The developmental basis for scaling of mammalian tooth size

When evolution leads to differences in body size, organs generally scale along. A well -known example of the tight relationship between organ and body size is the scaling of mammalian molar teeth. To investigate how teeth scale during development and evolution, we compared molar development from initiation through final size in the mouse and the rat. Whereas the linear dimensions of the rat molars are twice that of the mouse molars, their shapes are largely the same. Here, we focus on the first lower molars that are considered the most reliable dental proxy for size-related patterns due to their low within-species variability. We found that scaling of the molars starts early, and that the rat molar is patterned equally as fast but in a larger size than the mouse molar. Using transcriptomics, we discovered that a known regulator of body size, insulin -like growth factor 1 (Igf1), is more highly expressed in the rat molars compared to the mouse molars. Ex vivo and in vivo mouse models demonstrated that modulation of the IGF pathway reproduces several aspects of the observed scaling process. Furthermore, analysis of IGF1-treated mouse molars and computational modeling indicate that IGF signaling scales teeth by simultaneously enhancing growth and by inhibiting the cusp-patterning program, thereby providing a relatively simple mechanism for scaling teeth during development and evolution. Finally, comparative data from shrews to elephants suggest that this scaling mechanism regulates the minimum tooth size possible, as well as the patterning potential of large teeth.

Automated summary

When it comes to differences in body size due to evolution, organs tend to scale proportionally. This study focuses on the scaling of mammalian molar teeth, specifically looking at the development process in mice and rats. While the shapes of their molars are similar, the rat molars are twice the size of the mouse molars. The study reveals that insulin-like growth factor 1 (Igf1), a known regulator of body size, is more highly expressed in rat molars compared to mouse molars, and its modulation can reproduce the observed scaling process.