The present study was undertaken to determine scallop vitellogenin (Vtg) cDNA sequence, to identify Vtg synthesizing cell, and to analyze the regulation of Vtg mRNA expression. Clones containing partial cDNA sequence of Vtg were isolated from cDNA library of the scallop ovary by immunoscreening with the anti-scallop vitellin (Vn) serum. The deduced amino acid sequence of the clone containing the longest cDNA insert (1,689 bp) was identified as a member of the lipid transport protein family and exhibited about 20-35% identity with Vtgs of other oviparous animals. Northern blot analysis identified a single transcript longer than 10 kb in the ovary. Dot blot analysis of the ovary showed a high amount of Vtg mRNA during the growing stage and the level was retained until spawning stage. In situ hybridization demonstrated the expression of Vtg mRNA in the auxiliary cells closely associated with growing oocytes, suggesting that the synthesis of a major yolk protein in the scallop occurs through hetero-synthetic pathway without mediation through the blood flow but occurs de novo in the ovary. The content of Vtg mRNA in the ovarian tissue cultured in vitro with vitellogenesis promoting factor(VPF), which strongly promotes Vtg protein synthesis, from the cerebral plus pedal ganglion (CPG) showed no change. The transcription of Vtg mRNA appeared to be promoted by estradiol-17β (E2) not by VPF although VPF may enhance the translation of Vtg mRNA.
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