| Summary: | 博士 === 國立臺灣海洋大學 === 水產養殖學系 === 93 === 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.
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