Resolving the complexity of vitellogenins and their receptors in the tetraploid Atlantic salmon (Salmo salar): Ancient origin of the phosvitin‐less VtgC in chondrichthyean fishes

Egg yolk proteins are mainly derived from vitellogenin (Vtg), and serve as essential nutrients during early development in oviparous organisms. Vertebrate Vtgs are predominantly synthesized in the liver of maturing females, and are internalized by the oocyte after binding to specific surface receptors (VtgR). Here, we clarify the evolutionary history of vertebrate Vtgs, including the teleost VtgC, which lacks phosvitin, and investigate the repertoire of Vtgs and VtgRs in the tetraploid Atlantic salmon (Salmo salar). Conserved synteny of the vtg genes in elephant fish (Callorhinchus milii) strongly indicates that the vtg gene cluster was present in the ancestor of tetrapods and ray-finned fish. The shortened phosvitin in the VtgC ortholog of this chondrichthyean fish may have resulted from early truncation events that eventually allowed the total disappearance of phosvitin in teleost VtgC. In contrast, the tandem-duplicated VtgCs identified in the spotted gar (Lepisosteus oculatus) both contain the phosvitin domain. The Atlantic salmon genome harbors four vtg genes encoding the complete VtgAsa1, phosvitin-less VtgC, and truncated VtgAsb proteins; vtgAsa2 is a pseudogene. The three vtg genes were mainly expressed in the liver of maturing females, and the vtgAsa1 transcript predominated prior to spawning. The splice variant lacking the O-linked sugar domain dominated ovarian expression of vtgr1 and vtgr2. Strongly increased vtgAsa1 expression during vitellogenesis contrasted with the peaks of vtgr1 and vtgr2 in the previtellogenic oocytes, which gradually decreased over the same period. Recycling of the oocyte VtgRs is probably not sufficient to maintain receptor number during vitellogenesis.