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| Modern Genetic Analysis Chapter 16. The Genetic Basis of Development Additional Aspects of Pattern Formation Figure 16-29. The production of the C. elegans vulva from the equivalence group by cellcell interactions. (a) The parts of the vulva anatomy occupied by the descendants of the equivalence group cells. (b) The primary, secondary, and tertiary cell types are distinguished by the cell division patterns that they undergo. (c) Early in development, there is no signal from the anchor cell, and all the equivalence group cells are in the default tertiary cell state. (d) Later in development, the anchor cell sends a signal that activates a receptor tyrosine kinase signal transduction cascade. The equivalence group cell nearest the anchor cell receives the strongest signal and becomes the primary vulva cell. It then sends out lateral inhibition signals to its neighbors, preventing them from also becoming primary vulva cells and shunting them into the secondary vulva cell pathway. (Part b from R. Horvitz and P. Sternberg, Nature 351, 1991, 357. Parts a, c, and d adapted from I. Greenwald, Trends in Genetics 7, 1991, 366.)

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