Identification of a New Insulin-like Growth Factor 3 (igf3) in Turbot (Scophthalmus maximus): Comparison and Expression Analysis of IGF System Genes during Gonadal Development

Fish reproduction is closely related to the regulation of the brain and liver, making it essential to identify the factors that control this process. The turbot (Scophthalmus maximus) is an economically significant species that has been successfully breeding through industrial aquaculture. Investigation of factors into the involvement of gonadal development is crucial for artificial breeding. In this study, a new insulin-like growth factor 3 igf3 gene was cloned and characterized. Additionally, all three types of turbot IGFs contain a distinct IGF domain, with IGF3 and IGF2 being grouped with other teleosts, demonstrating a closely related genetic relationship. The expression analysis showed that igf3 mRNA is predominantly expressed in the gonad and brain (specifically in the pituitary and hypothalamus), suggesting its effects at multiple levels in the brain–pituitary–gonadal axis. Furthermore, the mRNA levels of igfs during gonadal development were examined. In the gonad and liver of female turbots, the expression levels of igfs mRNA significantly increased from stage II to VI during the process of oogenesis, including maturation and degeneration. In the testis and liver of male turbots, igf3 mRNA maintained high expression levels during the proliferation of spermatogonia at stages II and III. In addition, the highest levels of igf1 and igf2 were observed at the beginning of spermatogenesis and during sperm production at stages V and VI. All the results suggest that the IGFs were closely related to the gonadal development in turbot and improve a better understanding of the IGF system in the regulation of gonadal development in teleost.