Gender-specific expression of estrogen receptors (ER
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and ERβ),
growth hormone receptors (GHR1 and GHR2), insulin-like
growth factors (IGF-I and IGF-
II) and three vitellogenins (Vgs A–C) was examined in the liver, gonad, pituitary, and brain of sexually mature male, female, and 17β-estradiol (E
2)-treated male tilapia (
Oreochromis mossambicus). Reflecting greater
growth rate in male tilapia, hepatic expression of GHR1, GHR2, IGF-I and IGF-II as well as plasma IGF-I levels were higher in males than in females, whereas the expression of Vgs A–C and ER
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was higher in females. On the other hand, expression of all genes measured was higher in the ovary than in testis. Forty eight hours after E
2 injection (5 μg/g) into male fish, hepatic expression of most transcripts measured were altered to levels that were similar to those seen in females. The changes included decreased expression of GHR1, GHR2, IGF-I, and IGF-II, and increased expression of ER
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and Vgs A–C. E
2 treatment also increased Vg and decreased IGF-I in the plasma. Brain expression of ER
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, ERβ, GHR1, and IGF-I was higher in females than in males, whereas pituitary expression of GHR2 and IGF-I was lower in females; only brain expression of GHR1 was increased by E
2 treatment. These findings suggest that E
2 stimulates Vg production primarily through activation of ER
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and down-regulation of the GH/IGF-I axis, thus shifting energy from somatic
growth towards vitellogenesis at the level of the liver.