Project description:Hox and ParaHox genes encode transcription factors with conserved similar expression patterns in divergent animals. The Pdx (Xlox) homeobox gene, for example, is expressed in a sharp spatial domain in the endodermal cell layer of the gut in chordates, echinoderms, annelids and molluscs. The significance of comparable gene expression patterns is unclear because it is not known if downstream transcriptional targets are also conserved. We thus conducted experiments to show that a classic transcriptional target of Pdx1 in vertebrates, the insulin gene, is also a direct target of Pdx in the Pacific oyster. We report that oyster has a diversity of insulin-related genes including one co-expressed with Pdx in the endodermal layer of oyster digestive tissue. Transcriptome analysis reveals functional similarity of this tissue to vertebrate pancreas. Using ATAC-seq we identify a Pdx homeodomain binding site upstream of the endodermally-expressed oyster insulin-related gene and using cell culture demonstrate that oyster Pdx acts as a transcriptional activator through this site. These data argue that a classic homeodomain-target gene interaction dates back to the base of Bilateria.
