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Gonadotropin and gonadotropin receptors in protandrous black porgy, Acanthopagrus schlegeli :regulation and their relations to sex change
|Authors: ||Jin-Lien Du|
|Contributors: ||NTOU:Department of Aquaculture|
black porgy;gonadotropin;gonadotropin receptor;sex steroids;sex change
|Issue Date: ||2011-06-30T08:38:50Z
|Abstract: ||摘 要 本研究之目的在探討促性腺素 (gonadotropin, GTH) 與其受體(gonadotropin receptor, GTHR：包括FSHR與LHR) 在黑鯛性轉變過程的變化，及其調控作用對影響性轉變之機制。 2∼3年齡黑鯛血漿luteinizing hormone (LH) 季節性變化中，僅於非繁殖季出現LH波峰的族群，在第3繁殖季可維持雄性機能，而血漿LH維持於較低生理量或藉由處理給予大量的LH均會導致2∼3年齡黑鯛性轉變。 於黑鯛的非繁殖季，僅有低劑量Testosterone (T) 會促進血清LH的含量；進入繁殖季初期，雌二醇 (estradiol-17β, E2) 對LH為正迴饋作用，可促使血清LH含量、腦下垂體LHβ及GTHα mRNA的表現，至繁殖季節末，E2對黑鯛LH的調控作用轉為負迴饋作用，至生殖腺完全沒有產精能力時，則轉為正迴饋作用；低劑量的11-ketotestosterone (11-KT) 於生殖腺發育的較早期，對LH會造成抑制作用，但於隨著生殖腺的發育，會轉為短暫的正迴饋作用，作用效力很快消失，高劑量11-KT對LH為負迴饋作用，會導致血清LH含量下降，但對腦下垂體GTH mRNA基因表現則無影響；T處理的部份，於繁殖季初期，很快回復基礎值，但高劑量的T可促使腦下垂體GTHα mRNA的表現量；在繁殖季末，T及11-KT的處理，會導致LH顯著的變動，顯示此階段黑鯛LH對雄性素的調控作用相當敏銳，因此造成血清LH有迅速的變動，最終達成負迴饋作用，至生殖腺完全沒有產精能力時，雄性素對LH的調控作用會轉為正迴饋作用。綜合以上結果，我們推論黑鯛生殖腺發育階段，E2應是較強勢的主導者；T為非繁殖季LH之主要調控者，且其作用方式應是透過對腦下垂體的正迴饋作用，以促進LH之合成與分泌，11-KT對LH較無調控作用，其作用階層主要為生殖腺之精子生成作用。 在GTHR的研究中，我們發現兩性階段之精巢GTHR相當豐富，且於LHRH analog或性類固醇處理後，均可促使精巢組織的FSHR或LHR顯著增加，顯示兩性階段之精巢處於較活躍的狀態。 因此，於非繁殖季 (5到6月)，當黑鯛受到少量的T刺激後，一方面會促使黑鯛合成並分泌大量的LH，另外一方面，則會促進精巢組織之GTHR、ERγ及AR大量表現，以便接受隨後來自腦下垂體之GTH與生殖腺所合成的性類固醇，進而促使精巢組織得以成長、發育，讓精、卵巢並存的生殖腺逐漸往雄性功能的精巢組織發育；而當黑鯛處於非繁殖季LH較低含量時，卵巢組織具有較活躍的aromatase，因此，T會被aromatase轉為E2，一方面藉由與精巢組織的ER□結合，以抑制精巢之發育，並使精巢萎縮為痕跡器官，另一方面，繼續藉由卵巢組織的性類固醇生成作用，活絡整個生殖腺主軸，使卵巢組織得以持續發育 (發生在繁殖季前，9月之後)，因此，精、卵巢競爭的結果，最終使黑鯛達到性轉變。|
Abstract The objectives of the present study were to investigate the effects of gonadotropin and their receptors on the reproductive endocrine system and sex change in the protandrous black porgy, Acanthopagrus schlegeli. Analysis on the seasonal change of plasma LH revealed the highest peak during the non-spawning season. This increased LH may in turn, favour testis development in black porgy. Administration of sex steroids to black porgy during different season (non-spawning to post-spawning season) demonstrated the regulatory effect of sex steroids on LH. During the non-spawning season, only low T could stimulate an increase in LH concentration. In contrast, during spawning season, the sex steroids had a negative feedback on LH concentration which changed to a positive feedback during the post-spawning season. During pre-spawning season, E2 induced a significant increase in the levels of plasma LH, in addition to the increased expression of pituitary LHβ and GTHα mRNA. 11-KT resulted in a decreased plasma or serum LH concentration, but had no effect on the pituitary GTH mRNA, whereas T enhanced the transcription levels of LH mRNA. In contrast to the regulatory effect of sex steroids during the spawning and post-spawning season, there was no feedback effect on LH synthesis by sex steroids during puberty. Fish keeps the plasma LH in the lower base levels or deal with a large amount of LH will induce sex change in black porgy. Bisexual gonad (with ovarian and testicular tissue together) was observed in black porgy before sex change. Testicular tissue develops to become the dominant tissue during non-spawning to spawning season. The ovarian tissue becomes dominant in black porgy from post-spawning to non-spawning season and later regresses during the non-spawning to spawning season. Testicular tissue, but not ovarian tissue, grow and develop from bisexual gonad by the stimulation of LH. Different levels of GTH receptors (FSHR and LHR) on the testicular and ovarian tissues in the bisexual gonad maybe accounted for the differential sensitivity of these tissues to sex steroids. FSHR and LHR transcripts were significantly higher in the bisexual testicular tissue than ovarian tissue during the non-spawning season in 2+-year-old fish. The sensitivity of black porgy testicular tissue to the steroidogenic effect of FSHR and LHR increase during bisexual stage (non-spawning to pre-spawning season). Thus from the study, a model for the testis development has been proposed as follows: low testosterone during the non-spawning season stimulates an increased production of LH. Similarly, steroid treatment also stimulates an increase in the FSHR and LHR in the bisexual gonad. Thus, the increased LH, FSHR and LHR production may in turn stimulate the testis development during the non-spawning season.
|Appears in Collections:||[水產養殖學系] 博碩士論文|
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