Intra-pituitary relationship of follicle stimulating hormone and luteinizing hormone during pubertal development in Atlantic Bluefin tuna (Thunnus thynnus)

As part of the endeavor aiming at the domestication of Atlantic bluefin tuna (BFT; Thunnus thynnus), first sexual maturity in captivity was studied by documenting its occurrence and by characterizing the key hormones of the reproductive axis: follicle stimulating hormone (FSH) and luteinizing hormone (LH). The full length sequence encoding for the related hormone b-subunits, bftFSHb and bftLHb, were determined, revealing two bftFSHb mRNA variants, differing in their 50 untranslated region. A quantitative immuno-dot-blot assay to measure pituitary FSH content in BFT was developed and validated enabling, for the first time in this species, data sets for both LH and FSH to be compared. The expression and accumulation patterns of LH in the pituitary showed a steady increase of this hormone, concomitant with fish age, reaching higher levels in adult females compared to males of the same age class. Conversely, the pituitary FSH levels were elevated only in 2Y and adult fish. The pituitary FSH to LH ratio was consistently higher (>1) in immature than in maturing or pubertal fish, resembling the situation in mammals. Nevertheless, the results suggest that a rise in the LH storage level above a minimum threshold may be an indicator of the onset of puberty in BFT females. The higher pituitary LH levels in adult females over males may further support this notion. In contrast three year-old (3Y) males were pubertal while cognate females were still immature. However, it is not yet clear whether the advanced puberty in the 3Y males was a general feature typifying wild BFT populations or was induced by the culture conditions. Future studies testing the effects of captivity and hormonal treatments on precocious maturity may allow for improved handling of this species in a controlled environment which would lead to more cost-efficient farming.

We would like to express our gratitude to the personnel of Tuna Graso, S.A., and Malta Fish Farming Ltd., for their assistance during the sampling operations. In particular, we would like to acknowledge the assistance of Goran Brstilo and the divers from DRVENIK TUNA D.O.O., whose expertise and dedication were instrumental for the success of this project. Thanks are also due to Ivana Jovanović and the staff of the DRVENIK-TUNA in Croatia for their hospitality and assistance during the field experiments. We would like to express our gratitude to Dr. A. Colorni (IOLR-NCM, Israel) for careful reading and helpful comments that improve the quality of our manuscript. This work was undertaken as part of the research program “From capture based to SELF-sustained aquaculture and Domestication Of bluefin tuna, Thunnus Thynnus” (SELFDOTT) and was supported by a research grant from the European Community under the collaborative projects FP7-KBBE-2007-1 program (Grant Agreement number: 212797).

Peer reviewed