Comparison of developmental genome expression in rodent molars reveals extensive developmental system drift.

Abstract

Serial appendages are similar organs found at different places in the body, such as fore/hindlimbs or different teeth. They are bound to develop with the same pleiotropic genes, apart from identity genes. These identity genes have logically been implicated in cases where a single appendage evolved a drastically new shape while the other retained an ancestral shape, by enabling developmental changes specifically in one organ. Here, we showed that independent evolution involved developmental changes happening in both organs, in two well characterized model systems. Mouse upper molars evolved a new dental plan with two more cusps on the lingual side, while the lower molar kept a much more ancestral morphology, as did the molars of hamster, our control species. We obtained quantitative timelines of cusp formation and corresponding transcriptomic timeseries in the 4 molars. We found that a molecular and morphogenetic identity of lower and upper molars predated the mouse and hamster divergence and likely facilitated the independent evolution of molars lingual side in the mouse lineage. We found 3 morphogenetic changes which could combine to cause the supplementary cusps in the upper molar and a candidate gene, Bmper. Unexpectedly given its milder morphological divergence, we observed extensive changes in mouse lower molar development. Its transcriptomic profiles diverged as much as, and co-evolved extensively with, those of the upper molar. Consistent with the transcriptomic quantifications, two out of the three morphogenetic changes also impacted lower molar development. Moving to limbs, we show the drastic evolution of the bat wing also involved gene expression co-evolution and a combination of specific and pleiotropic changes. Independent morphological innovation in one organ therefore involves concerted developmental evolution of the other organ. This is facilitated by evolutionary flexibility of its development, a phenomenon known as Developmental System Drift. AUTHOR SUMMARYSerial organs, such as the different wings of an insect or the different limbs or teeth of a vertebrate, can develop into drastically different shapes due to the position-specific expression of so-called "identity" genes. Often during evolution, one organ evolves a new shape while another retains a conserved shape. It was thought that identity genes were responsible for these cases of independent evolution, by enabling developmental changes specifically in one organ. Here, we showed that developmental changes evolved in both organs to enable the independent evolution of the upper molar in mice and the wing in bats. In the organ with the new shape, several developmental changes combine. In the organ with the conserved shape, part of these developmental changes are seen as well. This modifies the development but is not sufficient to drastically change the phenotype, a phenomenon known as "Developmental System Drift", DSD. Thus, the independent evolution of one organ relies on concerted molecular changes, which will contribute to adaptation in one organ and be no more than DSD in another organ. This concerted evolution could apply more generally to very different body parts and explain previous observations on gene expression evolution.