MULTIPLE VITELLOGENINS AND THEIR RECEPTORS IN FISHES

Higher-order teleosts (Acanthomorpha) express three distinct forms of vitellogenin (VtgAa, VtgAb, and VtgC). The VtgAa and VtgAb are considered "complete" and contain a suite of five yolk protein domains (lipovitellin heavy chain, lipovitellin light chain, phosvitin, beta'-component, and c-terminal component), however the VtgC is "incomplete" and only contains the lipovitellin heavy and light chains. These egg yolk precursors are produced by the liver in response to estrogen and released into the circulatory system where they are taken up specifically by growing oocytes via receptor-mediated endocytosis. Two vitellogenin receptors (LR8 and Lrp13) have been characterized in fish ovaries. The Lrp13 localizes throughout the zona radiata and granulosa cells and it specifically binds VtgAa, whereas the LR8 localizes to the oolemma and zona radiata interna and binds VtgAb. To date, no known receptor binds VtgC and it might enter oocytes through the fluid phase of endocytosis. Additionally, VtgC localizes exclusively to lipid inclusions within growing oocytes, whereas VtgAb localizes to the ooplasm and yolk globules. The VtgAa has not yet been evaluated in this manner. The VtgAb primarily enters growing oocytes during early- to mid-vitellogenesis whereas the VtgAa enters oocytes from mid- to post-vitellogenesis. The VtgAb is typically the predominant form in the blood plasma and the yolk. The VtgC is steadily accumulated by oocytes beginning during pre-vitellogenesis and continues until post-vitellogenesis and its composition in the yolk can widely vary between species. For example, in striped bass VtgC accounts for 26% of the vitellogenin-derived yolk in post-vitellogenic oocytes, however in the white perch it comprises only 4%.

Lower-order teleosts possess several different types of complete vitellogenins, although, with the exception the Ostariophysian fishes, they all appear to be functionally similar despite the fact that both the LR8 and Lrp13 receptors are present in the ovary of at least the salmonids. These species also express the VtgC as well. Evidence suggests that the vtgAa and vtgAb paralogs of Acanthomorpha arose from an ancestor vtgA after a genome (or gene) duplication event. Gar have two complete vitellogenins and based on both phylogeny and genomic synteny analyses, these correspond to vtgA and vtgB orthologs, and both are likely present in other basal fish groups, such as sturgeons and bichir (Polypteriformes). The vtgB form has thus been lost in more derived fishes. The coelacanth vtgABI gene is flanked by eltd1, a gene adjacent to the vtgC locus in all fish genomes examined to date, thus supporting orthology of coelacanth vtgABI and teleost vtgC. The phosphvitin domain present in coelacanth VtgABI existed in the common ancestor with Actinopterygii, but was subsequently lost in the VtgC of more derived fishes.

Understanding the functions of these multiple vitellogenins is relevant to egg quality, since yolk components not only provide nutrition to embryos and larvae at specific developmental stages, but contribute to egg buoyancy as well and these processes may vary in relation to diverse early life histories and reproductive strategies among the expansive fish taxa.