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. The gene is expressed in cells of the blastopore of late blastulae and the archenteron invagination and stomodeum invagination regions of gastrulae. It appears to be crucial for gastrulation and mesoderm differentiation of embryos. Although this expression pattern is shared by most deuterostomes[AA1] , Brachyury expression was not reported in adult stage. Here we show that Brachyury of an indirect-developer, a 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 analyses and single cell RNA-seq (scRNA-seq) analysis. Iso-seq analysis showed that the gene expression occurred only in the caudal region of the adult but not in the anterior regions including stomochord. scRNA-seq analysis showed that a cluster that contained Brachyury-expressing cells composed of mesoderm-related cells, but  not associated with epidermis, nervous system, and muscle. Although further studies are required to examine roles of Brachyury at the adult stage, this study suggests a possible evolutionary link of the Brachyury-expressing caudal tip of hemichordates with the anus formation region of chordates.