Molecular characterization of dental development in a toothed archosaur, the American alligator Alligator mississippiensis

Abstract

Few skeletal structures are as informative of the adaptive natural history of vertebrate animals as their teeth. Understanding principles of tooth development is key to understanding evolution of the vertebrate dentition in general and emergence of multiple specialized tooth types in particular. Morphological and phylogenetic considerations suggest that crocodilians have the most primitive mode of dentition within extant tetrapods, displaying simple, conical, socketed, and continuously replaced teeth. Previous histological studies revealed several dental fates, including functional and non-functional teeth (rudiments) in the developing alligator embryos. We analyze expression of key odontogenic regulators and markers to better characterize the molecular patterning of crocodilian dentition. Importantly, we demonstrate that the morphologically distinct tooth types in Alligator mississippiensis are distinguishable by differences in their developmental programs. We also present evidence showing that tooth maturation is accompanied by dynamic gene expression in the epithelial and mesenchymal cells involved in tooth development. Our data reveal a significant morphological and genetic variation in early dental fates. We believe that this underlying developmental variation reflects modularity, or the ability of teeth to develop semi-autonomously along the alligator jaw. We propose that such modularity may have been a crucial for adaptive evolution within Amniota, allowing for the progressive modifications to tooth replacement, number, and shape.

Indeterminate Growth: Could It Represent the Ancestral Condition?

“…Developmental variation results from a combination of modularity and semi-autonomous tooth development. Modularity may have been key to the adaptive evolution of teeth in amniotes, permitting progressive modification in tooth shape, number, and replacement pattern, and the alligator may be a model for the "most primitive" extant tetrapod dentition (Weeks et al 2013). …”

Developmental mechanisms driving complex tooth shape in reptiles

“…In reptiles, structures resembling primary EKs have been indicated in the alligator, which has single cuspid teeth, and also in several squamate species with both single and multicuspid teeth . On the other hand, an evaluation of Shh and Fgf4 expression in crocodilians did not confirm the presence of an EK . Similarly, in squamates with single cuspid teeth, localized expression of key mammalian EK markers (eg, Bmp2 , Bmp4 , Axin2 , and Wnt10b ) was not observed .…”

“…1,[19][20][21] On the other hand, an evaluation of Shh and Fgf4 expression in crocodilians did not confirm the presence of an EK. 22 Similarly, in squamates with single cuspid teeth, localized expression of key mammalian EK markers (eg, Bmp2, Bmp4, Axin2, and Wnt10b) was not observed. 20,23 Interestingly, Bmp2 was found to be expressed in the enamel epithelial bulge cells in Eublepharis macularius, which exhibits two enamel ridges on the tooth surface.…”

Insight from Frogs: Sonic Hedgehog Gene Expression and a Re‐evaluation of the Vertebrate Odontogenic Band

“…Teeth are also able to form in the absence of an OB in several lineages. The first teeth to form in crocodylians are non‐functional and develop quite superficially, evaginating, depositing dentine, and then submerging into the mesenchyme (Westergaard and Ferguson, ; Weeks et al, ). These species lack an OB (Tokita et al , A Lainoff, personal communication), indicating that an OB is certainly not necessary for creating individually spaced, functional tooth precursors with most of their histomorphogenetic properties (Westergaard and Ferguson, ; Tokita et al, ; Wu et al, ).…”