris Organ, an assistant teaching professor in the Department of Earth Sciences in MSU's College of Letters and Science, worked with a team from 10 countries and six U.S. states along with Washington D.C.'s Smithsonian Institution to sequence the genome of the tuatara, a reptile Organ refers to as a "living fossil."

"The tuatara isn't a lizard, even though it looks like one. They evolved early among their group of amniotes, animals like lizards, birds and mammals," said Organ. "The group never really diversified much, so the tuatara is very similar anatomically to fossils that we see that go back 200 million years. The question that remains is, if the anatomy of the animal hasn't evolved very much, what about the genome?"

The project marks the first time the tuatara genome, which is roughly twice the size of a human genome, has ever been sequenced, which illuminates not only how the unique species evolved, but also offers some insights into human genetic lineage as well. Organ brought a paleontological perspective to the international team, helping to compare the genome of the animals living today to their prehistoric ancestors. The paper, "The tuatara genome reveals ancient features of amniote evolution," appeared in the scientific journal Nature on Aug. 5.

Lead author Neil Gemmell from the University of Otago in New Zealand said sequencing the genome allows scientists to learn just where the tuatara fits in the tree of life.

"If we consider a tree, with species diverging over time and splitting off into groups such as reptiles, birds and mammals, we can finally see with some certainty where the tuatara sits," Gemmell said. The sequencing of the tuatara genome places it on the same branch of the tree of life as snakes and lizards up until about 250 million years ago, when the tuatara's branch, the genus Sphenodon, split from the class branch squamata, the branch that includes snakes and lizards. The tuatara has been genetically unique ever since.

Tuatara are native only to the islands of New Zealand, and because of that they have experienced very little habitat change during their long evolutionary existence, said Organ. Rodents, as a counterexample, exist all over the world. They have adapted over time to survive in a multitude of habitats with varying weather, predators, threats and food sources. Tuatara have been exposed to very few such speciation events, meaning they have seen a remarkably slow pace of evolutionary change.

Coupled with a long lifespan -- a tuatara can live for more than 100 years -- this consistent habitat means tuatara have seen very slow evolutionary rates.

Tuatara predate modern snakes and lizards by around 100 million years, said Organ. They also do not use genetic sex chromosomes to determine the sex of offspring, instead determining sex based on the temperature of their surroundings. This makes them particularly sensitive to changing habitats or a warming climate, which could unbalance the male-to-female ratio and lead to significant declines in population.

The tuatara is also a culturally significant animal to the Maori native people of New Zealand, said Gemmell. The research team worked closely with indigenous communities in New Zealand as well as the New Zealand Institute for Plant and Food Research, the Ngatiwai Trust Board and the New Zealand Department of Conservation to explore how these novel insights into the tuatara could aid in its preservation in New Zealand.

In addition to having an unusually large genome for an amniote, the tuatara also bears unique, never-before-identified genetic elements discovered through this research. Through narrowing down what makes the tuatara unique, the team hopes to discover the root of the species' longevity and to identify the best way to preserve this unique reptile.

"Sequencing a genome is like piecing together a page of text using only sentence fragments," said Organ. "But because the tuatara genome is so large and has changed over time, it's like trying to piece together 'War and Peace,' and in a different dialect. Since the tuatara is so distantly related to anything that's alive today, we have genomic 'sentences' that don't overlap clearly, and that makes compiling this genome something new and exciting."
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Montana State University

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