Isolation and characterization of endostyle-specific genes in the ascidian Ciona intestinalis

The endostyle is a special organ in the pharynx of Urochordata, Cephalochordata, and Cyclostomata. It may have arisen in the common ancestor of these taxa, along with a shift to internal feeding for extracting suspended food from the water. In addition, the endostyle has a functional homology to the vertebrate thyroid gland. The endostyle is therefore one of the structures key to the understanding of the origin and evolution of chordates. In the present study, we isolated and characterized cDNA clones for four endostyle-specific genes, CiEnds1, CiEnds2, CiEnds3, and CiEnds4, of the ascidian Ciona intestinalis. Although the predicted amino acid sequences of the gene products CiENDS1, CiENDS2, and CiENDS3 showed no similarity to known proteins, their mean hydropathy profiles suggest that they are secretory proteins. In addition, CiENDS3 contained a unique repeat of 10 amino acids [R(QPCI)-(RRPC)I]. CiEnds1 and CiEnds2 were expressed in zone 6, a protein-secreting glandular element of the endostyle, and CiEnds3 was expressed in zone 2, another secretory zone. CiEnds4, a cytoplasmic actin gene, was predominantly expressed in zones 3 and 5, which are supporting elements of the endostyle. The amino acid sequences of CiENDS1 and CiENDS2 resembled each other. In addition, they resembled a zone-6-specific gene product (HrENDS2) of another ascidian, Halocynthia roretzi. The results suggest that these genes are conserved among ascidian species, and therefore they (as well as CiEnds3 for the protein with a unique motif) may be useful probes for further analyses of molecular mechanisms involved in endostyle development.