Chapter  14:  Gene Control in Development

Different proteins are expressed in different cells in the developing spinal cord. The Hedgehog protein (yellow) is an extracellular signal specifically expressed in the ventral-most region of the spinal cord, called the floor plate (see Chapter 23). Hedgehog controls the identity of different neuronal precursor cells in the ventral spinal cord. The more dorsal population expresses the homeobox protein Pax-6 (green), while the more ventral population expresses the homeobox protein Nkx2.2 (red). Hedgehog represses Pax-6 and induces Nkx2.2 in the ventral-most progenitor cells. Motoneurons are derived from both populations of progenitor cells and express IsI1 (blue), another homeobox transcription factor. [See J. Ericson et al., 1997, Cell 90: 169 – 180; photograph courtesy of T. M. Jessell.]

The ultimate goal of developmental biologists is to unravel the mystery of how a fertilized egg is transformed into a complex multicellular organism. This process requires execution of a complex developmental program whereby specific genes are activated in a precise time sequence and in the correct location, generating different types of tissues and the specific cell types composing them. Classical and molecular geneticists have discovered numerous genes that participate in the highly regulated programs that result in the development of plants and animals. Understanding the molecular basis for the action of such genes is one of the most actively studied areas in all of biology.

In Chapters 10 and 11, we examined various mechanisms for regulating gene expression. As a result of these controls, cells can respond to changes in their environment and different cell types can produce characteristic proteins. By far the most prominent mechanism for regulating the production of different proteins in different cells entails a vast array of DNA-binding proteins that act, often in various combinations, to either activate or repress gene transcription.

In this chapter, we take a closer look at the spatial and temporal control of gene expression during development. Genetic and molecular studies show that different cells express different sets of genes based on their developmental history, their patterns of cell division, their position in the developing organism, and their interactions with other cells. We focus on several well-studied cases of differential gene transcription to specify different cell types in yeast, animals, and plants to illustrate the general transcription-control strategies that regulate development. The interrelationships between transcriptional programs in development and signaling between cells are considered in Chapter 23.