The evolution of diverse animal forms followed the radiation of bilaterian phyla in the Cambrian. The toolkit for animal development expanded in the stem lineage of bilaterians and was largely present in Urbilateria. The ecological forces of the Cambrian may have facilitated the evolution of animal morphology using an established set of developmental regulatory genes and networks to build diverse body parts and body plans.
climate change and predation. The inferred presence of ancestral patterning mechanisms and differentiated cell types, even in a simple or primitive form, demonstrates that the potential for much greater morphological complexity and diversity was largely in place in Urbilateria and was subsequently released by environmental and ecological forces (Fig. 4.13).
The revelation of developmental regulatory similarities between long-diverged bilaterians has complicated the assessment of the evolutionary relationships between all of the structures that deploy a shared regulatory gene. For example, it was once believed that "eyes" had evolved independently in arthropods, molluscs, and chordates, based upon morphological and phylogenetic considerations. The discovery of the role of Pax6 (and other genes in the circuit) in the development of all sorts of eyes suggests that eyes did not, in developmental genetic terms, evolve repeatedly "from scratch." Regulatory circuits made up of the same sets of genes are not likely to have been constructed independently by chance, gene by gene, out of the entire repertoire of hundreds of transcription factors in the toolkit.
Expression of Dll in bilaterian phyla
Dll protein is expressed in body wall outgrowths of many bilaterian phyla, including representatives from all three major bilaterian clades (arthropods + onychophora, lophotrochozoans, and deuterostomes). (a) Dll expression (green) in an onychophoran embryo marks the head appendages and the lobopod walking legs (inset). (b) In a polychaete (annelid) embryo, Dll is expressed in the growing parapodia (inset) and other appendages. (c) Dll is expressed in cells at the distal tip of tube feet and spines of a newly metamorphosed sea urchin (echinoderm). Dll expression marks the distal tip of proximo-distal outgrowths in these phyla, suggesting that a P/D axis was defined in Urbilateria and was marked by Dll.
Source: Panganiban G, Irvine SM, Lowe C, et al. Proc Natl Acad Sci USA 1997; 94: 5162-5166.
In another example, the deployment of Dll during the development of body wall outgrowths in many bilaterian phyla (Fig. 4.14), as well as the function of hth/Meis and Exd/pbx genes in proximodistal limb patterning in arthropods and vertebrates, is not likely to have evolved de novo in each lineage. This similarity, however, does not mean that all bilaterian limbs are homologous, indeed, they are not. All bilaterian limbs did not evolve directly from an Urbilatarian appendage. Rather, they may be considered to be developmental "paralogs" of one another, products of the novel deployment and modification of an ancient and shared regulatory circuit in many different animal lineages over the course of animal evolution.
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