Box 61 Molecular insights into insect development

The formation of segments in the early embryo of Drosophila is understood better than almost any other complex developmental process. Segmentation is controlled by a hierarchy of proteins known as transcription factors, which bind to DNA and act to enhance or repress the production of specific messages. In the absence of a message, the protein for which it codes is not produced; thus ultimately transcription factors act as molecular switches, turning on and off the production of specific proteins. In addition to controlling genes below them in the hierarchy, many transcription factors also act on other genes at the same level, as well as regulating their own concentrations. Mechanisms and processes observed in Drosophila have much wider relevance, including to vertebrate development, and information obtained from Drosophila has provided the key to cloning many human genes. However, we know Drosophila to be a highly derived fly, and it may not be a suitable model from which to derive generalities about insect development.

During oogenesis (section 6.2.1) in Drosophila, the anterior-posterior and dorsal-ventral axes are established by localization of maternal messenger RNAs (mRNAs) or proteins at specific positions within the egg. For example, the mRNAs from the bicoid (bcd) and nanos genes become localized at anterior and posterior ends of the egg, respectively. At oviposition, these messages are translated and proteins are produced that establish concentration gradients by diffusion from each end of the egg. These protein gradients differentially activate or inhibit zygotic genes lower in the segmentation hierarchy - as in the upper figure (after Nagy 1998), with zygotic gene hierarchy on the left and representative genes on the right - as a result of their differential thresholds of action. The first class of zygotic genes to be activated is the gap genes, for example Kruppel (Kr), which divide the embryo into broad, slightly overlapping zones from anterior to posterior. The maternal and gap proteins establish a complex of overlapping protein gradients that provide a chemical framework that controls the periodic (alternate segmental) expression of the pair-rule genes. For example, the pair-rule protein hairy is expressed in seven stripes along the length of the embryo while it is still in the syncytial stage. The pair-rule proteins, in addition to the proteins produced by genes higher in the hierarchy, then act to regulate the segment polarity genes, which are expressed with segmental periodicity and represent the final step in the determination of segmentation. Because there are many members of the various classes of segmentation genes, each row of

cells in the anterior-posterior axis must contain a unique combination and concentration of the transcription factors that inform cells of their position along the anterior-posterior axis.

Once the segmentation process is complete each developing segment is given its unique identity by the homeotic genes. Although these genes were first discovered in Drosophila it has since been established that they are very ancient, and a more or less complete

Molecular insights into insect development

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