The Expression Patterns Of Two Vitellogenin Transcripts In The Zhikong Scallop, Chlamys Farreri During Ontogenesis

Vitellogenin (VTG) is the precursor of yolk protein vitellins (Vn), which serves as the energy resource for embryonic and larval development. For oviparous animals, yolk formation and accumulation are the important events in the process of oogenesis. As the major component of yolk materials, Vn plays important biological functions. So the vitellin precursor VTG possesses high scientific value.It has been widely and deeply studied in verterbrates, insects and crustaceans. However, these studies were restricted at adult level, especially in gonadal developmental cycles, the report about the information of VTG during ontogenesis was limited. In the present study, an important commercial species of aquatic, Zhikong scallop Chlamys farreri was selected as the research object. A small transcprit of Cf-vtg (named cf-vtg2) from Chlamys farreri was cloned by rapid amplification of cDNA end (RACE) technology and analyzed its sequence. Furthermore, the spatio-temporal expression pattern of Cf-vtg2 and the ovary Cf-vtg1 transcript during ontogenesis were analyzed and compared using Real-Time PCR and in situ hybridization methods. Moreover, the differentiation of gonads of C.farreri was studied by histology method.In this studied, Cf-vtg2 cDNA consisted of 1719 nucleotides and encoded 564 amino acid residues. It shared over 99% identity to the corresponding N-terminal region of the Cf-VTG1. RT-PCR showed that Cf-vtg1 mRNA was specific expressed for ovary, and Cf-vtg2 mRNA was expressed in hepatopancreas. Real-Time PCR and in situ hybridization showed that Cf-vtg(Cf-vtg1 and Cf-vtg2)were detected in unfertilized eggs and from fertilized eggs to D-shaped larvae, indicating a maternally Cf-vtg supplied. It showed the highest value in fertilized eggs and decreased gradually after fertilization. The expression level reached to the lowest value in D-shaped larvae and approximately 0 from umbo-larvae. Until the shell height reached to 7mm, Cf-vtg2 was first detected in junvenile hepatopancreas, and no significant difference was showed between male and female. In situ hybridization analysis showed that the Positive signals were observed uniformly distributed in the unfertilized eggs, and from fertilized eggs to trochophore. At D-larva period, the ubiquitous expression became restricted to the visceral mass region, and then vanished after the umbo-larva period. Cf-vtg mRNA can be first observed in parenchymal cells of hepatopancreas before and after sex differentiation, and in follicle cells in the ovary during the first gonadal developmental cycle, but not in the testis.