Evaluation of Seasonal Variation in Reproduction-Related Gene Expression in the Pacific Bluefin Tuna Thunnus orientalis Using Next-Generation Sequencing

Tuna aquaculture has rapidly expanded and developed in recent years as an important commercial enterprise in Japan, while becoming increasingly vital to the global economy. A reliable supply of hatchery-reared juvenile fish is imperative for tuna aquaculture, but the development of a more productive and sustainable juvenile production tuna supply model requires the rearing of suitable broodstock. Despite advances in farming technology for bluefin tuna, there is little new information on the molecular mechanisms of their reproduction. Given this deficiency, gene expression profiles of the adult bluefin  tuna brain—which is integral to gonadal development and maturation—were analyzed using next-generation sequencing. Brain samples were collected over different seasons, and complementary DNA libraries derived from mRNA from different brain regions were sequenced using a next-generation DNA sequencing on the Illumina platform. RNA-seq reads were analyzed to estimate gene expression levels using the whole genome sequence data of the Pacific bluefin tuna, which has recently been reported by our group. Several reproductive genes, such as the gene encoding gonadotropin-releasing hormone, were expressed in the anterior part of the mesencephalon, which includes the hypothalamus. Some unidentified genes were highly expressed during gonadal maturation. In order to gain insight into the molecular basis of reproductive maturation, gene expression data for the pituitary gland and other parts of the brain were compared. Some genes showed spatially and temporally restricted expression patterns, and were expressed only in specific brain regions or in a season-specific manner. These findings are potentially useful for the identification of a suitable female spawner, which would benefit current tuna aquaculture practices.