Conspecific vitellogenin induces the expression of vg gene in the Indian male walking catfish, Clarias batrachus (Linn.)

To understand the regulatory mechanism of Vg induced vitellogenin synthesis the vg gene expression in Indian male walking catfish, Clarias batrachus was investigated. Semipurified conspecific Vg containing Vg1 and Vg2 in a ratio of 2.7: 1.0 was administered into male catfish and vg cDNA (1.1 kb) was amplified from total RNA in liver by reverse transcriptase polymerase chain reaction (RT-PCR) using primers designed from published sequence of Clarias macrocephalus. The nucleotide and deduced amino acid sequence of the cDNA shared maximum similarities (98-100%) with the corresponding sequences of known E-2-induced Vg of C. batrachus and C. macrocephalus in the database. A 0.178 kb cDNA fragment nested within the 1.1 kb DNA was then amplified by real time PCR to evaluate the role of Vg on relative expression of vg gene. The result revealed a significant upregulation (1.79 fold) of vg mRNA at 6 h reaching maximum level (9.78 fold) at 24 h post Vg injection as compared to the saline control. Similarly, E-2-treatment also showed maximum mRNA expression (7.73 fold) at 24 h post injection. The findings suggest that like E-2, Vg itself can induce vg gene expression resulting in a significant increase in plasma Vg levels (9.11 +/- 0.73 mg/ml for Vg1 and 3.02 +/- 0.28 mg/ml for Vg 2) in male catfish where E-2 is lacking. The work provides further opportunity to study the regulatory mechanism of vg gene expression by Vg.