ISOLATION AND CHARACTERIZATION OF ENDOSTYLE-SPECIFIC GENES IN THE ASCIDIAN CIONA-INTESTINALIS

The endostyle is a special organ in the pharynx of Urochordata, Cephal ochordata, 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 functi onal homology to the vertebrate thyroid gland. The endostyle is theref ore one of the structures key to the understanding of the origin and e volution of chordates. In the present study, we isolated and character ized cDNA clones for four endo-style-specific genes, CiEnds1, CiEnds2, CiEnds3, and CiEnds4, of the ascidian Ciona intestinalis. Although th e predicted amino acid sequences of the gene products CiENDS1, CiENDS2 , and CiENDS3 showed no similarity to known proteins, their mean hydro pathy profiles suggest that they are secretory proteins. In addition, CiENDS3 contained a unique repeat of 10 amino acids IR(QPCI)(RRPC)I]. CiEnds1 and CiEnds2 were expressed in zone 6, a protein-secreting glan dular element of the endostyle, and CiEnds3 was expressed in zone 2, a nother secretory zone. CiEnds4, a cytoplasmic actin gene, was predomin antly 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 sugge st that these genes are conserved among ascidian species, and therefor e they las well as CiEnds3 for the protein with a unique motif) may be useful probes for further analyses of molecular mechanisms involved i n endostyle development.