FIGURE 6-1D The development of termini in Drosophila (Nüsslein-Volhard 1991). The Torso-like protein is present in the egg shell at the two ends of the egg, deposited there during oogenesis. After fertilization, the egg secretes several proteins into the space between the plasma membrane and egg shell. The proteins include proteases that are locally activated at the end by the Torso-like protein and release a ligand that binds locally to the transmembrane Torso receptor, a member of the RTK signal transduction family. The activated receptor locally activates Raf and MAPK, which phosphorylate a transcription factor locally, inhibiting its repression of genes and allowing local expression of the Tailless (Tll) and Huckebein (Hkb) genes involved in formation of the endoderm, terminal ectoderm, and gut involution during gastrulation. Thus, the Torso-like protein is involved in endoderm induction.

Hox Genes and the Drosophila Connection to Vertebrate Development

Even though researchers in other areas widely appreciated the breakthroughs in Drosophila development, they questioned the relevance of the information to vertebrate development. Vertebrates, as chordates, were thought to have branched from arthropods long ago and last shared a very simple common ancestor in the pre-Cambrian era (about 540 million years ago). The two groups were thought to have evolved their segmentation and heads independently. One of the first significant similarities between vertebrate and fly development came from work on homeotic genes, now called Hox genes. As mentioned before, the Hox genes are expressed in eight broad bands or spatial compartments in the anteroposterior dimension of the body shortly after gastrulation but prior to organogenesis and cytodifferentiation. Their encoded products make each spatial compartment different from the others.

The study of the eight Hox genes of Drosophila was primarily pioneered by E. Lewis from 1940 to 1970. For his work in that area, he shared the Nobel Prize



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