Endocrine control of gametogenesis and spawning induction in the carp

Abstract

The gonadotropic regulation of spermatogenesis and spermiogenesis is apparently mediated by 11-ketotestosterone (11-KT) secreted by Leydig cells. However, the acquisition of sperm motility requires the stimulation of the sperm duct by gonadotropin, or by 17α,20β-dihydroxy-4-pregnen-3-one (17,20-P) which is produced by spermatozoa from 17α-hydroxyprogesterone (17-OH-P). The theca of the ovarian follicle performs most of the steroidogenic steps culminating in the formation of testosterone which diffuses into the granulosa layer to be aromatized to estradiol. A parallel two-cell type mechanism operates towards final oocyte maturation: theca cells produce 17-OH-P which diffuses into the granulosa cells to be converted to 17,20-P. Nevertheless, this model does not apply to all teleost fish. The synthesis of vitellogenin (VTG) by the liver is regulated by estradiol in synergism with growth hormone. The uptake of VTG, through low-affinity/high-capacity binding sites on the plasma membrane of the oocytes is stimulated by GTH-I. The proteins forming the chorion are produced by the liver and carried in the circulation to be incorporated at the periphery of the oocyte. The cascade of hormones involved in final oocyte maturation includes GTH-II which stimulates the formation of 17,20-P, or other maturation-inducing steroids, which bind to receptors on the oocyte membrane and activate the maturation-promoting factor (MPF). This factor is composed of cyclin B and a kinase of the CDC2 type. Ovulation involves the formation of prostaglandins and synthesis of proteases which degrade part of the follicular wall to form a rupture site through which the egg can emerge. Contractility of the follicular wall may take part in the process of ovulation. There is ample evidence for gonadotropin duality in salmonids. The two GTHs share a common α-subunit but differ considerably in their β-subunits. Also in the carp two GTHs have been isolated from pituitaries of adult females, and although shown to be chemically distinct, their function is quite similar. However, gonadotropin duality in cyprinid fish has not been confirmed by other laboratories and further research is required to elucidate this point. The release of GTH from the pituitary is regulated by various brain hormones, the most active being gonadotropin-releasing hormone (GnRH), which stimulates the release, and dopamine, which is inhibitory. Spawning induction in cyprinid fish is currently carried out by the hypophyseal approach (hypophysation or administration of chorionic gonadotropin) or by the hypothalamic approach (GnRH superactive analogues in combination with dopamine antagonists — the Linpe method). Comparison between the two approaches under the conditions of Israeli hatcheries has shown that both are suitable for spawning induction in carp, provided that the GTH content of the pituitary extract is calibrated. However, the Linpe method is advantageous because of the availability and lower cost of the compounds required for hypothalamic manipulations. In addition, the hazard of spreading pathogens along with pituitary material is a point to be considered. Adapting the Linpe method to Israeli cyprinid culture has required research int selecting the most suitable GnRH analogue and the dopamine antagonist, and the most suitable mode of their administration. In addition, the research examined the proper time of injection, the response latency and its temperature dependence. The results of the study indicate that the response to spawning induction by either approach cannot be assumed to be the same in every country due to differences in the environment, hatchery procedures and carp lines. In order to establish an optimal procedure for spawning induction at a given site, a study under the specific local conditions should be carried out using previous results only as guidelines.