Figure 3.

The evolution of animal appendages. The cladogram depicts the relationships between a selected subset of animal phyla and is based upon a combination of sources (see ref. 17 for review; other phylogenies are possible but do not alter the basic inferences drawn here). Branch lengths are not scaled. Taxa for which Dll expression data is presented here are shown in bold along with the appearance of the various appendages in these groups. The appearance of various appendages within the phyla studied here are indicated. Only the jointed limbs of arthropods and tetrapod limbs arose by modification of a pre-existing appendage. The immediate ancestors of polychaetes, Onychophora, echinoderms, urochordates, and vertebrates are not thought to have borne appendages that would be homologous to the structures analyzed here. The deuterostome ancestor (b) may have had one Dll/Dlx gene since cephalochordates have only one (13). However, the presence of two Dlx genes in ascidians (15) leaves open the possibility that at least one Dlx duplication event occurred earlier in the deuterostome lineage. The ancestor of the appendage-bearing protostome clade and the deuterostomes possessed the Dll gene and all of the features of the proposed Urbilaterian (18), and may have borne appendages (a). The Dll gene predates this ancestor and is found in nematodes and expressed in the CNS, which may be an older site of function than body wall outgrowths. There is a report of a possible cnidarian Dll ortholog (19), but the similarity of the cnox3 homeodomain to Drosophila Dll (56%) is not significantly higher than that of nonorthologous homeodomains, and the Dll antibody does not recognize this gene product. We therefore believe the origin of Dll is unresolved.