Endostyle glandular element-specific expression of vWF-like genes in protochordates

The endostyle in invertebrate chordates is a characteristic pharyngeal trait for mucus protein secretion during suspension feeding, while it is generally recognized as a homologous organ of the vertebrate follicular thyroid with iodine-concentrating and thyroid peroxidase activities. Our recent study on disturbances in thyroid peroxidase activity in protochordates (ascidians and lancelets) revealed a mucus sheet malformation in their pharynx; however, the main component of mucus sheets remains unclear. We herein focused on von Willebrand factor-like (vWFL) genes, which have been identified as endostyle-specific genes in the ascidian Ciona. We found that homologous genes of vWFL were shared among chordates with endostyles, but were lost in vertebrates lacking endostyles. An RNA-seq analysis of protochordates revealed that vWFL transcripts were restricted and prominent in their endostyles. In addition, a gene expression analysis using in situ hybridization revealed that protochordate vWFLs shared an exclusive gene expression pattern that "localized to a specific glandular cell of the endostyle, even though it was not identical among orthologs". Therefore, the ancestral endostyle may have established a "glandular element with vWFL expression", and may subsequently have diverged the "glandular element" and "vWFL expression" in the respective chordate lineages.