Introduction: As in all vertebrates, reproduction in fish is regulated by GnRH control on gonadotrophic hormones (GtH) activity. However, the neuroendocrine factors that promote GnRH and GtH activity are unknown. In Nile tilapia (Oreochromis niloticus), sexual activity and reproduction ability depend on social rank; only dominant males and females reproduce. Here, this characteristic of dominant fish allows us to compare brain and pituitary gene expression in animals that do and do not reproduce, aiming to reveal mechanisms that regulate reproduction. Methods: an extensive transcriptome analysis was performed, combining two sets of transcriptomes: a novel whole-brain and pituitary transcriptome of established dominant and subordinate males, together with a cell-specific transcriptome of LH and FSH cells. Pituitary incubation assay validated the direct effect of steroid application on chosen genes and GtH secretion. Results: in most dominant fish, as determined behaviorally, the gonadosomatic index was higher than in subordinate fish, and the leading upregulated pituitary genes were those coding for GtHs. In the brain, various neuropeptide genes, including isotocin, cholecystokinin, and MCH, were upregulated; these may be related to reproductive status through effects on behavior and feeding. In a STRING network analysis combining the two transcriptome sets, brain aromatase, highly expressed in LH cells, is the most central gene with the highest number of connections. In the pituitary incubation assay, testosterone and estradiol increased the secretion of LH and specific gene transcription. Conclusions: the close correlation between behavioral dominance and reproductive capacity in tilapia allows unraveling novel genes that may regulate the HPG axis, highlighting aromatase as the main factor affecting the brain and pituitary in maintaining a sexually active organism.

Many fish species display clear temporal separation between prolonged periods of energetically costly gonadal development, and short spawning events. These two distinct processes are controlled by the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH), respectively. While it was suggested that a common secretagogue, gonadotropin-releasing hormone (GnRH), controls both functions, it is unclear how the secretion of the two gonadotropins is differentially regulated. Here, we usedin vivoandex vivocalcium imaging of zebrafish gonadotrophs to study the regulation of gonadotropin release in fish. We show that while LH cells are highly responsive to GnRH stimulation, the response of FSH cells is weak and inconsistent. By combining transcriptomics and calcium imaging, we found that FSH cells express the receptor for the satiety hormone cholecystokinin (CCK) and display a strong calcium response to its application, accompanied by FSH secretion, which designates this peptide as abona fideFSH secretagogue. From an evolutionary perspective, these findings indicate that the control of folliculogenesis and ovulation in fish was placed under different neural circuits, which respond to the specific somatic and environmental cues relevant to each process.