Effect of Increased Water Temperatures on the Expression of Gonadotropin-Releasing Hormone (GnRH), Kisspeptin and their Receptors in the Regulation of Hypothalamus-Pituitary-Gonadal (HPG) Axis in Nile Tilapia (Oreochromis niloticus)

Abstract

Temperature is a preeminent factor in the regulation of fish reproduction and can hinder gonadal development and maturation beyond a specific threshold. To understand the mechanism that controls reproduction under different acclimation temperatures, genes encoding for kisspeptin (kiss2), gonadotropin-releasing hormones (gnrh1) and their receptors (gpr54 and gnrh1r) in the brain along with gonadal maturation and gonadosomatic index (GSI) was studied in juvenile Nile tilapia (Oreochromis niloticus). The fish were subjected to three distinct acclimation temperatures including 31°C, 34°C and 37°C for a duration of 14 days. The findings showed that mRNA levels of kiss2, gpr54, gnrh1 and gnrh1r were relatively decreased at 37°C compared to 31ºC and 34ºC in the brain of O. niloticus. Histologically, the gonads of both sex had normal growth of gametes at control temperature (31ºC) compared to the other groups. At 37°C acclimation temperatures, the spermatogenesis and oocyte maturation were slowed down and atretic oocytes were found in the ovary. The GSI value was concomitant with other results, with a significant decrease at 37°C for both sexes. Taken together, the results imply that elevated temperatures beyond a specific threshold had a negative impact on reproduction by suppressing Kiss/GnRH system and eventually restraining normal growth and maturation of gametes.