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Antennapedia 2d thoracic

3d thoracic

Ultrabithorax 1st abdominal

Antennapedia 2d thoracic

3d thoracic

Ultrabithorax 1st abdominal for another gene. Subsequently, other (secondary) pair-rule genes, including fushi tarazu (ftz) and paired, become active in response to the Eve, Runt, and Hairy proteins, which are transcription factors, as well as to maternal and gap proteins. The outcome is a complex pattern of overlapping stripes.

In early embryos each segment primordium is about four cells wide along the anterioposterior axis, which corresponds to the approximate width of pair-rule expression stripes. With pair-rule genes active in alternating four-on four-off patterns, the repeat unit is about eight cells. Each cell expresses a combination of transcription factors that can potentially distinguish it from any of the other seven cells in the repeat unit. Complete segmentation of the embryo into repeat units in which each cell exhibits a unique transcriptional pattern depends on activation of a third set of genes, the segment-polarity genes. These genes, which include engrailed (en) and wingless (wg), also are expressed in stripes, but the stripes are narrower and appear once in each segment primordium. Because some segment-polarity genes encode components of cell-cell signaling systems, they are discussed in Section 15.5.

Figure 15-24a schematically depicts the distribution of some key regulators in the Drosophila embryo during the first few hours after fertilization. When all the segmentation genes have been turned on, the resulting single-cell accuracy of cell-fate specification is impressive, as the example in Figure 15-24b illustrates.

M FIGURE 15-24 Summary of sequential, spatially localized expression of selected genes in early development of the Drosophila embryo. (a) Maternal bicoid mRNA is localized at the anterior pole of the egg, but Bicoid protein, which is synthesized soon after fertilization, diffuses to form a gradient in the fly syncytium. In most cases, an mRNA and its corresponding protein are present in the same regions of the embryo. Specific combinations of Bicoid and various gap-gene products, including Hunchback, Kr├╝ppel, and Knirps, control transcription of the pair-rule genes such as fushi tarazu (ftz) and even-skipped (eve). Gap gene products are shown in discrete bands but actually they overlap. The pair-rule proteins demarcate 14 stripes corresponding to the parasegments (an offset form of the segment primordia). The segment-polarity gene engrailed (en) is expressed at the anterior end of each parasegment; it and other segment-polarity genes participate in patterning of each parasegment. Cellularization occurs after 2.5 hours, and gastrulation occurs at about 4.5 hours. By this time, each parasegment consists of four belts of cells. (b) Within a parasegment, each belt of cells (represented by a circle) is characterized by expression of a unique set of proteins encoded by pair-rule and segment-polarity genes. Shown here are the locations of three pair-rule proteins and two segment-polarity proteins in three parasegments (4-6). These expression patterns act as positional values that uniquely characterize each cell belt in a parasegment and determine where Hox genes such as Antennapedia (Antp) and Ultrabithorax (Ubx) are transcribed. Hox genes give the repeating body segments their distinct shapes and appendages (indicated at the bottom). Hox gene expression is regulated by gap, pair-rule, and segment polarity genes.

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