Extensive sequence conservation among insect, nematode, and vertebrate vitellogenins reveals ancient common ancestry

The eggs of most oviparous animals are provisioned with a class of protein called vitellogenin (Vg) which is stored as the major component of yolk. Until recently, deduced amino acid sequences were available only from vertebrate and nematode Vgs, which proved to be homologous. The sequences of several insect Vgs are now known, but early attempts at pairwise alignments with vertebrate and nematode Vgs have been problematic, leading to conflicting conclusions about how closely insect Vgs are related to the others. In this paper we demonstrate that insect Vg sequences can be confidently aligned with one another along their entire lengths and with multiple vertebrate and nematode Vg sequences along most of their spans. Although divergence is high, conservation among insect, vertebrate, and nematode Vg sequences is widespread with a preponderance of glycine, proline, and cysteine residues among strictly conserved amino acids, establishing conclusively that Vgs from the three phyla are homologous. Areas of least-certain alignment are primarily in and around insect and vertebrate polyserine domains which are not homologous. Phylogenetic reconstructions of Vgs based on sequence identities indicate that the insect lineage is the most diverged and that the mammalian serum protein, apolipoprotein B-100, arose from a Vg ancestor after the nematode/vertebrate divergence.