Expression and localization of vitellogenin genes (VTG) and receptor (VGR) in the gonad development of silver pomfret Pampus argenteus

Vitellogenesis is the main event of oocyte growth in oviparous animals, which is mainly manifested by the accumulation of vitellogenin (VTG). The accumulation of vitellogenin depends mainly on the absorption of exogenous vitellogenin, which enters oocyte through endocytosis mediated by its receptor (VGR). We investigated the expression and localization of VTG and VGR during gonad development of Pampus argenteus. The qPCR results show that vtgs were not expressed in male fish, but in the ovary and liver of female fish; the expression levels went up at first and then down. The expression levels of vgr in the testis were low and only 1%-3% of that in ovary. ELISA results show that during the ovarian development of P. argenteus, VTG in liver, serum, and ovary all showed a trend from increasing to decreasing. However, VTG in liver peaked in Stage IV, and in serum and ovary peaked in Stage V, reflecting changes in the characteristics of VTG in the liver (synthesis), blood (transport), and ovaries (accumulation). During gonad development, VGR in the ovaries first increased and then decreased, reaching a peak in Stage V, in contrast to vgr mRNA expression. The VGR content in the testis was extremely low and stable, consistent with vgr mRNA. Immunohistochemistry results show that the location and intensity of VTG and VGR positive signals were synchronized with the changes of their protein content, which revealed that VTG was mainly synthesized in the liver cytoplasm, secreted into the blood, and transported to ovary in Stage III. VGR is highly expressed in oocytes in Stage II. In Stage III, a large amount of VTG reaches the ovary, when VGR begins to translate and is subsequently transported to the plasma membrane of the oocyte. Therefore, the positive signal of VGR was stronger near the plasma membrane of oocytes in Stages I and II. By using qPCR, ELISA, and immunohistochemistry, the synthesis, transport, and accumulation of vitellogenin were elucidated and the mechanism of its endocytosis on egg membrane mediated by VTG during the development of P. argenteus was revealed preliminarily.

Automated summary

This study investigated the expression and localization of VTG and VGR during gonad development of Pampus argenteus. The results showed that vtgs were expressed in the ovary and liver of female fish, while vgr expression in the testis was low. ELISA results indicated that VTG levels increased and then decreased during ovarian development, with peaks in the liver, serum, and ovary at different stages. VGR expression in the ovaries increased and then decreased, reaching a peak in Stage V.