Advancing Puberty in the Black Carp (Mylopharyngodon Piceus) and the Striped Bass (Morone Saxatilis)
Authors: Yaron Zvi , Schreibman Martin P. , Elizur Abigail , Zohar Yonathan 
Published: December 2021

The black carp (bc)GtH IIb cDNA was amplified and isolated, cloned and sequenced. Comparison of the bcGtH IIb deduced a.a. sequence with that of GtH IIb from other teleosts revealed high homology to cyprinid species and a lower homology to salmonid or perciform fish. The gene coding for the GtH IIb was isolated and sequenced. Three bc recombinant phages which hybridized to the goldfish GtH Ib cDNA probe were isolated and are currently being characterized. The region coding for the mature GtH IIb was expressed in a bacterial expression vector resulting in the production of a recombinant protein. In vitro folding resulted in a protein only 1.3% of which displaced the native common carp GtH II in a RIA. Therefore, the common carp GtH RIA was utilized for the physiological studies at the current phase of the project. Two non-functional sites were identified along the brain-pituitary gonadal axis in the immature black carp. The pituitary is refractory to GnRH stimulation due to a block proximal to the activation of PKA and PKC probably at the level of GnRH receptors. The gonads, although capable of producing steroids, are refractory to gonadotropic stimulation but do respond to cAMP antagonists, indicating a block at the GtH receptor level. Attempts to advance puberty in 2 and 3 y old black carp showed that testosterone (T) stimulates GtH synthesis in the pituitary and increases its sensitivity to GnRh. A 2 month treatment combining T+GnRH increased the circulating GFtH level in 3 y old fish. Addition of domperidone to such a treatment facilitated both the accumulation of GtH in the pituitary and its response to GnRH. The cDNA of striped bass GtH a, Ib and IIb subunits were amplified, isolated, cloned and sequenced, and their deduced a.a. sequences were compared with those of other teleosts. A ribonuclease protection assay was developed for a sensitive and simultaneous determination of all GtH subunits, and of b-actin mRNAs of the striped bass. GnRH stimulated dramatically the expression of the a and GtH IIb subunits but the level of GtH Ib mRNA increased only moderately. These findings suggest that GtH-II, considered in salmonids to be involved only in final stages of gametogenesis, can be induced by GnRH to a higher extent than GtH-I in juvenile striped bass. The native GtH II of the striped bass was isolated and purified, and an ELISA for its determination was developed. The production of all recombinant striped bass GtH subunits is in progress using the insect cell (Sf9) culture and the BAC-TO-BAC baculovirus expression system. A recombinant GtH IIb subunit has been produced already, and its similarity to the native subunit was confirmed. The yield of the recombinant glycoprotein can reach 3.5 mg/ml after 3 days culture. All male striped bass reach puberty after 3 y. However, precocious puberty was discovered in 1 and 2 y old males. Females become vitellogenic during their 4th year. In immature 2 y old females, T treatment elevates the pituitary GtH II content while GnRH only potentiates the effect. However, in males GnRH and not T affects GtH accumulation in the pituitary. Neither GnRH, nor T treatment resulted in gonadal growth in 2 y old striped bass, indicating that either the accumulated GtH II was not released, or if released, the gonads were refractory to GtH stimulation, similar to the situation in the immature black carp. In 3 y old female striped bass, 150 day GnRHa treatment resulted in an increase in GSI, while T treatment, with or without GnRHa, resulted in a decrease in oocyte diameter, similar to the effect seen in the black carp. Further attempts to advance puberty in both fish species should take into account the positive effect of T on pituitary GtH and its negative effect of ovarian growth.