INTERNAL AND EXTERNAL FACTORS CONTROLLING REPRODUCTION IN THE AFRICANCATFISH, CLARIAS-GARIEPINUS

The African catfish, Clarias gariepinus, is a highly appreciated speci es for aquaculture, because of its favourable food conversion, its res istance to diseases, its relatively low requirements for water quality , the possibility for high stocking density and the excellent meat qua lity. For those reasons even in the Netherlands, there is a modest, bu t Europe's largest and still expanding, African catfish farming activi ty. Although this species grows and matures in captivity, there is no spontaneous reproduction. We could demonstrate that the failure to rep roduce resides in the brain-pituiary-gonad axis. Hormones required for oogenesis and spermatogenesis are being produced in sufficient quanti ties. However, final oocyte maturation, ovulation, spermiation and spa wning behaviour do not occur, due to the lack of a gonadotropin surge. In nature, the prespawning gonadotropin surge is induced by environme ntal factors such as the water level in the spawning area. Under farmi ng conditions the environmental cues are hard to identify and/or to mi mic. In combination with unavoidable stress this causes a blockade of the release of gonadotropin releasing hormone (GnRH). Consequently, go nadotropin surge release fails to occur, which is enforced by an effec tive hypothalamic dopaminergic inhibition. The gonadrotropin surge ind uces the conversion of 17 alpha OH-progesterone into 17 alpha hydroxy- 20 beta-dihydroprogesterone, the final maturation inducing substance. Based on these data, several protocols for artificial propagation coul d be developed. They include either a treatment with a GnRH analogue i n combination with a dopamine receptor antagonist, a treatment with ho mologous gonadotropin or HCG, or a treatment with 17 alpha OH-progeste rone. Since a number of years we have used the African catfish as a mo del for fundamental research on fish reproductive endocrinology. Till now one gonadotropic hormone (GTH) could be demonstrated. Its amino ac id composition and sequence was analysed and appeared to be homologous with known forms of the maturational GTH (GTH-II). Specific radioimmu no assays for the complete hormone and its alpha- and beta-subunit res pectively, have been developed. cDNAS encoding the subunits have been cloned. They are applied now for Northern blotting and in situ hybridi zation. GnRHs were fully characterised (a specific catfish-GnRH and ch icken-GnRH-II). Specific antibodies against these peptides were raised and the cDNAS encoding the hormone precursor molecules were cloned an d used for respectively immunocytochemical localisation and radioimmun oassays, and in situ hybridisation. The importance of the two GnRH for ms for gonadotropin release was studied. Chicken-GnRH-II appears to be 10 to 100 times more potent than catfish GnRH, probably due to its hi gher receptor affinity. Catfish GnRH, however, is present in the brain and pituitary about 100 times more than chicken GnRH-II. Steroid horm one synthesis by ovaries, testis and seminal vesicles was analysed. Th e sex steroids that play a role in the negative feedback control of go nadotropin release were identified (11 -keto-testosterone and testoste rone) and their interaction with hypothalamic dopamine metabolism was demonstrated as one of the possible mechanisms of action. Several ster oid conjugates from the seminal vesicles were shown to have pheromonal activities, involved in reproductive behaviour. They induce under cer tain physiological conditions attraction between conspecifics and sync hronization of ovulation.