15-P004 Anterior specification and head segmentation in the parthenogenetic and viviparous pea aphid Acyrthosiphon pisum: Expression of hunchback, orthodenticle, empty spiracles, and deformed

sequences in molecular phylogenetic trees is consistent with early observations that polychaetes tend to have relatively short branches in such trees, and hence are useful taxa with which to reconstruct gene family evolution. Also it is clear that the traditional molecular model systems for protostomes (e.g. Drosophila melanogaster and Caenorhabditis elegans), which are restricted to the Ecdysozoa, have undergone extensive gene loss during evolution. These ecdysozoan systems, in terms of gene content, are thus more derived from the bilaterian ancestral condition than lophotrochozoan systems like the polychaetes, and are thus less suitable models for deducing bilaterian ancestral states despite the array of powerful genetic and mechanistic manipulation techniques in these ecdysozoans.