We are excited to share our work on molar #EvoDevo in mouse and hamster
, led by the amazing team of Marie Sémon and Sophie Pantalacci. We find an inverted hourglass pattern, with a maximum of evolutionary divergence at the bell stage.
https://www.biorxiv.org/content/10.1101/2025.01.23.634446v1
Molar EvoDevo is interesting because there is a clear link between form and function, and variation has a strong adaptive potential. Moreover, as tooth development is well characterised, we can link patterns to gene properties and to developmental processes.
We obtained #RNAseq from tooth germs over the embryonic and postnatal period where the major events of morphogenesis occur, from bud, to cap, to bell stage until differentiation and enamel/dentin secretion, and obtained clusters of coexpressed genes per stage.
As expected under coevolution, upper and lower molars of the same species have very similar gene expression.
As expected from rapid molar evolution, mouse and hamster gene have rather different gene expression.
More surprising, the coexpression clusters are most divergent in middle molar development.
We found a similar pattern in selection on protein coding genes, with lower selective pressure (dN/dS) and higher positive selection (branch-site test) in middle development of molars.
What explains this pervasive pattern? There are major differences both in proportions of highly expressed genes, and in expression specificity of genes. Highly expressed genes are known to be under stronger selection, which is also expected for more broadly expressed genes.
Broad or narrow expression were measured both from tooth single-cell (cell-type specificity) and from organ-level bulk data (organ-specificity) @bgeedb. In a first approximation broadly expressed genes are expected to be more pleiotropic, thus under stronger selection.
Interestingly, while highly expressed genes are frequent both in early and late molar development, broadly expressed genes are frequent early but not late. Thus early and late conservation have different causes.
While early conservation has been observed for other organs, and is consistent with the embryo-wide hourglass pattern (mid-development is early organogenesis), there are few observations of such an inverse hourglass, and it appears the first at the dN/dS level.
This pattern appears driven both by the evolution of developmental processes intrinsic to teeth and by negative and positive selection essentially extrinsic to the teeth, via the level of gene pleiotropy.