GnRHa-mediated stimulation of the reproductive endocrine axis in captive Atlantic bluefin tuna, Thunnus thynnus.

A controlled-release implant loaded with GnRH agonist (GnRHa) was used to induce spawning in Atlantic bluefin tuna (Thunnus thynnus) during two consecutive reproductive seasons. The fish were implanted underwater and sampled between days 2 and 8 after treatment. At the time of GnRHa treatment, females were in full vitellogenesis and males in spermiation. There was a rapid burst of pituitary luteinizing hormone (LH) release at day 2 after treatment in GnRHa-treated fish, and circulating LH remained elevated up to day 8 after treatment. In contrast, control fish had significantly lower levels in the plasma, but higher LH content in the pituitary, as observed in many other cultured fishes that fail to undergo oocyte maturation, ovulation and spawning unless induced by an exogenous GnRHa. Plasma testosterone (T) and 17ß-estradiol (E(2)) were elevated in response to the GnRHa treatment in females, while 11-ketotestosterone (11-KT) but not T was elevated in males. Even though oocyte maturation and ovulation did occur in GnRHa-induced fish, no significant elevations in 17,20ß-dihydroxy-4-pregnen-3-one (17,20ß-P) or 17,20ß,21-trihydroxy-4-pregnen-3-one (20ß-S), in either the free, conjugated or 5ß-reduced,3α-hydroxylated forms was observed in fish sampled within 6 days after treatment. Interestingly, a significant peak in plasma free 17,20ß-P levels occurred in both males and females at day 8 after treatment. Histological sections of the ovaries in these females contained oocytes at the migrating germinal vesicle stage, suggesting the role of this hormone as a maturation-inducing steroid in Atlantic bluefin tuna. In conclusion, the GnRHa implants activated effectively the reproductive endocrine axis in captive Atlantic bluefin tuna broodstocks, through stimulation of sustained elevations in plasma LH, which in turn evoked the synthesis and secretion of the relevant sex steroids leading to gamete maturation and release.

Proliferation and apoptosis of male germ cells in captive Atlantic bluefin tuna (Thunnus thynnus L.) treated with gonadotropin-releasing hormone agonist (GnRHa).

The effects of administration of gonadotropin-releasing hormone agonist (GnRHa) on proliferation and apoptosis of male germ cells were evaluated on Atlantic bluefin tuna (Thunnus thynnus L.) reared in captivity. Fish (n=19) were treated with a sustained-release delivery system loaded with GnRHa during the natural spawning season of 2004 and 2005 (June-July). Untreated Control fish (n=17) and adult wild spawners were used for comparison. Fish were sacrificed 2-8 d after GnRHa implantation and body weight and gonad weight were recorded, and gonads and blood were taken. Germ cell proliferation and apoptosis were evaluated through the immunohistochemical detection of proliferating cell nuclear antigen (PCNA) and the terminal deoxynucleotidyl transferase-mediated d'UTP nick end labelling (TUNEL) method, respectively. Plasma 11 ketotestosterone (11-KT) levels were measured using an ELISA method. Mean gonado-somatic index and seminiferous lobule diameter did not differ between GnRHa-treated and Control fish, and were significantly lower in captive-reared individuals than in wild spawners. Significant increases in 11-KT plasma levels and spermatogonial mitosis, along with a reduction of germ cell apoptosis were demonstrated in GnRHa-treated fish compared to Controls. The results suggest that GnRHa administration was effective in enhancing germ cell proliferation and reducing apoptosis in captive males through the stimulation of luteinizing hormone (LH) release and testicular 11-KT production.

Gonadotropins beta-GtHI and beta-GtHII from the gilthead seabream, Sparus aurata.

Full-length cDNA sequences encoding the beta-subunits of the gonadotrophins GtHI and GtHII were isolated, cloned, and sequenced from a single gilthead seabream (Sparus aurata) pituitary using RACE PCR. Beta-GtHI and beta-GtHII degenerate PCR primers were designed according to regions of high amino acid sequence homology between the chum salmon and the bonito beta-GtHI or beta-GtHII. DNA sequence analysis of the cloned PCR products confirmed the presence of the predicted complete coding region as well as 5' and 3' untranslated sequences. The deduced amino acid sequences of beta-GtHI and beta-GtHII from the gilthead seabream were compared to GtH sequences from a number of teleosts. Seabream beta-GtHI shows the highest homology to the bonito beta-GtHI (76%) and the striped bass (74%), while seabream beta-GtHII is 98% homologous to the yellow fin porgy, 87% homologous to the bonito, and 93% homologous to the striped bass beta-GtHII. Northern blot analysis showed the transcripts of the gilthead seabream beta-GtHI and beta-GtHII to be about 700 and 740 bases, respectively. During the spawning season, beta-GtHI is expressed at higher levels in males than in females while beta-GtHII is expressed at similar levels in both sexes.

Detection and identification of a pathogenic marine Mycobacterium from the European seabass Dicentrarchus labrax using polymerase chain reaction and direct sequencing of 16S rDNA sequences.

Mycobacteriosis has become a major concern for the commercial mariculture of the European sea bass Dicentrachus labrax in Israel. The disease remains asymptomatic for a long time, is virtually impossible to eradicate with antibiotics, stunts the growth of the fish and renders the fish unmarketable. The pathogen was identified as Mycobacterium marinum by direct sequencing and analysis of approximately 600 bp of the pathogen ribosomal encoding DNA (rDNA). The polymerase chain reaction technique was evaluated as a diagnostic tool for detecting the infection in D. labrax and found to be highly specific and sensitive.

Relationship between the Unicellular Red Alga Porphyridium sp. and Its Predator, the Dinoflagellate Gymnodinium sp.

Contamination of algae cultivated outdoors by various microorganisms, such as bacteria, fungi, algae, and protozoa, can affect growth and product quality, sometimes causing fast collapse of the cultures. The main contaminant of Porphyridium cultures grown outdoors in Israel is a Gymnodinium sp., a dinoflagellate that feeds on the alga. Comparison of the effects of various environmental conditions, i.e., pH, salinity, and temperature, on Gymnodinium and Porphyridium species revealed that the Gymnodinium sp. has sharp optimum curves, whereas the Porphyridium sp. has a wider range of optimum conditions and is also more resistant to extreme environmental variables. The mode of preying on the alga was observed, and the specificity of the Gymnodinium sp. for the Porphyridium sp. was shown. In addition, Gymnodinium extract was shown to contain enzymatic degrading activity specific to the Porphyridium sp. cell wall polysaccharide.