Ascidian homologs of mammalian thyroid peroxidase genes are expressed in the thyroid-equivalent region of the endostyle

The endostyle is a pharyngeal organ for the internal filter feeding of uroc hordates, cephalochordates, and larval lamprey. This organ is also consider ed to be homologous to the follicular thyroid gland of higher vertebrates. Thyroglobulin (Tg) and thyroid peroxidase (TPO) are specifically expressed in the thyroid gland of higher vertebrates, and they play an important role in iodine metabolism for the synthesis of thyroid hormones. Previous histo chemical observations showed that iodine-concentrating and peroxidase activ ities were detected in zones 7, 8, and 9 of the ascidian endostyle, suggest ing that; these zones contains cells that are equivalent to those in the ve rtebrate follicular thyroid. In order to investigate the molecular developm ental mechanisms involved in the formation and function of the endostyle, w ith special reference to the evolution of the thyroid gland, in the present study, we isolated and characterized cDNA clones for TPO genes, CiTPO from Ciona intestinalis and HrTPO from Halocynthia roretzi. Northern blot and i n situ hybridization analyses revealed that; the expression of the ascidian TPO genes was restricted to zone 7, one of the elements equivalent to the thyroid. These results provide the first evidence at the gene expression le vel for shared function between a part of the ascidian endostyle and the ve rtebrate follicular thyroid gland. (C) 1999 Wiley-Liss, Inc.