The Interaction between the Brain and the Pituitary in Social State of Tilapia

Social rank is associated with differences in behavior and physiology. Dominant individuals at the top of the hierarchy often have better health, more access to resources, and higher reproductive success. Social status also affects hormones, receptors, and neuropeptides, however, the specific roles hormones play in determining social status in most species are not well understood. Reproduction is controlled by the conserved Hypothalamus-Pituitary-Gonad axis and is tightly linked to social rank in many cichlid species. A Nile tilapia male can switch rapidly and reversibly between dominant and subordinate phenotypes. Dominant males are characterized by their territoriality and high level of aggression when only dominant males and females reproduce. This characteristic of dominant fish allows us to compare brain and pituitary gene expression in fish that reproduce and those that do not, to reveal how reproduction is controlled. An extensive transcriptome analysis was performed, combining two sets: a novel whole-brain and pituitary transcriptome of established dominant and subordinate males, as well as a cell-specific transcriptome of luteinizing-hormone (LH) and follicle stimulating-hormone (FSH) cells. Ascension in the social hierarchy was found to increase steroid hormones, FSH and LH, and gonadal size. 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. Brain aromatase – the enzyme that converts testosterone to estradiol – was highly expressed in LH cells. Testosterone and estradiol increased the secretion of LH and specific gene transcription from tilapia pituitary.