542-488 million years ago
Tunicates possess homologs of many vertebrate hormones and anatomical structures which seem to be homologs of vertebrate pituitary and thyroid glands.
Tunicates possess genes for all the major peptide hormone receptors (such
as insulin and gonadotropins), except growth hormone. Tunicates lack steroid
hormones (and the P450 enzymes which synthesize them) but they do possess
nuclear receptors, such as those which bind thyroid hormones and retinoic
acid (which protostomes lack). (Dehal, 2002; Christiaen, 2002). Preliminary
evidence suggests that the neural gland of tunicates produces hormones
similar to prolactin, beta-endorphin, and MSH and thus is homologous to
the pituitary gland. (Burighel, from Harrison, 1997, p. 279). The tunicate
Pitx (pituitary homeobox) gene homolog is expressed in the neural
complex (part of the embryonic pharynx) comparable to where the pituitary
forms in vertebrates (Christiaen, 2002).
The endostyle of tunicates, lancelets, and lampreys are homologous and all are a site for iodination. The endostyle of larval lampreys develops into the thyroid gland (Stach, 2000; Ruppert, from Harrison, 1997, p. 440). In tunicates, the endostyle sequesters iodine and produces calcitonin, thyroid peroxidase (which synthesizes thyroid hormones in vertebrates), and iodothyronine deiodinases (which convert thyroxine to T3) (Burighel, from Harrison, 1997, p. 244; Christiaen, 2002; Dehal, 2002).