Sexually Dimorphic Responses to Growth Hormone and Luteinizing Hormone within the Somatotropic and Reproductive Axes of Tilapia

Abstract

Sexual dimorphism in body size is a common phenomenon among vertebrates, including teleost fishes. The hormonal underpinnings of this phenomenon, however, have not been fully clarified and may be associated with interactions between the endocrine factors that control growth and reproduction. Given their well‐described patterns of sexually dimorphic growth, we used Mozambique tilapia (Oreochromis mossambicus) as a model to examine how the activities of luteinizing hormone (LH) are modulated by growth hormone (GH), and conversely, how LH affects targets of GH. We hypothesized that GH would induce factors within the GH/insulin‐like growth factor (GH/IGF) axis that favor growth, while LH would attenuate growth‐promoting factors and stimulate factors that favor reproduction. We analyzed gonad morphology and gene expression in hypophysectomized tilapia injected with ovine GH (oGH) and LH (oLH) either alone or in combination. Gonadosomatic index (GSI) was reduced in females following hypophysectomy; the combined administration of oGH and oLH restored GSI to control levels. Hypophysectomy regressed the ovary and the combined treatment of oGH and oLH restored ovarian structure. Gonadal gh receptor (ghr2) and estrogen receptor β (erβ) were higher in females, whereas igf1, igf3, erα, and androgen receptor ß (arß) were higher in males of control fish. The inhibitory effects of hypophysectomy were most pronounced on igf3transcripts, and these levels were restored by both oGH and oLH alone, or in combination, in males. In females, the combination of both hormones was required to recover gonadal igf3 levels. A hypophysectomy‐induced decrease in arβ was restored by a combination of oGH and oLH in females, while there were no effects of oGH or oLH on arβ in males. In muscle, ghr2 was more responsive to oGH in males, while igf2 was more responsive to oLH in females. oGH and oLH both inhibited a hypophysectomy‐induced increase in hepatic igf2 in males. Our results indicate that genes associated with growth and reproduction exhibit sexually dimorphic responses to GH and LH in tilapia, and therefore, provide insight into the endocrine bases of sexually dimorphic growth in fishes.