Head Thoracic Abdominal segments segments

The identity of each individual segment is established.

The identity of each individual segment is established.

21.5 In an early Drosophila embryo, the major body axes are established, the number and orientation of the body segments are determined, and the identity of each individual segment is established. Different sets of genes control each of these three stages.

Dorsal protein (as in cells on the dorsal side of the embryo), activates a gene called decapentaplegic, which specifies dorsal structures. In this way, the ventral and dorsal sides of the embryo are determined.

Stages in the early development of fruit flies and the genes that control each stage

Developmental Stage

Establishment of main body axes

Determination of number and polarity of body segments

Establishment of identity of each segment


Egg polarity genes Segmentation genes Homeotic genes

21.6 Dorsal protein in the nuclei helps to determine the dorsal-ventral axis of the Drosophila embryo. (a) Relative concentrations of Dorsal protein in the cytoplasm and nuclei of cells in the early Drosophila embryo. (b) Micrograph of a cross section of the embryo showing the Dorsal protein, darkly stained, in the nuclei along the ventral surface.

Determination of the anterior-posterior axis Establishing the anterior-posterior axis of the embryo is a crucial step in early development. We will consider several genes in this pathway (Table 21.3). One important gene is bicoid, which is first transcribed in the ovary of an adult female during oogenesis. Bicoid mRNA becomes incorporated into the cytoplasm of the egg and, as it is passes into the egg, bicoid mRNA becomes anchored to the anterior end of the egg by part of its 3' end. This anchoring causes bicoid mRNA to become concentrated at the anterior end (Figure 21.7a). (A number of other genes that are active in the ovary are required for proper localization of bicoid mRNA in the egg.) When the egg has been laid, bicoid mRNA is translated into Bicoid protein. Because most of the mRNA is at the anterior end of the egg, Bicoid protein is synthesized there and forms a concentration gradient along the anterior- posterior axis of the embryo, with a high concentration at the anterior end and a low concentration at posterior end. This gradient is maintained by the continuous synthesis of Bicoid protein and its short half-life.

The high concentration of Bicoid protein at the anterior end induces the development of anterior structures such as the head of the fruit fly. Bicoid—like Dorsal—is a morphogen. It stimulates the development of anterior structures by binding to regulatory sequences in the DNA and influencing the expression of other genes. One of the most important of the genes stimulated by Bicoid protein is hunchback, which is required for the development of the head and thoracic structures of the fruit fly.

The development of the anterior-posterior axis is also greatly influenced by a gene called nanos, an egg-polarity

Table 21.2 Key genes that control development of the dorsal-ventral axis in fruit flies and their action

Gene dorsal cactus toll twist decapentaplegic

Where Expressed


Ovary Ovary

Embryo Embryo

Action of Gene Product

Affects expression of genes such as twist and decapentaplegic

Traps Dorsal protein in cytoplasm

Alters Dorsal protein, allowing it to dissociate from Cactus protein and move into nuclei of ventral cells Takes part in development of mesodermal tissues

Takes part in development of gut structures


Some key genes that determine the anterior-posterior axis in fruit ^

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