Correlation between gonad-inhibiting hormone and vitellogenin during ovarian maturation in the domesticated Penaeus monodon

Vitellogenesis in crustaceans is a process by which a major yolk protein, vitellin (Vn) is proteolytically produced from vitellogenin (Vg) and deposited into developing oocytes. Vitellogenesis is regulated by a gonad-inhibiting hormone (GIH) produced in the eyestalk ganglia. In this study, Vg and GIH mRNA expression and their physiological concentrations at the protein level were examined during ovarian maturation in domesticated broodstock of Penaeus monodon. GIH mRNA was expressed at the highest level in the eyestalk ganglia of the shrimp with immature ovary while the GIH peptide was actively released into the hemolymph. The release of GIH dropped dramatically at stage I of the ovary onwards conforming to its negative regulatory function on Vg synthesis. Vg mRNA expression study confirmed that Vg was synthesized in both the ovary and the hepatopancreas of P. monodon. The expression of Vg increased as ovarian maturation progressed similarly to that demonstrated in the wild broodstock. Vg protein was found in the hemolymph since stage I of ovarian maturation suggesting a rapid release of Vg into the hemolymph before deposition into oocytes as shown by a significant increase of Vn in the ovary at the following stage. Unlike previous studies in wild P. monodon broodstock, Vn was localized to follicle cells of late perinucleolar oocytes and to both follicle cells and ooplasm of the vitellogenic oocytes of domesticated broodstock. We speculate that the incorporation of Vn from follicle cells to the oocytes occurred more slowly in domesticated shrimp; this may account for the retarded reproductive maturation of the domesticated broodstock comparing with the wild broodstock. Our study thus provides insights on vitellogenesis in domesticated P. monodon that will be useful for improvement of their reproductive maturation.