Figure 21–17 The organization of the four egg-polarity gradient systems in Drosophila. Nanos is a translational repressor that governs the formation of the abdomen. Localized Nanos mrNa is also incorporated into the germ cells as they form at the posterior of the embryo, and Nanos protein is necessary for germ-line development. Bicoid protein is a transcriptional activator that determines the head and thoracic regions. toll and torso are receptor proteins that are distributed all over the membrane but are activated only at the sites indicated by the coloring, through localized exposure to the extracellular ligands Spaetzle (the ligand for toll) and trunk (the ligand for torso). toll activity determines the mesoderm and torso activity determines the formation of terminal structures.
Figure 21–18 The origins of the Drosophila body segments. (a) at  3 hours, the embryo (shown in side view) is at the blastoderm stage and no segmentation is visible, although a fate map can be drawn showing the future segmented regions (color). (B) at 10 hours, all the segments are  clearly defined (t1: first thoracic segment; a1: first abdominal segment). See Movie 21.3. (c) the segments of the Drosophila larva and their correspondence with regions in the embryo. (D) the segments of the Drosophila adult and their correspondence with regions in the embryo.
three Groups of Genes control Drosophila Segmentation along the a-p axis
The body of an insect is divided along its A-P axis into a series of segments. The segments are repetitions of a theme with variations: each segment forms highly specialized structures, but all built according to a similar fundamental plan (Figure 21–18). The gradients of transcription regulators set up along the A-P axis in the early embryo by the egg-polarity genes are the prelude to creation of the segments. These regulators initiate the orderly transcription of segmentation genes, which refine the pattern of gene expression to define the boundaries and ground plan of the individual segments. Segmentation genes are expressed by subsets of cells in the embryo, and their products are the first components that the embryo’s own genome contributes to embryonic development; they are therefore called zygotic-effect genes, to distinguish them from the earlier-acting maternal-effect genes. Mutations in segmentation genes can alter either the number of segments or their basic internal organization.
The segmentation genes fall into three groups according to their mutant phenotypes (Figure 21–19). It is convenient to think of the three groups as acting in sequence, although in reality their functions overlap in time. First to be expressed is a set of at least six gap genes, whose products mark out coarse A-P subdivisions of the embryo. Mutations in a gap gene eliminate one or more groups of adjacent segments: in the mutant Krüppel, for example, the larva lacks eight segments. Next comes a set of eight pair-rule genes. Mutations in these genes cause a series of deletions affecting alternate segments, leaving the embryo with only half as many segments as usual; although all the mutants display this two-segment periodicity, they differ in the precise pattern. Finally, there are at least 10 segment-polarity genes, in which mutations produce a normal number of segments but with a part of each segment deleted and replaced by a mirror-image duplicate of all or part of the rest of the segment.
In parallel with the segmentation process, a further set of genes—the homeotic selector, or Hox, genes—serves to define and preserve the differences between one segment and the next, as we describe shortly.
The phenotypes of the various segmentation mutants suggest that the segmentation genes form a coordinated system that subdivides the embryo progressively into smaller and smaller domains along the A-P axis, each distinguished by a different pattern of gene expression. Molecular genetics has helped to reveal how this system works.
a hierarchy of Gene regulatory Interactions Subdivides the Drosophila embryo