Daily rhythms in the reproductive axis of Nile tilapia (Oreochromis niloticus): Plasma steroids and gene expression in brain, pituitary, gonad and egg

Abstract

Control of reproduction is one of the key issues to extend tilapia aquaculture. In most fish species, reproduction is a rhythmic process which ensures that reproduction timing matches the most favorable environmental conditions to maximize the survival of offspring. Although the existence of daily spawning rhythms has been reported for a number of fish species, the daily rhythms in neuroendocrine factors along the BPG (brain-pituitary-gonadal) axis remains mostly unknown. The objective of this paper was to investigate the existence of daily rhythms in BPG gene expression and plasma sex steroids in tilapia (Oreochromis niloticus). To this end, 36 males and 36 females were kept in separate tanks at 29 ± 0.5 °C and in a 12 h:12 h light/dark cycle. After 4 weeks of acclimation, male and female tilapia were mixed into six groups at a female:male ratio of 6:6. Two weeks later, fish were anesthetized and sampled every 4 h during a 24-hour cycle, and blood, brain, pituitary, gonad and eggs were collected for later analyses. The expression levels of the genes from the brain (gnrh1, gnrh2, gnrh3, kiss2, gnih), pituitary (fshβ and lhβ), gonad (star, cyp11a, cyp17, cyp19a1a and amh) and eggs (zp3b and fyna) were analyzed by qPCR. ELISA was used to analyze testosterone and estradiol in male and female plasma, respectively, and maturation-inducing hormone (MIH) in plasma in both sexes. The results revealed timely-coordinated daily rhythms along the BPG axis of tilapia. The expression of the brain genes showed that their acrophase was distributed throughout the LD cycle. However, a coordinated and harmonious pattern of daily rhythms appeared on the following levels of the BPG axis. The peaks in expression of the pituitary genes were reached at night, while the gonadal genes and plasma sex steroids peaked in the light phase. These findings can be useful for improving breeding protocols for tilapia aquaculture and for defining optimum timing for reproduction in captivity.