Identification of pennaceous barbule cell factor (PBCF), a novel gene with spatiotemporal expression in barbule cells during feather development.

Bird contour feathers exhibit a complex hierarchical structure composed of a rachis, barbs, and barbules, with barbules playing a crucial role in maintaining feather structure and function. Understanding the molecular mechanisms underlying barbule formation is essential for advancing our knowledge of avian biology and evolution. In this study, we identified a novel gene, pennaceous barbule cell factor (PBCF), using microarray analysis, RT-PCR, and in situ hybridization. PBCF is expressed in barbule cells adjacent to the ramus during pennaceous barbule formation, where these cells fuse with the ramus to establish the feather's branching structure. PBCF expression occurs transiently after melanin pigmentation of the barbule plates but before the expression of barbule-specific keratin 1 (BlSK1). Orthologues of PBCF, predicted to be secreted proteins, are conserved across avian species, with potential homologues detected in reptiles, suggesting an evolutionary lineage-specific adaptation. Additionally, PBCF is expressed in non-vacuolated notochord cells and the extra-embryonic ectoderm of the yolk sac, hinting at its broader developmental significance. The PBCF gene produces two mRNA isoforms via alternative splicing, encoding a secreted protein and a glycophosphatidylinositol (GPI)-anchored membrane-bound protein, indicating functional versatility. These findings suggest that PBCF may be involved as an avian-specific extracellular matrix component in cell adhesion and/or communication, potentially contributing to both feather development and embryogenesis. Further investigation of PBCF's role in feather evolution and its potential functions in other vertebrates could provide new insights into the interplay between development and evolution.