Replaying evolutionary transitions from the dental fossil record

The evolutionary relationships of extinct species are ascertained primarily through the analysis of morphological characters. Character inter-dependencies can have a substantial effect on evolutionary interpretations, but the developmental underpinnings of character inter-dependence remain obscure because experiments frequently do not provide detailed resolution of morphological characters. Here we show experimentally and computationally how gradual modification of development differentially affects characters in the mouse dentition. We found that intermediate phenotypes could be produced by gradually adding ectodysplasin A (EDA) protein in culture to tooth explants carrying a null mutation in the tooth-patterning gene Eda. By identifying development-based character inter-dependencies, we show how to predict morphological patterns of teeth among mammalian species. Finally, in vivo inhibition of sonic hedgehog signalling in Eda null teeth enabled us to reproduce characters deep in the rodent ancestry. Taken together, evolutionarily informative transitions can be experimentally reproduced, thereby providing development-based expectations for character-state transitions used in evolutionary studies. Gradual changes that occur to mammalian tooth morphology across evolutionary time were modelled in vitro and in vivo by modulation of signalling pathways in the mouse, and computer modelling was used to provide further analysis of the parameters influencing tooth morphology. Tooth development down the ages Evolution is conventionally presented as a parade of adult morphology as it changes through time, and teeth are often the best (or only) lines of evidence that palaeontologists have at their disposal. Dental traits are therefore widely used to determine relatedness of fossil species. Now Jukka Jernvall and colleagues have used developmental biology to replay the past 50 million years of the evolution of rodent teeth. Mice unable to produce the morphogen ectodysplasin have very generalized teeth. The addition of small amounts of ectodysplasin to mutant teeth explants reproduces the known changes in the morphology of rodent teeth over evolutionary time. The authors have constructed a computational model of tooth development, with an interface (ToothMaker) that provides a new a tool for resolving how species are related.