![]() ![]() ![]() ![]() In flies, two proteins, Extradenticle and Homothorax, are co-expressed in the part of the leg closest to the body and they establish an initial proximal-distal axis in the developing limb. Research in the fly Drosophila melanogaster has identified a number of genes and signaling pathways that guide the development of the legs along the three axes of the body: anterior-posterior (head-to-tail), dorsal-ventral (top-to-bottom), and proximal-distal, which runs from the body to the tips of the limbs (reviewed in Estella et al., 2012). Ask a developmental geneticist, however, and the answer will not be so clear. A human leg appears to have little in common with an insect leg, apart from the fact that both are used for walking, so most zoologists think that the limbs of vertebrates and arthropods evolved independently and are not, therefore, homologous structures. ![]()
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