Expression profile and localization of vitellogenin mRNA and protein during ovarian development in turbot (Scophthalmus maximus).

Egg yolk generation is a common physiological process in oviparous animals. To understand oogenesis and reproductive capacity, it is necessary to characterize vitellogenins (Vtgs), which are the precursors of major egg yolk proteins (Yps). Therefore, to improve our understanding of the entire process of egg yolk generation in female turbot (Scophthalmus maximus), we obtained full-length cDNAs of vtg genes, examined gene expression in the female liver and ovary, and analyzed Vtg synthesis in the ovary. Three distinct complete polypeptide sequences were identified and designated as VtgAa, VtgAb, and VtgC, which confirmed the multiplicity of the vtg gene in turbot and showed that it follows a "three vtg model". The expression of these three vtg genes in the female liver was far higher than that in other tissues, including the ovary. The expression of all three vtg genes was extremely low before vitellogenesis, and then increased and was maintained at a high level until the degradation stage, which was in accordance with changes in the concentration of estradiol-17β (E2) and the gonadosomatic index. Compared with the liver, the ovary had a higher E2 level and lower vtg expression, suggesting that some other factors limit high vtg expression in the ovary of turbot. Transcripts of vtgAb and the Yps derived from them were both detected in oogonia and primary oocytes, which showed that these might possess the ability to perform autosynthesis of yolk. These findings add to our understanding of the reproductive physiology of Vtg synthesis in turbot.