The role of thyroid hormones during the development of eye pigmentation in the Pacific bluefin tuna (Thunnus orientalis).

Thyroid hormones (THs) are essential for the embryonic and post-embryonic development of fish. We studied the role of THs during the early, post-embryonic, development of Pacific bluefin tuna. Embryos were treated with L-thyroxine (T(4)) or the anti-thyroid drug methimazole (MMI), and reared in microtitre plates for 3 days. Immersion in MMI, but not T(4), led to retardation of retinal pigment epithelium (RPE) pigmentation 3 days post-hatching (dph). Concurrent immersion in T(4) and MMI had no effect of RPE pigmentation. We also measured the expression of TRalphaA, TRalphaB, and TRbeta mRNA using real-time RT-PCR. Treatment with MMI significantly reduced TRbeta mRNA expression. Taken together these results suggest that the development of RPE pigmentation is mediated by TH, most likely via TRbeta.