The tuatara genome reveals ancient features of amniote evolution

Evolution of vertebrates (all animals having a spine) is a common topic for these GEITP pages. The tuatara is a land vertebrate — unique to New Zealand. Because these GEITP pages had NEVER before heard of this animal, it was worth pasting a picture 😊 [see bottom of this email]. The tuatara is the only living member of the archaic reptilian order Rhynchocephalia (Sphenodontia), which last shared a common ancestor with other reptiles ~250 MYA (million years ago; see Figure 1 of attached article). [Recall that dinosaurs became extinct at ~65 MYA.] The tuatara species thus represents an important link to the now-extinct “stem reptiles” from which dinosaurs, modern reptiles, birds, and mammals evolved; this makes the sequenced tuatara genome important for our understanding of the evolution of amniotes (any animal whose embryo develops in an amnion and chorion and has an allantoic membrane — such as mammals, birds, and reptiles).

The tuatara is also a species of importance in other contexts: [a] it is a special treasure for the Māori, who believe that this animal is a “guardian of special places”; [b] it is a critically important species that is vulnerable to extinction — owing to habitat loss, predation, disease, and other factors; [c] it displays various morphological and physiological innovations that have puzzled scientists since its first description. These structures include a unique combination of features that are shared variously with lizards, turtles and birds — which left its taxonomic classification in doubt for many decades. This taxonomic mystery has largely been addressed using molecular approaches, but the evolutionary timing of divergence of the tuatara from the lineage that forms modern squamates (lizards and snakes), rate of evolution of tuatara, and number of species remain open to discussion. Lastly, there are aspects of tuatara biology that are unique within reptiles, or atypical of reptiles; these include a unique form of temperature-dependent sex determination (females produced below ~22 °C, males above ~22 °C), extremely low basal metabolic rates, and remarkable longevity [average lifespan ~60 years, some live to be well over 100 years old (one male reproduced successfully for the first time at 111 years of age with an 80-year-old female). Some experts believe that captive tuatara could live as long as 200 years].

Authors [see attached article & editorial] found the tuatara’s 5-Gb (5 gigabase; 5 billion bases) genome to be among the largest of vertebrate genomes so far assembled (e.g. human genome is ~3.2 Gb). Phylogenetic analyses indicate that the tuatara lineage diverged from that of snakes and lizards ~250 MYA. This lineage shows a moderately slow rate of molecular evolution, punctuated with instances of more rapid evolution. The genome sequence analysis identified expansions of proteins, nonprotein-coding RNA families, and repeat elements, the latter of which show a mixture of reptilian and mammalian features. These data in the tuatara genomic sequence provide a valuable resource for deep comparative analyses of tetrapods [any of various (mostly land-dwelling) vertebrates having four feet, legs, or leglike appendages; they also breathe air with lungs. Some, such as whales and snakes, have lost one or both pairs of limbs], as well as a valuable resource for tuatara biology and species conservation.


Nature 20 Aug 2020; 584: 403-409

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