An ancient reptile challenges some modern hypotheses about molecular evolution rates

Student post submitted by Nathan S. Sunderson.

A recent study by Jennifer Hay and colleagues at the Allan Wilson Centre for Molecular Ecology and Evolution in New Zealand has found that unexpectedly, the Tuatara (Sphenodon punctatus ) may have some of the fastest rates of molecular evolution seen in the animal kingdom. The two species of tuatara have long been a fascination in the realm of herpetology because they have changed little since the time of the dinosaurs and are the sole living members of a sister clade to the familiar squamata (snakes and lizards). Tuataras have many “primitive” characteristics such as low metabolism, slow growth, and slow reproduction: all of which are expected to correspond to slow mutation rates. How can an animal with such historically slow morphological evolution have high rates of molecular evolution?

By sequencing the mitochondrial DNA hypervariable regions (HVRs) of the control region of modern and fossilized tuataras and radiodating the fossils examined, the group calculated an evolution rate of 1.56 (0.83-2.34) substitutions per nucleotide per million years. This is the highest rate seen among the 10 animal species studied to-date using similar statistical techniques.

These findings contradict the assertions that: 1) there is an inverse relationship between generation time and molecular evolution rate, 2) rate of evolution corresponds to body size, and 3) there is a direct correlation between metabolic and evolutionary rates. The most important implication of this study is that the rates of neutral molecular and phenotypic evolution may be decoupled. Indeed, previous studies on other living fossils (coelacanth and horseshoe crab) suggest that they may similarly have high evolutionary rates. The group has suggested that perhaps genome structure and replication processes (not metabolism, body mass, and generation time) are the key to understanding the diversity of evolutionary rates among animals.

Reference:

HAY, J., SUBRAMANIAN, S., MILLAR, C., MOHANDESAN, E., LAMBERT, D. (2008). Rapid molecular evolution in a living fossil. Trends in Genetics, 24(3), 106-109. DOI: 10.1016/j.tig.2007.12.002