In this chapter, we have focused on large-scale as well as the inter- and intracellular events that take place during embryogenesis and the formation of adult structures. In particular, we discussed how the adult body plan is laid down by a cascade of gene expression, and the role of cell-cell communication in development. Based on your knowledge of these topics, answer several fundamental questions: (a) How do we know how many genes control development in an organism like Drosophila? (b) What experimental evidence demonstrates that molecular gradients in the egg control development? (c) How did we discover that selector genes specify which adult structures will be formed by body segments? (d) How did we learn about the levels of gene regulation involved in vulval development in \(C .\) elegans?

To know how many genes control development in an organism like Drosophila, we can refer to the extensive research that has been done on its genome. Scientists have performed various genetic screens to identify the genes involved in the development of Drosophila. These screens often involve inducing mutations and observing the effects on the developing embryo or adult structures. By examining the phenotypes of mutant Drosophila, scientists have been able to identify and classify the genes that control development into different categories, such as segmentation genes, homeotic genes, and others.

The experimental evidence for molecular gradients controlling development comes from studies of various organisms, including Drosophila. In these experiments, scientists have observed the concentration gradients of specific proteins or mRNAs within the developing embryo, which influence the expression of other genes and ultimately dictate cell fates. One well-known example is the Bicoid protein gradient in Drosophila, which acts as a morphogen to control anterior-posterior patterning. Scientists have shown that altering the Bicoid gradient, either by injecting additional protein or by reducing its expression, can change the pattern of genes expressed and the overall body plan of the developing embryo.

The discovery of selector genes, which specify which adult structures will be formed by body segments, came from studies of the homeotic (Hox) genes in Drosophila. These genes were initially identified through mutations that produced homeotic transformations, in which one body part is transformed into another, often strikingly different, body part. Further research on Hox genes revealed that their expression is spatially and temporally regulated and that they act by controlling the expression of downstream target genes responsible for specific adult structures. This led to the understanding that Hox genes play a crucial role in determining segment identity and adult structures during development.

Our understanding of the levels of gene regulation involved in vulval development in Caenorhabditis elegans comes from several approaches. One approach is genetic analysis, where mutants with abnormal vulval development are isolated, and the genes affected by these mutations are identified. This has led to the discovery of complex gene regulatory networks controlling vulval cell fate specification, including the roles of signal transduction pathways (e.g. Ras, Notch) and transcription factors (e.g., LIN-12). Another approach is studying the expression patterns and functions of these genes during embryogenesis and larval development. This has provided valuable insights into the timing and spatial regulation of gene expression in the developing vulva, as well as the molecular mechanisms by which these genes interact to control cell fate decisions.