Supplementary material from "Exacerbated sonic hedgehog signalling promotes a transition 
from chemical pre-patterning of chicken reticulate scales to mechanical skin folding"

Many examples of self-organised embryonic patterning can be attributed to chemically-mediated systems comprised of interacting morphogens. However, mechanical patterning also contributes to the emergence of biological forms. For example, various studies have demonstrated that diverse patterns arise from elastic instabilities associated with the constrained growth of soft materials, which generate wrinkles, creases and folds. Here, we show that between days 12 and 13 of in-ovo development, transient sonic hedgehog pathway agonism in the chicken embryo, through a single intravenous injection of smoothened agonist (SAG), abolishes the Turing-like chemical patterning of reticulate scales on the ventral footpad and promotes a transition to mechanical labyrinthine skin folding. Using in-situ hybridisation, nanoindentation and labelling of proliferating cells, we confirm that skin surface folding is associated with the loss of signalling placode pre-patterning as well as increased epidermal growth and stiffness. Using additional in-ovo hydrocortisone treatments, we also demonstrate that experimentally-induced hyper-keratinisation of the skin mechanically restricts SAG-induced folding. Finally, we verify our experimental findings with mechanical growth simulations built from volumetric light sheet fluorescence microscopy data. Overall, we reveal that pharmacological perturbation of the underlying gene regulatory network can abolish chemical skin appendage patterning and replace it with self-organised mechanical folding.