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 pro tein called vitellogenin (Vg) which is stored as the major component o f yolk. Until recently, deduced amino acid sequences were available on ly from vertebrate and nematode Vgs, which proved to be homologous. Th e sequences of several insect Vgs are now known, but early attempts at pairwise alignments with vertebrate and nematode Vgs have been proble matic, leading to conflicting conclusions about how closely insect Vgs are related to the others. Ln this paper we demonstrate that insect V E sequences can be confidently aligned with one another along their en tire lengths and with multiple vertebrate and nematode Vg sequences al ong most of their spans. Although divergence is high, conservation amo ng insect, vertebrate, and nematode Vg sequences is widespread with a preponderance of glycine, proline, and cysteine residues among strictl y conserved amino acids, establishing conclusively that Vgs from the t hree phyla are homologous. Areas of least-certain alignment are primar ily in and around insect and vertebrate polyserine domains which are n ot homologous. Phylogenetic reconstructions of Vgs based on sequence i dentities indicate that the insect lineage is the most diverged and th at the mammalian serum protein, apolipoprotein B-100, arose from a Vg ancestor after the nematode/vertebrate divergence.