First Data on Sexual Dimorphic Growth of Cultured Pacific Bluefin Tuna, Thunnus orientalis (Temminck et Schlegel), and Its Sex Manipulation by Treatment with an Aromatase Inhibitor

Abstract

Tuna (genus Thunnus), particularly Pacific bluefin tuna (T. orientalis; PBT), are commercially important fish in the aquaculture industry worldwide. The objective of this study was to investigate sexual dimorphism in the growth performance of aquaculture-produced PBT and develop techniques for its sex manipulation, for the first time in tuna. A comparison of the body size between sexes revealed that male-cultured PBTs were larger than females at harvest. We also confirmed that cyp19a1a (encoding a gonadal aromatase) expression increased specifically in the genotypic female PBT gonads during sex differentiation. This suggested that aromatase plays an important role in ovarian differentiation and that the suppression of aromatase activity may effectively induce masculinization in genotypic females. Therefore, we administered letrozole—an aromatase inhibitor (AI)—into sexually undifferentiated PBT through the oral route. AI administration resulted in a 100% sex reversal of genotypic females into phenotypic males at the molecular level. Our results provide the basis for future studies on the establishment of mono-sex male production technology in PBT, which would help improve the productivity of closed-cycle PBT aquaculture. Furthermore, this study offers important insights into the understanding of the sex-wise growth of tuna species in aquacultural settings, and developing sex manipulation techniques.

Transcriptome characterization of gonadal sex differentiation in Pacific bluefin tuna, Thunnus orientalis (Temminck et Schlegel)

“…Asterisks indicate significant differences between sexes at each age ( P < 0.05, Welch’s t -test). Cyp19a1a expression data were previously reported 1 . …”

“…Tuna, specifically bluefin tuna, has a high market demand and value and is of substantial commercial interest to fisheries and aquaculture operations worldwide. Despite the ecological and commercial importance of tuna, the mechanisms by which its reproductive system develops remain poorly understood, mainly owing to the difficulty in obtaining artificial seedlings that can be used for experiments and their rearing 1 . A detailed understanding of the biological basis for tuna reproduction is necessary to achieve several goals in fisheries and aquaculture operations, such as effective assessments of population fluctuations in wild-stock management 2 and efficient broodstock management in captivity to obtain high-quality fertilized eggs 3 , 4 .…”

“…A detailed understanding of the biological basis for tuna reproduction is necessary to achieve several goals in fisheries and aquaculture operations, such as effective assessments of population fluctuations in wild-stock management 2 and efficient broodstock management in captivity to obtain high-quality fertilized eggs 3 , 4 . Hence, we recently studied the vital reproductive aspects of Pacific bluefin tuna ( T. orientalis ; hereafter, PBT), as a model of tuna species, based on our closed-cycle aquaculture technology 1 , 5 – 8 .…”

“…We also showed the vital role of aromatase-mediated estrogen biosynthesis in ovarian differentiation in PBT. Aromatase inhibitor (AI) administration resulted in sex reversal of genotypic female to phenotypic male 1 . We confirmed testicular differentiation of the AI-treated genotypic female PBT gonads at the molecular level through histological observation, gene expression analysis, and serum sex steroid measurement 1 .…”

“…Aromatase inhibitor (AI) administration resulted in sex reversal of genotypic female to phenotypic male 1 . We confirmed testicular differentiation of the AI-treated genotypic female PBT gonads at the molecular level through histological observation, gene expression analysis, and serum sex steroid measurement 1 . However, the detailed molecular mechanisms of gonadal sex differentiation in general, and testicular differentiation in particular, remain unclear.…”