Repeated evolution of the mouse tooth: an eco-evo-devo perspective – BIGTOOTH

The strength of the project is to bring together complementary approaches around the model of the repeated evolution of the mouse tooth. Main methods applied in the project are:
- Geometric morphometrics (2D and 3D). These methods allow the quantification of tooth shape variation, within and among wild and laboratory populations.
- Together with breeding with controlled pedigrees, heritability of dental morphological traits will be assessed.
- Sampling of wild populations. A trapping campaign allowed bringing wild mice in the lab, in order to breed them in controlled conditions and assess the relationship between body size and dental morphology.
- Methods of developmental biology (gene expression patterns, cell tracing on dental rows in culture) allow the experimental test of the hypothesis that characteristic properties of the tooth development favor the repeated evolution of the phenotype of interest.

An extensive documentation of the dental morphology in wild populations, based on existing collections, confirmed the frequent occurrence of the dental phenotype of interest, especially in insular contexts, but also in fossil populations. This recurrent evolution follows an evolutionary direction matching a direction of main variance observed within populations. This suggests that underlying genetic and developmental mechanisms, preexisting within populations, tend to channel inter-population evolution along preferred directions. This leads to a reevaluate the interpretation of parallel evolution, even of a pronounced phenotype: this evolution may be “easy” to occur, because being facilitated by developmental properties characteristic of the species.
This hypothesis is currently being experimentally validated. Comparing the development between upper and lower teeth (the elongation of the tooth occurring on the upper tooth) demonstrates in mice with the “normal” dental phenotype that the upper tooth shows a peculiar development of the anterior zone, a characteristic that could be mobilized to favor the anterior elongation of the tooth.

Several insular models will be investigated in details, coupling the characterization of tooth shape using geometric morphometrics and population genetics. This will improve our comprehension of the evolutionary dynamics of tooth shape in natural contexts.
Breeding experiments will complement these studies by providing heritability estimates for tooth morphology, and by assessing the relationship between growth patterns of the mice and dental morphology.
Finally, the hypotheses regarding developmental mechanisms favoring this dental evolution will be tested by comparing the development of the first molar between mice with the normal phenotype and mice displaying the dental phenotype of interest.

1. Claude J. Log-shape ratios, procrustes superimposition, elliptic fourier analysis: three worked examples in R. Hystrix, the Italian Journal of Mammalogy (in press)
2. Renaud, S., E.A. Hardouin, B. Pisanu & J.-L. Chapuis. 2013. Invasive house mice facing a changing environment on the Sub-Antarctic Guillou Island (Kerguelen Archipelago). Journal of Evolutionary Biology, 26: 612-624.
3. Renaud, S. & J.-C. Auffray. 2013. The direction of main phenotypic variance as a channel to morphological evolution: case studies in murine rodents. Hystrix, the Italian Journal of Mammalogy. (sous presse).
4. Stoetzel, E., C. Denys, J. Michaux & S. Renaud. 2013. Mus in Morocco: a Quaternary sequence of intraspecific evolution. Biological Journal of the Linnean Society, 109: 599–621.

The phenotypic variation is a key feature in evolution, being the target of the screening by selection as well as of the sorting by random factors. The phenotypic variation is the product of the genetic variance, expressed and modulated by developmental processes. Can these developmental systems contribute to orient evolution, by producing preferably certain phenotypes? Our project addresses this key topic in evolutionary biology by focusing on the model of the dental pattern in the mouse. Our preliminary works evidenced a remarkable result: the repeated evolution of a prestyle at the anterior part of the first upper molar, in various cases where mice tend to display a size increase: several cases of insular evolution as well as laboratory strains selected for a large size. Our project aims at deciphering the processes beyond this parallel evolution. (1) Morphometric analyses will quantify the evolutionary patterns involved; compare the morphological signature in the various independent cases of evolution of the prestyle; evaluate how much phenotypic evolution is influenced by the phylogenetic context, and finally if the morphological trend of anterior elongation of the tooth is already present as a component of the intra-population variance, suggesting a latent potential of the developmental system. (2) Experiments of developmental biology will analyse the detailed morphogenesis of the tooth and relate the phenotypic variation observed on teeth to the underlying developmental processes. (3) Our preliminary results show that the occurrence of the prestyle is favoured in big mice, but not all. To understand this coupling between tooth shape and size, we will analyse the growth patterns in several strains. Some pathways of size increase should tend to favourably trigger the apparition of the prestyle: for instance, an early size increase during embryonic growth has good chances to impact the development of the tooth that occurs at this period. (4) To validate these results largely based on laboratory strains, trapping of insular mice from Corsica is planned, in order to breed them in controlled conditions and analyse in these mice and their offspring the coupling between growth, size and tooth phenotype.

These aspects will be tackled at the intra-specific scale by mobilising existing collections documenting numerous cases of insular evolution, and various laboratory strains documenting various genetic pathways for size increase (independent selection from standard laboratory strains, and/or punctual mutations). Yet, a striking contradiction emerges: the occurrence of the prestyle seems coupled to a large size within the domestic mouse, whereas it characterizes the smallest of their relatives within the genus of the mouse: the pigmy mice Nannomys. This could be due to the inter-specific evolution of the developmental systems themselves. The integration within the project of the study of two strains of pigmy mice will allow tackling this problem of extrapolation from the micro- to the macro-evolutionary scale.