ZIC3 shapes digit morphogenesis in avian

Through the evolutionary diversification of birds, a variety of digit morphologies have evolved as adaptations to diverse environments, with the regression of the anterior digits being a remarkable phenomenon. Transcriptomic analysis of chicken limb development has revealed molecular signatures in the anterior digits, among which Zic family member 3 (Zic3) stands out as a unique highly expressed transcription factor. However, the function of Zic3 in digit development in birds remains unclear. In this study, we investigated the expression pattern of Zic3 across five phylogenetically diverse avian species. Our analysis revealed a consistent Zic3 expression pattern in species from Neoaves and Palaeognathae, except for the ostrich. Ostrich hindlimbs exhibited increased Zic3 expression and an extended expression range from the first toe to the second toe. By locally expanding the expression domain of Zic3 in chicken anterior hindlimb buds we obtained a phenotype similar to that of early ostrich feet, characterized by significant shortening of digits I and II. We further performed mRNA Sequencing (mRNA-Seq) of the Zic3-overexpressed autopods and found that Zic3 inhibited skeletal development through multiple pathways, including the Wnt signaling pathway, ECM-receptor interaction and Focal adhesion. Our results reveal a pivotal role for Zic3 in anterior digits and identify its downstream regulatory mediators in birds.