Can neural crest cell gene expression explain correlated trait evolution in wall lizards?

In this study I aim to test the hypothesis that phenotypic co-variation in the common wall lizard is caused by the developmental biology of neural crest cells.

The common wall lizard Podarcis muralis is a familiar sight throughout southern Europe. Generally, P. muralis is small and brown but in western Italy they have evolved a new phenotype with a suit of exaggerated trait states (a syndrome) which is now spreading and hybridizing with surrounding populations of the the ancestral phenotype. Lizards with the syndrome phenotype are more aggressive and territorial, have more pronounced dentition, bigger heads and display a characteristic coloration with increased blackness. Furthermore, this syndrome seems to have evolved in parallel in at least five other species of wall lizards. What is it that make these traits go together?

Generally correlated trait evolution is theoretically unlikely considering the extreme dimensionality of phenotypes. Never the less it is observed in nature. Usually, it is explained by referring to one or more of three mechanisms: correlated selection, genetic correlations between traits or development reducing the dimensionality of phenotype variability. These are not mutually exclusive and can even cause one another. In the case of the common wall lizard previous studies have been unable to find genetic correlations between traits such as a “supergene” or similar. Rather, since the traits involved in the syndrome seem to all share developmental origins in the same embryonic stem cell population, neural crest cells (NCCs), we have good reason to think that the developmental biology of this specific cell type is important for understanding the repeated evolution of the syndrome.

To study this, I collect embryos from captive lizards with and without the syndrome phenotype. I then dissociate all cells in the embryo and use fluorescence activated cell sorting (FACS) to sort out the NCCs. The NCCs are then sequenced using state of the art technology for single cell transcriptomics (scRNAseq). This allows me to do direct comparisons of specific NCC derived cell types between the two phenotypes.