Cloning of cDNA encoding vitellogenin and its expression in red sea urchin, Pseudocentrotus depressus

Both male and female red sea urchins, Pseudocentrotus depressus, accumulate a large quantity of the major yolk protein (MYP) in the nutritive phagocytes of immature gonads before the initiation of game-togenesis. To examine the accumulation mechanism of this protein in the gonad, we cloned full-length cDNA encoding vitellogenin (Vg; the MYP precursor in the coelomic fluid), and investigated its expression in various tissues of immature adults. The nucleotide sequence of Vg contains an open reading frame of 4050 bp encoding 1349 amino acids. The deduced amino acid sequence near the N-terminal showed 25% homology to the vertebrate transferrin family. Vitellogenin mRNA was detected in the ovary, testis, stomach, intestine and rectum by Northern blot analysis, with the highest level of mRNA expression in the gonad. Weak expression was also detected in the esophagus and coelomocytes by RT-PCR. In situ hybridization demonstrated that nutritive phagocytes, which exclusively fill the lumina of the immature gonad, contained Vg mRNA. These results suggested that the MYP stored in the immature gonads is synthesized and accumulated mainly within the nutritive phagocytes.