The developmental origins of heterodonty and acrodonty as revealed by reptile dentitions
by: Salomies, L., Eymann, J., Ollonen, J., Khan, I., & Di-Poï, N.
Summarized by Kat Cool, a fourth-year geology student studying at the University of South Florida. She is pursuing her major with a geophysics emphasis and a minor in Geographic Information Systems and Technology. She is also the proud owner of three bearded dragons that have inspired her interest in this article. In the future she hopes to study meteorology at the graduate level and hopefully specialize in severe weather forecasting.
What data were used? Discovering evolutionary mechanisms for dental changes could have implications in phylogenetics, taxonomy, and ecological identification of animals that are extinct as well as those still here today. This could be especially useful in key taxa groups that have a poor fossil record or a more mysterious evolutionary history. One group of reptiles the lepidosaurs (snakes and lizards), are a perfect candidate for this research due to their diversity in dental structures. Mutations in the genetic codes of lepidosaurs could provide key insight to the mechanisms behind their dental evolution. One mutation commonly seen is variation of the ectodysplasin (EDA) pathway. This mutation can be observed in many vertebrate species, including humans, and causes changes in the appearance of hairs, feathers, scales, nails, and teeth. The subject for this study group will be the humble bearded dragon (Pogona vitticeps), due to the different stages of EDA mutations one can easily observe. Since this mutation is also known to influence tooth development, scientists decided to look at the dental structure of these morphs as well.
Methods: To analyze the tooth development of bearded dragons, scans were taken to take a closer look at the EDA mutation both during embryonic development and after the hatchlings have emerged from the egg. Then 3D-rendered bearded dragon skulls were compared at 14 days after hatching. The teeth of the wild-type bearded dragon, the leather-back (Sca/+), and the silk-back (Sca/Sca) were then compared based on the appearance (or lack thereof) of pleurodont and acrodont teeth (Figure 1 for images and descriptions of teeth; Figure 2 for images of the lizards).
Results: It was found that wild-type hatchlings had eight acrodont teeth and one small pleurodont tooth per jaw. There is also a central egg tooth on the middle jaw bone (premaxilla) that is replaced with a pleurodont tooth soon after hatching. However, it was found that both scaleless bearded dragons (Sca/+ and Sca/Sca) often did not have pleurodont teeth on their premaxillary bone, leading to about half of juvenile dragons with an EDA mutation having few or no teeth on this bone at all. It was also found that bearded dragons with EDA mutations had fewer teeth in total than the wild-type dragons, as well as wider teeth. These observations were more evident in the silk-back juveniles (Sca/Sca).
Why is this study important: At the time of these results, the scaleless bearded dragon was the first known example that researchers had found of a gene mutation that resulted in position changes in teeth. These results provide a contrasting prospective to results found when studying the dental structures of mice. While the research with mice indicated that vertebrate tooth position was based on a complex model of gene expression patterns, the scaleless bearded dragon data suggests tooth identity can be produced with the modification of a simple gene.
The big picture: The simple modifications of the EDA gene had very observable effects on the position of the teeth. Though more research is necessary, this study shows that through observations of living species today, mechanisms of dentition diversity can be discovered through many different approaches to better understand evolutionary development. Though it is sometimes a long, slow process that can span across millions of years, it can also sometimes be isolated to a change in a single gene at a specific moment.
Citation: Salomies, L., Eymann, J., Ollonen, J., Khan, I., Di-Poï, N. (2021) The developmental origins of heterodonty and acrodonty as revealed by reptile dentitions. Science Advances 7(51). DOI: 10.1126/sciadv.abj7912