Cloning and expression analysis of the vitellogenin gene in the scallop Chlamys farreri and the effects of estradiol‐17β on its synthesis

Vitellogenin (VTG), the precursor of yolk protein, plays an important role during vitellogenesis and oogenesis. However, little is known about the vitellogenin gene in marine bivalves. We cloned the full‐length cDNA of Chlamys farreri vitellogenin (Cf‐vtg) using a homologous cloning strategy and rapid amplification of cDNA ends. The full‐length cDNA consisted of 7604 nucleotides with an open reading frame encoding 2296 amino acid residues. The deduced amino acid sequence shared high similarity with the VTG of other species, particularly in the N‐terminal region. We analyzed the spatiotemporal expression of Cf‐vtg transcripts by semiquantitative and quantitative real‐time PCR. Cf‐vtg was primarily expressed in the ovary, and levels were 1.917±0.035 (mean±S.E.) and 2.570±0.252 times higher in growing and mature stages, respectively, than in the proliferative stage. There was negligible expression at the resting stage. Interestingly, we observed weak expression in the female hepatopancreas. The level was 4.50±0.90% (mean±S.E.) that in the ovary during the proliferative stage. We did not detect any expression in testis or in other tissues. Using in situ hybridization, Cf‐vtg transcripts were localized to the follicle cells surrounding the oocytes in the ovary. Moreover, synthesis of Cf‐vtg transcripts in the ovary increased when estradiol‐17β was injected in vivo into the gonads of early growing stage. We conclude that VTG synthesis is heterosynthetic in C. farreri. Also, we hypothesize that Cf‐VTG synthesis occurs primarily in the follicle cells and is mediated by estradiol‐17β.