Differential sensitivity to <scp>SHH</scp> signaling and neural crest‐mediated <i>Gas1</i> expression regulate jaw size during development and evolution

BACKGROUND: Developmental control of jaw size is crucial to prevent birth defects and facilitate evolutionary adaptation. We have shown that jaw size is established by neural crest mesenchyme (NCM), which are progenitor cells that migrate into the mandibular primordia and produce the jaws. NCM relies on multiple signaling pathways including Sonic Hedgehog (SHH) to mediate interactions with mandibular epithelium and promote jaw outgrowth. We investigated if NCM-mediated regulation of the SHH pathway underlies species-specific evolution of jaw size. RESULTS: We analyze expression of SHH pathway members over time and find that Growth Arrest-Specific 1 (GAS1), which is a SHH co-receptor, is expressed 20-75-fold higher in mandibular primordia of duck relative to those of quail. We generate quail-duck chimeras and demonstrate Gas1 expression is NCM-regulated. Gain- and loss-of-function experiments reveal species-specific sensitivity to SHH signaling, especially for Gas1. Gas1 overexpression and knockdown in NCM alters cell number and jaw size, and differentially affects genes involved in the SHH and WNT pathways, the cell cycle, and others. We also uncover intriguing differences in the Gas1 promoter and coding sequence between duck and quail. CONCLUSIONS: Our work suggests changes to Gas1 expression and function may modulate jaw size during development, disease, and evolution.