Fig. 7: A summary of findings and working model from this study for mouse molars and bat limbs.

a Two pairs of serial organs where one organ underwent drastic shape changes as compared to the relative conservation of the other. b Developmental stages covered by transcriptome data with key changes associated with supplementary cusps (molars) and webbing (limbs). Transcriptomes are dominated by co-evolution. Such shared changes (e.g. smaller signalling centers in molars) are necessary but not sufficient. In combination with specific changes (e.g. mesenchyme size) they allow to reach a threshold for organ-specific adaptation. In mouse molars, 3 changes (increased mesenchyme size, increased bucco-lingual asymmetry and smaller inhibitory signalling centers) combine to induce extra cusps on the lingual side of the upper molar only. In bat limbs, Fgf8 and the BMP inhibitor Grem1 combine to suppress interdigital apoptosis in the forelimb only⁴⁴. In mouse, a combined increase of FGF signaling and decrease of BMP signaling is necessary to suppress apoptosis and induce webbing. c Patterns of developmental divergence. top: Upper molar supplementary cusps develop last. We could expect divergence to accumulate during development. Lower molar divergence was expected to be much smaller. bottom: The observed levels of upper and lower molar divergence are comparable and the onset of the three morphogenetic changes described in b happen early in development.