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▲ FIGURE 15-21 Role of Nanos protein in excluding maternally derived Hunchback (Hb) protein from the posterior region of Drosophila embryos. (a) Both nanos (red) and hunchback (hb) (blue) mRNAs derived from the mother are distributed uniformly in the fertilized egg and early embryo. Nanos protein, which is produced only in the posterior region, subsequently inhibits translation of maternal hb (blue) mRNA posteriorly. (b) Diffusion of Nanos protein from its site of synthesis in the posterior region establishes a posterior ^ anterior Nanos gradient. A complex of Nanos and two other proteins inhibits translation of maternal hb mRNA. As a consequence, maternally derived Hb protein is expressed in a graded fashion that parallels and reinforces the Hb protein gradient resulting from Bicoid-controlled transcription of the embryo's hb gene (see Figure 15-20). [See C. Wreden et al., 1997, Development 124:3015.]

not only to DNA to promote transcription of the embryonic hunchback gene but also to caudal mRNA (which encodes another Drosophila protein having a role in early-patterning events) and regulates its translation.

Toll-like Signaling Activates an Ancient Defense System in Plants and Animals

Before continuing our account of signaling and gene control in development, we digress briefly to consider the connection between innate immunity and the Toll-Dorsal signaling pathway discussed previously (see Figure 15-17c). Recall that this pathway has many parallels with the NF-kB pathway in mammals, which is intimately involved in immune responses. Discovery of these parallels was the first hint that Toll signaling might function in nondevelopmental contexts. More recently, researchers have found that the Toll receptor, its lig-and Spätzle, and other pathway components are required for the expression of an antifungal peptide (drosomycin) in fly larvae and adults. Stimulation of a different Toll-like receptor triggers the production of an antibacterial peptide (diptericin) in flies. Discovery of a mammalian Toll-like receptor that controls the production of anti-inflammatory cy-tokines stimulated exploration of the whole set of similarities.

Toll signaling now appears to be one of the most evolu-tionarily conserved processes known. All the components between Toll and the activation of Dorsal have been largely conserved. There are 8 Drosophila proteins related to Toll plus Toll itself and 10 Toll-related human proteins that control the production of a wide variety of antimicrobial pep-tides in flies and cytokines in mammals. These molecules provide a rapid, nonspecific defense against infection by a wide array of pathogens. The adaptive immune response mounted by vertebrates, involving antibodies and T cells, is directed against specific pathogens but is slower to develop than the innate, nonspecific response.

Most remarkably, parts of the Toll pathway and its function in immunity are readily recognizable in plants. For instance, Arabidopsis has about 100 proteins containing domains similar to the cytosolic domains of Toll that transduce the intracellular signal. At least some of these proteins are required for resistance to tobacco mosaic virus, and an Arabidopsis protein similar in sequence to I-kB is required for resistance to downy mildew fungus. However, the Tolllike signaling in Arabidopsis appears to act through transcription factors that are unrelated to Dorsal or NF-kB.

Because the Toll-based innate immunity system appears to be present in both plants and animals, it may be more than a billion years old. In the course of their long evolution, at least some of the genes having roles in the basic survival function of immunity have been adapted to serve as developmental regulators—a nice example of biological parsimony in the use of genetic resources.

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