Ascidian homologs of mammalian thyroid transcription factor-1 gene are expressed in the endostyle

The endostyle is a special organ in the pharynx of urochordates, cephalocho rdates and cyclostomes. During evolution of the primitive chordates, the en dostyle was organized in their common ancestor(s) with a shift to internal feeding for extracting suspended food from the water. In addition, the endo style has an iodine-concentrating activity and is therefore thought to be f unctionally homologous to the vertebrate thyroid gland. Human TITF1 and mou se titf1 are members of the Nkx-2.1/TTF-1 gene subfamily, which encode an N K-2 type homeodomain transcription factor. The genes are expressed in the t hyroid gland and are essential for thyroid-specific structural gene express ion. In the present study, we isolated cDNA clones for ascidian homologs of titf1 from Halocynthia roretzi and Ciona intestinalis, and examined whethe r the genes are expressed in the ascidian endostyle. Results clearly indica ted that both the H. roretzi homolog Hrtitf1 and the C. intestinalis homolo g Cititf1 are expressed specifically in the endostyle. The present finding therefore provide molecular evidence for the functional relationship betwee n the ascidian endostyle and vertebrate thyroid gland. However, the genes a re expressed in the supporting element regions but not in the putative iodi ne-concentrating regions of the endostyle.