ABSTRACT
The sustainability of the commercial aquaculture undoubtedly depends on the continuous and predictable supply of quality seeds of the cultivated species. Propagation of fish culture has been impeded by the inadequate supply of fish seed-in terms of both quality and quantity. Efforts have been made to establish captive broodstock and induce their spawning. However, it was demonstrated that in captivity, a majority of farmed fish species fail to reproduce successfully, primarily due to the absence of the release of luteinizing hormone (LH) into the circulation. LH is released after the binding of gonadotropin-releasing hormone (GnRH) to its specific receptor(s) present on the gonadotrophic cells of the pituitary gland. Current spawning induction methods use nonendogenous GnRH agonists. Three different endogenous forms of GnRH (chicken GnRH-ll, salmon GnRH and seabream GnRH) were discovered in the brain of perciform species. It was further demonstrated that only one form of the GnRH, seabream GnRH (sbGnRH) reaches the pituitary in abundance. The correlation of sbGnRH levels with gonadal development and its specific anatomical delivery system for sbGnRH suggest that this form is the most relevant endogenous form that induces LH release, final oocyte maturation and spawning. However, sbGnRH elicited a very low level of LH release compared to the chicken (c)
GnRH-ll form in in vivo studies carried out in seabream and striped bass. To understand the molecular mechanisms controlling reproduction, cDNAs and genes for all three forms of GnRHs have been isolated from perciform species. Very recently, a full-length eDNA for the pituitary GnRH receptor was cloned from striped bass. A major portion of pituitary GnRH-R eDNA was also cloned from gilthead seabream. This chapter discusses current spawning methods in the light of possible isoforms of the GnRH receptor with reference to the presence of multiple GnRHs. It concludes with a summary of the relevance of molecular interaction of GnRH-GnRH receptor(s) in modern aquaculture biotechnology.
