Transcriptomic evidence for Brachyury expression in the caudal tip region of adult Ptychodera flava (Hemichordata).

Most metazoans have a single copy of the T-box transcription factor gene Brachyury. This gene is expressed in cells of the blastopore of late blastulae and the archenteron invagination region of gastrulae. It appears to be crucial for gastrulation and mesoderm differentiation of embryos. Although this expression pattern is shared by most deuterostomes, Brachyury expression has not been reported in adult stages. Here we show that Brachyury of an indirect developer, the hemichordate acorn worm Ptychodera flava, is expressed not only in embryonic cells, but also in cells of the caudal tip (anus) region of adults. This spatially restricted expression, shown by whole-mount in situ hybridization, was confirmed by Iso-Seq RNA sequencing and single-cell RNA-seq (scRNA-seq) analysis. Iso-Seq analysis showed that gene expression occurs only in the caudal region of adults, but not in anterior regions, including the stomochord. scRNA-seq analysis showed a cluster that contained Brachyury-expressing cells comprising epidermis- and mesoderm-related cells, but which is unlikely to be associated with the nervous system or muscle. Although further investigation is required to examine the roles of Brachyury in adults, this study provides important clues for extending studies on Brachyury expression involved in development of the most posterior region of deuterostomes.