Bilaterian origins: Significance of new experimental observations

Several recent laboratory observations that bear on the origin of the Bilat eria are reviewed and interpreted in light of our set-aside cell theory for bilaterian origins. We first discuss new data concerning the phylogeny of bilaterian phyla. Next, we use systematic, molecular, and paleontological l ines of evidence to argue that the latest common ancestor of echinoderms pl us hemichordates used a maximal indirect mode of development, furthermore, the latest common ancestor of molluscs and annelids was also indirectly dev eloping. finally, we discuss new data on Hox gene expression patterns which suggest that both sea urchins and polychaete annelids use Hox genes in a v ery similar fashion. Neither utilizes the complete Hox complex in the devel opment of the larva per se, while the Hox complex is expressed in the set-a side cells from which the adult body plan is formed. Our current views on t he ancestry of the bilaterians are summarized in phylogenetic terms, incorp orating the characters discussed in this paper. (C) 2000 Academic Press.