a figure from the essay
Fossils, Genes and the Evolution of Animal Limbs
by Neil Shubin, Clif Tabin and Sean Carroll

Figure 6 (from page 118 of Shaking the Tree: Readings from Nature in the History of Life edited by Henry Gee)

A cladogram of selected metazoans shows the distribution of major genes involved with appendage development. Homologous signals are deployed in similar locations in the limb primordia of arthropods (a, Drosophila) and vertebrates (b, chick). Equivalent orientations (dorsal up, anterior left) of a chick left wing bud and a Drosophila left wing imaginal disk are shown. Sonic hedgehog in the chick, and its homologue hedgehog in the fly are produced in a posterior domain. These factors induce the expression of secondary patterning signals in the appendages: overlapping expression of Bmp-2 in the chick and adjacent expression of dpp, the Bmp-2 homologue, in the fly. Dorsal cell fates are specified in both systems by LIM homeodomain transcription factors expressed throughout the dorsal half of the appendage primordia: Lmx-1 in the chick, and apterous in the fly. The outgrowth of both appendages is driven by a specialized group of cells (the AER in the chick and the wing margin in the fly) running along the anteroposterior axis at the junction of the dorsal and ventral compartments. These key groups of cells are specified in both the chick and the fly by the border between dorsal cells expressing the gene fringe and ventral cells not expressing fringe. For simplicity in viewing, conserved genes in signal transduction (such as the hedgehog receptor, patched) and other parallels between the two systems (such as the expression of Wnt genes) are not shown. Distal-less orthologues (Dll in Drosophila and Dlx in the chick) are expressed in a wide variety of animal appendages, including the lobopods of onychophorans, the tube feet of echinoderms, and the wings of birds and flies (light shading). The limbs of these taxa are not homologous as appendages because phylogenetically intermediate groups do not possess comparable structures. This suggests at least two phylogenetic possibilities: either similar genetic circuits were convergently recruited to make the limbs of different taxa, or these signalling and regulatory systems are ancient and patterned a different structure (presumably another type of outgrowth) in the common ancestor of protostomes and deuterostomes.⁹⁹

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About the book: Shaking the Tree: Readings from Nature in the History of Life edited by Henry Gee, published by the University of Chicago Press.