An international team of researchers has sequenced and analyzed the genome of the tuatara (Sphenodon punctatus), the only living member of the reptilian order Rhynchocephalia, once widespread across the ancient supercontinent Gondwana.
The tuatara is an iconic terrestrial vertebrate that is unique to New Zealand. It lives on 35 islands scattered around the New Zealand coast of and was recently reintroduced to the mainland.
The species measures up to 80 cm (31.5 inches) from head to tail-tip and has a mass of up to 1.5 kg. Its color ranges from olive green to brown to orange-red, and it can change color over their lifetime. It sheds its skin once per year.
It has a spiny crest along the back, especially pronounced in males. It also has two rows of teeth in the upper jaw overlapping one row on the lower jaw, which is unique among living species. Its diet consists of beetles, spiders, crickets, small lizards and, occasionally, sea birds.
The tuatara is the only living member of the archaic reptilian order Rhynchocephalia (Sphenodontia), which last shared a common ancestor with other reptiles at about 250 million years ago.
There are aspects of tuatara biology that are unique within, or atypical of, reptiles. These include a unique form of temperature-dependent sex determination, which sees females produced below, and males above, 22 degrees Celsius (71.6 degrees Fahrenheit); extremely low metabolic rates; and considerable longevity.
The tuatara represents an important link to the now-extinct reptiles from which dinosaurs, modern reptiles, birds and mammals evolved, and is thus important for our understanding of tetrapod vertebrate evolution.
“The tuatara is the last surviving species of a reptile group that roamed the Earth with the dinosaurs and remarkably, its genome shares features with those of mammals such as the platypus and echidna,” said Professor David Adelson, a researcher in the Department of Molecular and Biomedical Science at the University of Adelaide.
Professor Adelson and colleagues analyzed the tuatara genome and revealed an unusual architecture, half-way between mammal and reptile.
“Our research confirms that tuatara diverged from the ancestor of lizards and snakes about 250 million years ago,” said Dr. Matthieu Muffato, a researcher at EMBL-EBI.
“This long period of independent evolution explains why we found the tuatara genome to be so unlike those of other vertebrates.”
The sequence of the tuatara genome revealed a number of aspects of this creature’s lifestyle.
Although the tuatara is predominantly a nocturnal animal, its DNA carries a high number of genes involved in color vision, which might help day-active juveniles escape from their predators.
“The tuatara genome is considerably bigger than the human genome, and it has a unique constitution,” said Dr. Fergal Martin, also from EMBL-EBI.
“It contains a lot of repetitive DNA segments that are unique to the species and have no known function.”
The scientists also demonstrated that some sequences of DNA that move or jump location, referred to as ‘jumping genes,’ found in the tuatara are most similar to those found in platypus while others are more similar to those in lizards.
“The tuatara genome contained about 4% jumping genes that are common in reptiles, about 10% common in monotremes (platypus and echidna) and less than 1% common in placental mammals such as humans,” Professor Adelson said.
“This was a highly unusual observation and indicated that the tuatara genome is an odd combination of both mammalian and reptilian components.”
“The unusual sharing of both monotreme and reptile-like repetitive elements is a clear indication of shared ancestry albeit a long time ago,” said Dr. Terry Bertozzi, a scientist in the South Australian Museum.
The team’s results were published in the journal Nature.
N.J. Gemmell et al. The tuatara genome reveals ancient features of amniote evolution. Nature, published online August 5, 2020; doi: 10.1038/s41586-020-2561-9