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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Forming Complex Pattern: Establishing Positional Information Figure 16-18. The signaling pathway that leads to the gradient of nuclear versus cytoplasmic localization of DL protein. (a) A cross section of a Drosophila embryo showing the blastoderm cells inside the plasma membrane and the space (perivitelline space) between the inside boundary of the eggshell (perivitelline membrane) and the plasma membrane, where the SPZ ligand is produced on the ventral side of the embryo. (b) The SPZ ligand binds to the TOLL receptor, activating a signal transduction cascade through two proteins called TUB and PLL, leading to the phosphorylation of DL and its release from CACT. DL then is able to migrate into the nucleus, where it serves as a transcription factor for DV cardinal genes. (Adapted from H. Lodish, D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell, Molecular Cell Biology, 3d ed. Scientific American Books, 1995.)

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