Abstract
The endocrine regulation of reproduction in a multiple spawning flatfish with an ovary of asynchronous development remains largely unknown. The objectives of this study were to monitor changes in mRNA expression patterns of three gonadotropin hormone (GTH) subunits (FSHβ, LHβ and CGα) and plasma GTH levels during ovarian maturation of half-smooth tongue sole Cynoglossus semilaevis. Cloning and sequence analysis revealed that the cDNAs of FSHβ, LHβ and CGα were 541, 670 and 685 bp in length, and encode for peptides of 130, 158 and 127 amino acids, respectively. The number of cysteine residues and potential N-linked glycosylation sites of the flatfish GTHs were conserved among teleosts. However, the primary structure of GTHs in Pleuronectiformes appeared to be highly divergent. The FSHβ transcriptional level in the pituitary remained high during the vitellogenic stage while plasma levels of FSH peaked and oocyte development was stimulated. The LHβ expression in the pituitary and ovary reached the maximum level during oocyte maturation stages when the plasma levels of LH peaked. The brain GTHs were expressed at the different ovarian stages. These results suggested that FSH and LH may simultaneously regulate ovarian development and maturation through the brain-pituitary-ovary axis endocrine system in tongue sole.
Highlights
The two gonadotropin hormones (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), secreted by the pituitary gland, are key modulators of gametogenesis and gonadal steroidogenesis in almost all vertebrates, including fish
It is generally accepted that FSH and LH of fish have complementary functions during reproduction; FSH is mainly involved in the control of puberty and gametogenesis, whereas LH predominates at final gonadal maturation, ovulation or spermiation [2,5,6,7,8]
The gonadosomatic index (GSI) value was low, when the ovaries were at the previtellogenic and beginning of vitellogenic stages
Summary
The two gonadotropin hormones (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), secreted by the pituitary gland, are key modulators of gametogenesis and gonadal steroidogenesis in almost all vertebrates, including fish. These GTHs contain a common glycoprotein α subunit (CGα) that forms a heterodimer with a hormone-specific β subunit (FSHβ or LHβ) [1,2]. Expression patterns of GTHs in female fish spawning single batches of eggs, have shown that the FSHβ transcription increased during the vitellogenic stage and decreased during final maturation of gametes. Expression patterns of GTHs have been reported during the ovarian cycle of several multiple batch spawners [11,12,13,14,15,16]
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