| Mozambique tilapia (Oreochromis mossambicus) is a sexually dimorphic species, with male fish growing faster and larger than female fish. Compared with males, females produce large amounts of the yolk precursor proteins, vitellogenin (Vg), concomitant with suppression of the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. The primary objective of this dissertation was to investigate the modulation of the neuroendocrine system, specifically the GH/IGF-I axis, by gonadal steroid hormones, in particular by 17beta-estradiol (E2).; Multiple forms of genes encoding Vg, estrogen receptors, and GH receptors have recently been characterized in several teleost species including the tilapia. In male tilapia, E2 treatment rapidly increased hepatic expression of Vgs A, B, C and estrogen receptor alpha, while plasma IGF-I and hepatic expression of IGF-I and GH receptors 1 and 2 were suppressed. Few differences were seen in tissues other than the liver. These results suggest that E2 stimulates Vg production primarily through activation of estrogen receptor alpha, while down-regulating the GH/IGF-I axis at the level of GH receptor.; Vitellogenin production in male fish has been linked to both environmental estrogens and commercial fish diets. Injections of heptachlor and o,p'-DDE, pesticides shown to bind estrogen receptors, did not increase plasma Vg levels in male tilapia, however hepatic expression of Vgs were increased by o,p'-DDE 5 and 10 days post injection. Unlike E2, neither pesticide produced consistent effects on estrogen receptor expression or on the GH/IGF-I axis. Male tilapia fed daily on commercial Trout Chow have plasma Vg levels approximately 10 times lower than females. Male fish fed a fishmeal-free diet of squid and vegetables had dramatically lower Vg levels after 10 days. After 40 days, expression of Vgs was reduced by 99%, although estrogen receptor a expression did not change. There was no change in vitellogenesis in females fed the same diet. These results suggest that Vg in male tilapia is induced by estrogens in the feed, likely from the fishmeal component, via up-regulation of Vg gene expression. |
