7) Developmental Pathways are evolutionarily conserved (by Rey J. Rosa Morales)

      Do you know that the genes that regulate morphogenesis function in hierarchies or networks is termed developmental pathways or developmental circuits? These genes encode signaling proteins that relay molecular signals between cells, transcription factors, which respond to signaling pathways by increasing (up-regulating) or decreasing (down-regulating) transcription at target genes, and structural genes, which encode the proteins that actually do the work of development and physiology (e.g., enzymes and cytoskeletal proteins). Several developmental pathways that control the formation of major organs or appendages seem to be largely controlled by highly conserved transcription factors. The Distalless gene, for example, encodes a transcription factor that governs the development of body outgrowths that differentiate into very diverse structures in different phyla.

     A famous example of such a gene is eyeless, originally discovered in a classic Drosophila mutant with greatly reduced or missing eyes. Mutations in the mammalian homologue of eyeless, which is called Pax6, also cause reduction of the eyes. Pax6 eyeless activates the transcription of a hierarchy of regulatory proteins that control the development and differentiation of the eye. Expression of Pax6 eyeless is localized to the developing eye in embryos (see Figure 1). Amazingly, when researchers genetically engineered Drosophila to express eyeless in various parts of the body where it is not normally expressed (ectopic expression), they discovered that this gene was sufficient to induce the development of ectopic eyes at these positions (see Figure 2). Even more astounding is the functional conservation of the Pax6/eyeless gene between vertebrates and invertebrates: mouse and squid Pax6 genes can induce ectopic eyes when expressed in Drosophila. 

Figure 1. Pax6/eyeless gene in embryo. When the gene is present, it develops normally (upper panel). (low panel) Non-functional or altered Pax6/eyeless caused partial absence of the eyes. (Photo courtesy of learn.genetics.utah.edu)

Figure 2. Ectopic expression of Pax6/eyeless in Drosophila melanogaster. (upper panel) Red colored images shows the eyes at the antenna base. (low panel) Scanning electron microscope (SEM). At the Right low panel, a zoom of showing the morphology as the large, normal compund eye nearby. (Images courtesy of W. G. Gehrin and Nadean Brown).    

References:

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2. Oliver, G., A. Mailhos, R. Wehr, ; N. G. Copeland; A. Jenkins, and P. Gruss. 1995. Six3, a murine homolog of the sine oculis gene, demarcates the most anterior border of the developing neural plate and is expressed during eye development. Development. 121: 1045-l055.
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4. Xu, Z. P., Y. Woo, H. Her, D. R. Beier, and R. l. Maas. 1997. Mouse Eya homologues of the Drosophila eyes absent gene require Pax6 for expression in lens and nasal placode. Development ]24: 219-231.