The Multiple Vitellogenin System of the Striped Bass, Morone saxatilis: A Genomics and Proteomics Study

The present study provides the first characterization of multiple egg yolk precursors, vitellogenins (Vtgs), in striped bass ( Morone saxatilis), an aquaculture species of economic importance in North America. Striped bass express three distinct forms of Vtgs (VtgAa, VtgAb, and VtgC) that are produced by the liver, secreted into the blood, and taken up specifically by growing oocytes where they are proteolytically processed and stored as yolk. Full-length cDNAs encoding these Vtgs were sequenced and their derivative yolk protein products characterized in striped bass liver, plasma, and ovary using various immunobiochemical and mass spectrometry procedures. Peak accumulation by the oocyte of VtgAb and VtgAa occur during early vitellogenesis and mid-vitellogenesis, respectively, whereas VtgC is steadily accumulated throughout vitellogenesis. Patterns of stored yolk proteins present in the oocyte following uptake are similar to that described for other fishes with the exception that previously unreported proteolytic variants of lipovitellins (Lvs) were detected. Major yolk protein products undergo proteolysis during maturation in striped bass, however extensive degradation of lipovitellins derived from VtgAa into free amino acids was not observed as described for other estuarine fishes that spawn floating eggs. These results suggest that disparate accumulation and processing of multiple Vtgs by oocytes is regulated both by rates of hepatic secretion and oocyte uptake and that these processes may vary between fish species with different reproductive life histories and strategies. As these yolk components not only provide nutrition to embryos and larvae at specific developmental stages, they also contribute to oocyte hydration and egg buoyancy as well. This process of vitellogenesis may be related to egg quality in striped bass. Proportional Vtg composition accumulated in post-vitellogenic oocytes and ovulated eggs, however is not significantly correlated with egg quality evaluated as production of eggs bearing viable 4 and 24-hour embryos in striped bass.