Reproductive physiology of pejerrey fish Odontesthes bonariensis. Basic and applied aspects.

GUSTAVO M. SOMOZA; LEANDRO A. MIRANDA; LEONARDO G. GUILGUR; PABLO H. STROBL-MAZZULLA; JUAN I. FERNANDINO; MARTÍN BLASCO

Vittoria

Congreso; Aqüimerco 2004, AquaCiência 2004: Integração e Sustentabilidade I Congresso da Sociedade Brasileira de Aqüicultura e Biologia Aquática (AQUABIO). Congresso do Capítulo Latino Americano da World Aquaculture Society.; 2004

Sociedade Brasileira de Aqüicultura e Biologia Aquática (AQUABIO). Congresso do Capítulo Latino Americano da World Aquaculture Society.

The pejerrey, Odontesthes bonariensis is an atherinid fish native of the inland waters of Buenos Aires Province (Argentina). This species has also been introduced in many countries for aquaculture and game fishing purposes. Although, pejerrey aquaculture is considered of regional importance, it has not been well developed yet. Now, the laboratory is working on reproductive physiology, sex determination/differentiation and aquaculture of this species. One of the main topics of our group is to study central regulation of reproduction. The neuropeptide Gonadotropin Releasing Hormone (GnRH) is the major regulator of reproduction in vertebrates acting as a first signal from the hypothalamus to pituitary gonadotropes. In this context, three GnRH molecular variants were detected in the brain of pejerrey: sGnRH, cGnRH-II and a novel variant named pjGnRH. The structure of  this form was established by  isolating the peptide from the brain and determining its primary structure by chemical sequencing and mass spectrometry. This form is the only GnRH that reaches the pituitary gland and its biologic activity was tested in vitro. These three variants were distributed over specific brain areas. The characterization of cDNA codifying for these three variants was also performed and compared to those characterized in other teleost species. Pejerrey also has a marked thermolabile sex determination (TSD); for instance, in this species the proportion of females gradually changes from 100% at 15–19ºC to 0% at 29ºC when larvae are reared at different temperatures between 1 and 5 weeks after hatching. Our group has demonstrated that follicle stimulating hormone (FSH) and luteinizing hormone (LH) expressing cells appear just before gonadal differentiation in correlation with an increase in the number of GnRH neurons and pituitary innervation by GnRH. These evidences  suggest that the synthesis and release of gonadotropins may be regulated by GnRH originated from hypothalamic neurons before or just at the moment of sex determination. It is important to stress that the number of LH, FSH and GnRH neurons differed according to the incubation temperature, suggesting a correlation with TSD. Then, gonadotropins can regulate the production of sexual steroid by the gonads and/or the interrenal gland during sex determination and/or early gonadal differentiation. Then, the role of the cytochrome P450 brain aromatase, an enzyme responsible for the conversion of androgens to estrogens, is being studied. Furthermore, some genes known to be involved in the mammalian sex-determining pathway: SOX9, DMRT1 and SF1, were characterized in pejerrey. It is important to note that DMRT1 was found to be expressed only in testis. Our laboratory is also working in order to establish pejerrey to seed production. Different studies are being conducting to control reproduction in captivity. Pejerrey broodstock is currently being reared at the IIB-INTECH aquatic facilities and they normally spawn under natural conditions. However two aspects need to be improved: the low volume of expressible milt and the synchronization of spawning. In relation with these points the effectiveness of the administration of hCG, heterologue pituitary extracts, injection of GnRH superactive analogues, and implantation of sustained release GnRH pellets were proved with success.