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Identifier 000334828
Title Ο ρόλος του θυρεοειδή κατά την οντογένηση και την αναστροφή του φύλου στην τσιπούρα (Sparus aurata)
Alternative Title The role of the thyroid during ontogeny and sex reversal in the gilthead sea bream
Author Szisch, Vera
Thesis advisor Παυλίδης, Μιχάλης
Abstract It is well known, that the thyroid hormones thyroxine and triiodothyronine (TH, T4, T3) have important effects on many physiological processes, such as development, metabolism and reproduction in all vertebrates. They play a crucial role during ontogeny and without TH most organisms will not develop into the physiological adult form. As in all vertebrates, TH are essential also during the early development of fishes, with the most striking example the metamorphosis of flatfish, which is known to be TH-dependent. As far as the the regulation of sexual maturation of teleosts is concerned, it seems that thyroid hormones are involved, but further studies are necessary. Among all vertebrates, teleosts are the only group where the fascinating phenomenon of sex reversal occurs. The gilthead sea bream (Sparus aurta) belongs to those species, being a protandrous hermaphrodite and which could be considered as a model organism for the sparid family because of the wide range of knowledge as far as the general biology of this species is concerned. The aim of the present study was to determine the role of the thyroid system during ontogeny and sex reversal in the gilthead sea bream, two very important stages in the life-cycle of this species. To achieve this goal, the pattern of changes of thyroid hormone concentrations in combination with two important factors of the thyroid system were investigated for the first time: (1) the three types of deiodinases (DIO1, DIO2, DIO3), enzymes which regulate the peripheral levels of the biologically active form T3, and (2) the two types of thyroid hormone receptors (TR-α, TR-β), which are nuclear receptors through which TH excert their physiological action by regulating gene expression. Concerning the impact of TH on ontogeny in sea ream, TH concentrations as well as gene expression patterns support the hypothesis that thyroid hormones are essential especially during the first stages of early development (embryo, hatching) but also during metamorphosis, excerting their action in a stage-dependent manner. The present results support also the hypothsis, that water temperature influences the function of thyroid system in fish, since changes in gene expression of both, DIOs as well as TRs were observed in larvae held in different temperatures. Water temperature also affected growth rate in sea bream larvae, with a different growth performance in different temperatures. During sex reversal, the testis degenrates and at the same time the gonad develops into a functional ovary resulting in sexual mature females. The gene expression profiles of the deiodinases and thyroid hromone receptors in correlation with the pattern of changes in the ovary leads to the conclusion, that TH are involved in ovary development and maturation. In contrast, the results of all parameters studied in testis seem to be confusing, since the levels of the «activators» of the thyroid system (TH, DIO1, DIO2) increase, while thyroid hormone receptor expression decreases. In conclusion, the present results agree with some hypotheses, while they arise new ones concerning the functional level of thyroid hormone action. In any case the present results set the basis for further investigations leading to a deeper understanding of thyroid hormone action during ontogeny and sex reversal in teleosts.
Language Greek
Subject thyroid hormones
θυρεοειδικές ορμόνες
Issue date 2008-09-22
Collection   Faculty/Department--Faculty of Sciences and Engineering--Department of Biology--Doctoral theses
  Type of Work--Doctoral theses
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