Photoperiodic regulation of plasma gonadotropin levels in previtellogenic greater amberjack (Seriola dumerili).

In Seriola species, exposure to a long photoperiod regime is known to induce ovarian development. This study examined photoperiodic effects on pituitary gene expression and plasma levels of follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh) in previtellogenic greater amberjack (Seriola dumerili). The fish were exposed to short (8L:16D) or long (18L:6D) photoperiod. The water temperature was maintained at 22 °C. Compared with the short-photoperiod group, plasma Fsh levels were higher on days 10 and 30 in the long-photoperiod group, but plasma Lh levels did not significantly differ. On day 30, pituitary Fsh- and Lh-β subunit gene expressions were also higher in the long-photoperiod group than the short-photoperiod group, whereas α-subunit gene expressions were higher on days 20 and 30. Throughout the experiment, average gonadosomatic index and plasma E2 levels did not significantly differ between the two groups. This study clearly demonstrated that a long photoperiod induced Fsh release in the previtellogenic fish followed by upregulation of pituitary Fsh and Lh subunit gene expressions. An increase in plasma Fsh levels may be a key factor that mediates the photoperiodic effect on the initiation of ovarian development.