W4 f5

Gooseberry

Mutation of gooseberry causes the posterior half of each segment to be replaced by a mirror image of the anterior half of an adjacent segment.

21.8 Segmentation genes control the differentiation of the Drosophila embryo into individual segments. The gap genes affect large sections of the embryo. The pair-rule genes affect alternate segments. The segment-polarity genes affect the polarity of segments.

these genes cause part of each segment to be deleted and replaced by a mirror image of part or all of an adjacent segment. For example, mutations in the gooseberry gene cause the posterior half of each segment to be replaced by the anterior half of an adjacent segment.

The gap genes, pair-rule genes, and segment-polarity genes act sequentially, affecting progressively smaller regions of the embryo. First, the egg-polarity genes activate or repress the gap genes, which divide the embryo into broad regions. The gap genes, in turn, regulate the pair-rule genes, which affect the development of pairs of segments. Finally, the pair-rule genes influence the segment-polarity genes, which guide the development of individual segments.

Homeotic genes After the segmentation genes have established the number and orientation of the segments, homeotic genes become active and determine the identity of individual segments. Eyes normally arise only on the head segment, whereas legs develop only on the thoracic segments. The products of homeotic genes activate other genes that encode these segment-specific characteristics. Mutations in the homeotic genes cause body parts to appear in the wrong segments.

Homeotic mutations were first identified in 1894, when William Bateson noticed that floral parts of plants occasionally appeared in the wrong place: he found, for example, flowers in which stamens grew in the normal place of petals. In the late 1940s, Edward Lewis began to study homeotic mutations in Drosophila, which caused bizarre rearrangements of body parts. Mutations in the Antennape-dia gene, for example, cause legs to develop on the head of a fly in place of the antenna (I Figure 21.9).

Homeotic genes create addresses for the cells of particular segments, telling the cells where they are within the regions defined by the segmentation genes. When a homeotic gene is mutated, the address is wrong and cells in the segment develop as though they were somewhere else in the embryo.

Homeotic genes are expressed after fertilization and are activated by specific concentrations of the proteins produced by the gap, pair-rule, and segment-polarity genes. The homeotic gene Ultrabithorax (Ubx), for example, is activated when the concentration of Hunchback protein (a product of a gap gene) is within certain values. These concentrations exist only in the middle region of the embryo; so Ubx is expressed only in these segments.

The homeotic genes encode regulatory proteins that bind to DNA; each gene contains a subset of nucleotides, called a homeobox, that are similar in all homeotic genes. The homeobox consists of 180 nucleotides and encodes 60 amino acids that serve as a DNA-binding domain; this domain is related to the helix-turn-helix motif (See Figure 16.2a). Homeoboxes are also present in segmentation genes and other genes that play a role in spatial development.

There are two major clusters of homeotic genes in Drosophila. One cluster, the Antennapedia complex, affects the development of the adult fly's head and anterior thoracic segments. The other cluster consists of the bithorax complex and includes genes that influence the adult fly's posterior thoracic and abdominal segments. Together, the bithorax and Antennapedia genes are termed the homeotic complex (HOM-C). In Drosophila, the bithorax complex contains three

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When the major axes of the fruit-fly embryo have been established, segmentation genes determine the number, orientation, and basic organization of the body segments.

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21.9 The homeotic mutation Antennapedia substitutes legs for the antenna of a fruit fly. (a) Normal, wild-type antenna. (b) Antennapedia mutant.

Antennapedia complex pb Dfd Scr

Chromosome 3

Anterior-

Antennapedia complex pb Dfd Scr

Chromosome 3

labial proboscipedia Deformed

Sex combs Antennapedia reduced

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