Nature
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To the casual observer, tuataras look much like lizards, aside from lacking a penis. However, their evolution and genetics make them arguably the world's most distinctive vertebrate, and a new study shows their genome has some features in common with those other oddities of the animal kingdom, the monotremes.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Tuataras are among the world's great survivors, even if they're now restricted to islands off New Zealand, changing little over 250 million years ago. It's been known for some time that, despite appearances, they are very different genetically from anything else alive today, and a paper in Nature confirms they are no closer to the lizards and snakes they resemble than to mammals or birds.
Professor David Adelson of the University of Adelaide has found that tuatara's posses “jumping genes”, or transposable elements, that more closely resemble those of the platypus than the reptiles with which they are usually classified.
"The tuatara genome contained about 4 percent jumping genes that are common in reptiles, about 10 percent common in monotremes (platypus and echidna) and less than 1 percent common in placental mammals such as humans," Adelson said in a statement. "The unusual sharing of both monotreme and reptile-like repetitive elements is a clear indication of shared ancestry albeit a long time ago,"
Adelson and a large team found evidence of several rounds of population expansion and decline recorded in the tuatara genome. Comparisons from different locations led the authors to reject previous theories that a population from islands in the Cook Strait are so different from their northern counterparts they represent another species. Nevertheless, Adelson told IFLScience that the island population is distinct enough it might justify being treated as a “conservation unit”.
Coming to grips with the tuatara genome was a challenge because at 5 billion base pairs it is much larger than most animals, although still dwarfed by the axolotl.
“No one has found a relationship between the size of an animal's genome and its complexity,” Adelson said to IFLScience, but the task of aligning repetitive DNA elements for identification is related to the square of the genome's size.
Adelson describes the findings as a “cautionary tale” for those trying to reconstruct the ancestral species from which mammals, dinosaurs, and surviving reptiles all came. The tuatara is genetically unusual enough to change this picture, but we don't know what other branches from this distant ancestor would have done the same if they had survived as long.
Some scientists have speculated tuataras may be the closest surviving relatives of certain dinosaurs, but Adelson said this is hard to confirm since “we can't sequence a dinosaur.”
Tuataras have great significance in Maori culture, where they are considered to guard special places. Adelson expressed pride in the fact that none of the research was done without consultation with Maori cultural guardians of surviving populations, including final publication.
“I hope this is something that will be emulated,” he told IFLScience.
