| Abstract |
The gilthead seabream (Sparus aurata) is a protandrous hermaphrodite teleost. In Crete, the initial sexual maturity as a male takes place at the end of the second year of life and then the fish begin to undergo sex reversal. Sex reversal in fish is controlled by genetic, hormonal, environmental and social factors. Thyroid hormones regulate a wide range of functions, such as metabolism, metamorphosis, development, growth, reproduction and have been shown to be essential to the development and maturation of gonads. Its possible that thyroid hormones also play a role in sex reversal. In the present study, we have determined the levels of thyroid hormones in plasma, liver and gonads of a reared population of gilthead seabream aged 2+ from April to December 2005. Based on our Knowledge up to date, the levels of thyroid hormones in tissues of this species have not been determined and there has been no study regarding the role of these hormones in sex reversal. The objectives of the study were 1) the description and characterization of the sex reversal phases using histological techniques 2) the determination of thyroid hormone levels in plasma, liver -the main peripheral tissue of T3 production- and gonads, both seasonally and during the different phases of sex reversal and 3) the study of the effect of antithyroid drug, methimazole, in the peripheral regulation of the thyroid and in sex reversal. Sex reversal started before the end of the first reproductive period and developed rapidly. In mid July all fish were found in phase Φ5 (81-100% ) and in mid December the ratio of females to males was approximately 1:2,5. It is likely that social factors determined this ratio. It was shown that plasma T3 levels were significantly higher than those of T4, which supports the hypothesis that plasma T3 levels are higher than those of T4 in teleosts. During summer, there was an extended enzymatic conversion of T4 to T3 in peripheral tissues. On the contrary, the decreased plasma T3 levels in mid December possibly reflected the fact that during this period, the energy was directed to the gonads. Seasonal variations of tissue thyroid hormones mainly reflect the metabolic needs of fish for growth and reproduction and changes in activity of the 5-monodeiodinase enzyme. In gilthead seabream, the liver is considered to be important tissue for T3 production, justifying the high levels of plasma T3. The levels of the biologically active hormone T3 were high in the ovary during the whole study and these results show that in the ovarian tissue an important enzymatic conversion of T4 to T3 exists. During the gradual growth of ovarian tissue (Φ3-Φ5 phases), only a minor increase of plasma T3 was observed. In the liver, T3 levels were significantly fluctuated indicating that T3 is essential for the gradual formation of ovarian tissue and that the liver contributes to this process. The levels of T4 in the ovarian tissue increased along with the appearance of this tissue in the gonads excluding the last reversal phase (Φ5) where an important decrease was observed while T3 levels remained high but stable. The current results showed that T3 hormone exists in significant quantities during all sex reversal phases in gilthead seabream. Methimazole was a effective drug for the production of hypothyroid individuals of gilthead seabream, preventing not only T4 synthesis in the thyroid but also its metabolism in peripheral tissues (liver, gonads).
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Ενδοκρινική ρύθμιση της αναστροφής του φύλου στην τσιπούρα (Sparus aurata): ο ρόλος των θυροειδικών ορμονών
