Hox genes in brachiopods and priapulids and protostome evolution

Understanding the early evolution of animal body plans requires knowledge b oth of metazoan phylogeny and of the genetic and developmental changes invo lved in the emergence of particular forms. Recent 18S ribosomal RNA phyloge nies suggest a three-branched tree for the Bilateria comprising the deutero stomes and two great protostome clades, the lophotrochozoans(1) and ecdysoz oans(2). Here, we show that the complement of Nor genes in critical protost ome phyla reflects these phylogenetic relationships and reveals the early e volution of developmental regulatory potential in bilaterians. We have iden tified Hox genes that are shared by subsets of protostome phyla. These incl ude a diverged pair of posterior (Abdominal-B-like) genes in both a brachio pod and a polychaete annelid, which supports the lophotrochozoan assemblage , and a distinct posterior Hox gene shared by a priapulid, a nematode and t he arthropods, which supports the ecdysozoan dade. The ancestors of each of these two major protostome lineages had a minimum of eight to ten Hox gene s. The major period of Hox gene expansion and diversification thus occurred before the radiation of each of the three great bilaterian clades.