Figure 1. Hox cluster evolution. The genes are classified into four groups: ‘anterior’ (green), ‘zen’ (yellow), ‘central’ (blue) and ‘posterior’ (red), based upon sequence comparisons within the conserved (homeobox) region. Many taxa have gained hox genes by duplication of either single genes or whole clusters; others have lost hox genes. The clusters shown include mouse (vertebrate), sea urchin (echinoderm), Drosophila melanogaster (arthropod), Caenorhabditis elegans (nematode), Helobdella triserialis (annelid), Eleutheria hydrozoa (cnidarian). The Urbilateria cluster is hypothetical, and is based upon the range of orthologues detected in descendants. Empty boxes indicate hox genes whose existence is expected but not proven. Modified from Finnerty and Martindale (1998) The evolution of the Hox cluster: insights from outgroups. Current Opinion in Genetics and Development 8: 681–687.

Figure 2. The conserved dorsal–ventral patterning system of arthropods and vertebrates. Idealized body plans of (a) an arthropod (protostome), (b) a vertebrate (deuterostome). In the deuterostome, axial inversion has been accompanied by the development of a new mouth (deutero, second; stomo, mouth). (c) Fate map of Drosophila cellular blastoderm, viewed in transverse section, and showing the location of dpp and sog expression. During subsequent gastrulation, the mesoderm invaginates, bringing the two lateral bands of neurogenic ectoderm into ventral apposition. (d) Fate map of Xenopus blastula seen in a lateral view of the surface. The distribution of Bmp-4 and chd transcripts changes as gastrulation proceeds, and that shown here corresponds with an early stage. During gastrulation, the mesoderm and endoderm layers invaginate through the blastopore so that the whole surface becomes ectodermal. Modified from Wolpert L (1998) Principles of Development, Current Biology Ltd., Oxford University Press.

Figure 3. Hypothetical scenarios showing possible evolutionary histories of developmentally regulated genes, their expression patterns, their developmental mechanisms, and the structures to which they give rise. These four traits are shown mapped on to phylogenetic trees. In most cases (b–e), the genes are more ancient than the anatomical structures that become dependent upon them. Modified from Abouheif 1997.