Mechanisms of egg yolk formation and implications on early life history of white perch (Morone americana)

Abstract

The three white perch (Morone americana) vitellogenins (VtgAa, VtgAb, VtgC) were quantified accurately and precisely in the liver, plasma, and ovary during pre-, early-, mid-, and post-vitellogenic oocyte growth using protein cleavage-isotope dilution mass spectrometry (PC-IDMS). Western blotting generally mirrored the PC-IDMS results. By PC-IDMS, VtgC was quantifiable in pre-vitellogenic ovary tissues and VtgAb was quantifiable in pre-vitellogenic liver tissues however, neither protein was detected by western blotting in these respective tissues at this time point. Immunohistochemistry indicated that VtgC was present within pre-vitellogenic oocytes and localized to lipid droplets within vitellogenic oocytes. Affinity purification coupled to tandem mass spectrometry using highly purified VtgC as a bait protein revealed a single specific interacting protein (Y-box binding protein 2a-like [Ybx2a-like]) that eluted with suramin buffer and confirmed that VtgC does not bind the ovary vitellogenin receptors (LR8 and Lrp13). Western blotting for LR8 and Lrp13 showed that both receptors were expressed during vitellogenesis with LR8 and Lrp13 expression highest in early-and mid-vitellogenesis, respectively. The VtgAa within the ovary peaked during post-vitellogenesis, while VtgAb peaked during early-vitellogenesis in both white perch and the closely related striped bass (M. saxatilis). The VtgC was steadily accumulated by oocytes beginning during pre-vitellogenesis and continued until post-vitellogenesis and its composition varies widely between striped bass and white perch. In striped bass, the VtgC accounted for 26% of the vitellogenin-derived egg yolk, however in the white perch it comprised only 4%. Striped bass larvae have an extended developmental window and these larvae have yolk stores that may enable them to survive in the absence of food for twice as long as white perch after hatch. Thus, the VtgC may play an integral role in providing nutrients to late stage fish larvae prior to the onset of exogenous feeding and its composition in the egg yolk may relate to different early life histories among this diverse group of animals.