Non-Contiguous and Multi-Regional Expression of Pancreas-Related Genes along the Lancelet Gut

Deuterostomes generally share a functional digestive system involving the sequential steps of ingestion, digestion, absorption, and elimination along their one-way gut. Previous gene expression studies on pancreas-related exocrine digestive enzymes (XDEs), pancreas-related transcription factors (TFs), and ParaHox genes in the ascidian <i>Ciona robusta</i> showed non-contiguous and multi-regional pancreatic features along the <i>Ciona</i> gut. The combination of classical in situ hybridization and recent RNA-seq techniques in lancelets has allowed the fundamental features of the gut in basal chordates (lancelets and ascidians) to be examined in more detail. In the present study, we investigated the spatial expression of pancreas-related XDE and TF genes with a focus on their regionality in the adult lancelet gut. Whole-mount in situ hybridization using <i>Branchiostoma japonicum</i> revealed non-contiguous and multiple pancreatic regions in the lancelet gut. Public bulk RNA-seq data on lancelets (<i>Branchiostoma belcheri</i>, <i>B. floridae</i>, and <i>B. lanceolatum</i>) also supported the multi-regional distribution of pancreatic features along the lancelet gut. In addition, single-cell RNA-seq data indicated that the lancelet hepatic caecum comprised multiple cell clusters with the differential gene expression of pancreas-related XDEs, <i>Pdx</i>, and <i>Cdx</i>. Gene knockout experiments on ascidian <i>Pdx</i> suggested that "the multi-regional expression of pancreas-related XDE genes" in basal chordates is associated with Pdx-independent regulation.