Expression of endostyle-specific genes in the ascidian Halocynthia roretzi

The endostyle is a special organ in the pharynx of Urochordata, Cephalochordata and Cyclostomata. This organ may have arisen in their common ancestor with a shift to internal feeding for extracting suspended food from the water. In addition, the endostyle has functional homology to the vertebrate thyroid gland. The endostyle is therefore another key structure in the understanding of the origin and evolution of chordates. Following a previous report of the pharyngeal gill-specific genes, we report here the isolation and characterization of cDNA clones for endostyle-specific genes HrEnds1 and HrEnds2 of the ascidian Halocynthia roretzi. These cDNA clones were obtained by differential screening of an endostyle cDNA library and a pharyngeal gill cDNA library with total endostyle cDNA probes. Both transcripts were abundant in the library; each represented about 10% of the cDNA clones of the library. The HrEnds1 transcript was small in size, about 600 bp in length. Although the predicted amino acid sequence of the gene product showed no similarity to known proteins, mean hydropathy profiles suggested that HrENDS1 is a type Ib protein or secreted protein. The HrEnds2 transcript was about 2.5 kb in length. Although the HrEnds2 gene product showed no sequence similarity to known proteins, mean hydropathy profiles suggested that HrENDS2 is a secreted protein. The transcripts of both genes were not detected in embryos, larvae and early juveniles but were evident in 1-month-old young adult after several compositional zones were organized in the endostyle. In situ hybridization revealed that distribution of transcripts of both genes was restricted to zone 6, the protein-secreting glandular element of the endostyle. These genes may be useful for further analysis of molecular mechanisms involved in endostyle development.