Hepatic and extrahepatic expression of vitellogenin genes in the zebrafish, Danio rerio

Vitellogenins (Vtgs) are yolk precursor proteins in oviparous species and are cleaved into three portions-lipovitellin I (LVI), phosvitin (PV), and lipovitellin II (LVII)-in oocytes in vertebrates. In the present study, we found that the zebrafish genome contains at least seven vtg genes (vtg1-7) encoding heterogeneous vitellogenins with three distinct types of Vtgs: type I (Vtg1, 4-7) contains all the three major portions but lacks the C-terminal half of LVII; type II (Vtg2) is the only one including intact three portions; type III (Vtg3) lacks both PV and the LVII C-terminal half. The seven vtgs were located in two different chromosomes: one (vtg3) in LG11 and the rest closely linked in LG22, probably arisen from local gene duplication events. All of the seven vtgs are predominantly expressed in female liver and can be induced in male liver by 17beta-estradiol (E2). The level of vtg1 mRNA was about 100x and 1000x higher than those of vtg2 and vtg3 mRNAs. We also found vtg mRNAs in several non-liver tissues, but the expression level is generally <10% of that in the liver. In situ hybridization experiments confirmed that the extrahepatic expression was actually in adipocytes associated with several organs such as the intestine, ovary, and E2-induced testis.