The Insect Body Plan

The genetic regulatory hierarchies that establish the major features of the Drosophila melanogaster body plan are of interest for two reasons: (i) because they are among the best understood in any animal and illustrate potentially general principles, and (ii) because they provide a basis for comparison between Drosophila and other insects and arthropods. The structure of these regulatory hierarchies is critical to mechanisms of arthropod body plan evolution. The major features of the insect body plan that have both developmental and evolutionary significance include the following:

• Segmentation

• The organization of segments into distinct regions forming the head, thorax, and abdomen

3rd instar (2d)

Figure 3.4

The Drosophila life cycle

In Drosophila, the development of the segmented, motile larva from a fertilized egg takes about one day. After two larval instars, the imaginal discs of the late third larval instar that will give rise to adult tissues are well developed. Morphogenesis and differentiation of adult tissues take place during the pupal stage, before the adult emerges (eclosion).

3rd instar (2d)

Figure 3.4

The Drosophila life cycle

In Drosophila, the development of the segmented, motile larva from a fertilized egg takes about one day. After two larval instars, the imaginal discs of the late third larval instar that will give rise to adult tissues are well developed. Morphogenesis and differentiation of adult tissues take place during the pupal stage, before the adult emerges (eclosion).

• The identities of paired, jointed appendages on different head and thoracic segments

• The limbless abdomen

• The two pairs of dorsal flight appendages

Fruit flies are holometabolous insects, meaning that they undergo a complete metamorphosis between their larval and adult forms during the pupal stage. The larval body plan differs from the adult body plan (for a view of the fly life cycle, see Fig. 3.4). Here we examine regulatory mechanisms involved in both larval and adult patterning.

From egg to segments: the anteroposterior coordinate system

In just one day following fertilization, the Drosophila embryo develops from a single nucleus in a huge yolk-filled egg about 0.5 mm long into a highly organized segmented, motile, feeding larva with a complex nervous system and the future adult tissues growing as imaginal structures within it. Genetic screens identified five tiers of regulatory genes involved in organizing body pattern along the primary anteroposterior (A/P) axis of the developing embryo. As discussed in Chapter 2, the maternal effect, gap, pair-rule, segment polarity, and homeotic genes have large and distinct effects on the patterning of the A/P axis.

Genetic and molecular analyses of the regulatory interactions within and among these five tiers of genes have elucidated both the general logic of and many specific molecular interactions that operate within the A/P patterning hierarchy. These molecular mechanisms, in turn, illustrate several important general concepts about the function and transcriptional regulation of pattern-regulating genes during animal development.

Figure 3.5 shows the basic outline of the A/P axis regulatory hierarchy. The generation of the periodic, segmental organization of the larva from an initially aperiodic egg involves

Maternal

Cad Bcd Nos

Primary pair-rule

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