A Tooth for a Tooth: Evolutionary Development of Dental Structure Based on Common Mutations of the Bearded Dragon

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).

Figure 1 (A) shows diagrams of different tooth structures. The jawbone, dental pulp, dentine, and enamel are color coded in each of five tooth structures. Bearded Dragons have two of these types of teeth. The first is pleurodont teeth which are shown in the diagram by having one side of the tooth connected to the jawbone while the other side is exposed. The second type of teeth in this diagram of interest to us are the acrodont teeth. The acrodont teeth are shown in the diagram by having the jawbone go about halfway up on both sides of the tooth.(B) A phylogenetic tree showing families and subfamilies of Acrodonta connected by lines based on their relationship. The tree stems off into two main groups: The Agamidae and the Chamaeleonidae. The Lepidosauria reptiles are on the Agamidae side of the tree. (C) 3D-rendered skulls of lizards representing the main Acrodonta subfamilies. Each skull shows an X ray of the bones and teeth in the lizard’s skull from a straight on angle as well as from the side of a lizard’s skull. The anterior pleurodont teeth are highlighted in red and the acrodont teeth are noncolored
Figure 1 (A) Different tooth attachment types in vertebrates. There are two main types of teeth present in these reptiles: Pleurodont teeth are set on the inside jaws, while acrodont teeth are generally larger and attach to the jaw by connective tissue. Pleurodont teeth are accepted as the norm for Lepidosauria; however, there are certain families like Agamidae, Chamaeleonidae, and Trogoniphidae that show an understudied mix of pleurodont and acrodont teeth or a singular acrodont tooth. Another unique feature of living lepidosaurs is the lack of or absence of tooth replacement in acrodont teeth. (B) Phylogenetic tree showing families and subfamilies of Acrodonta. The Lepidosauria clade includes the Rhynchocephalia order with the single surviving species, the Tuatara, as well as the Squamata order with many living members like lizards and snakes (C to J) 3D-rendered skulls of lizards representing the main Acrodonta subfamilies


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).

Figure 2 Two female bearded dragons sitting next to each other on a pillow. They are laying on their stomach with their heads looking forward. Their round abdomen consolidates by their hind legs where their long tails extend out of frame. The bearded dragon on the left is a wild type bearded dragon. She is yellow and brown in color and her spikes are much more apparent than the bearded dragon on the right. The bearded dragon on the right is more orange and tan and lacks the same spiky texture as the other bearded dragon.
Figure 2. An image of a wild-type bearded dragon (left) and a leather-back bearded dragon or Sca/+ (right). If you are familiar with bearded dragon morphology, the EDA mutation is responsible for two of the most well-known morphs: the ‘leather-back’ and the ‘silk-back’. The leather-back bearded dragon (Sca/+) has one copy of the EDA mutation, resulting in reduced scale size than the bearded dragon’s without this mutation (also called wild type in this study). This reduction in scales creates a leathery appearance, earning these little mutants the colloquial name ‘leather-back’. The silk-back (Sca/Sca) bearded dragon has two copies of the EDA mutation: one from each parent. The result is a more extreme version of the features observed in the leather-back dragons: instead of reduced scales there appears to be an absence of scales all together.

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

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