Interactive Fly, Drosophila

EGF receptor

Egfr and the structuring of the abdomen

The adult abdominal epidermis develops during the pupal stage from groups of cells called histoblast nests, which differ from imaginal discs in two important respects: (1) abdominal histoblasts do not invaginate during embryogenesis, but remain part of the larval epidermis and secrete larval cuticle, and (2) they do not proliferate during the larval stages. After pupariation, histoblasts multiply rapidly and migrate to replace the polyploid larval epidermal cells (LEC). As the individual nests grow and merge, LEC are destroyed only upon contact with histoblasts, so that the continuity of the pupal epidermis is maintained at all times. The replacement of LEC by histoblasts is completed by 40-42 hours after puparium formation (APF). The epidermis of each abdominal segment is produced by three bilateral pairs of histoblast nests: the anterior dorsal nests produce the tergite; the posterior dorsal nests form the flexible intertergal cuticle, and the ventral nests produce the sternite and pleura. In addition, a spiracular nest produces a small patch of epidermis around each spiracle. Dorsally, each segment is composed of a sclerotized, pigmented tergite and flexible, unpigmented intertergal cuticle that is normally folded underneath the tergite. All cells in the sternites, pleura and tergites secrete 3-4 trichomes per cell. The wide-based, curved trichomes secreted by pleural cells are distinct from the thinner, straighter sternal and tergal trichomes. In addition, sternites and tergites, but not the pleura, contain arrays of bristles. Dorsoventral patterning is also present within tergites, since the dark pigment band at the posterior edge of each tergite is wider medially than laterally. Some segments deviate from the typical pattern. For example, the first abdominal segment (A1) lacks a sternite. Also, in the male, A7 lacks both a sternite and a tergite, A6 lacks bristles on its sternite, and A5 and A6 have uniformly darkly pigmented tergites (Kopp, 1999 and references).

The adult abdominal epidermis is formed during the first 40-42 hours of pupal development. At pupariation, the abdominal epidermis is composed predominantly of the polyploid LEC, which are easily distinguishable from the much smaller, diploid histoblasts. At this stage, the anterior dorsal histoblast nest (aDHN) contains 13-19 cells; the posterior dorsal nest (pDHN) contains 5-8 cells, and the ventral nest (VHN) contains 9-13 cells. Histoblasts begin to proliferate and migrate to supplant the LEC soon after pupariation. At 18-20 hours APF, the aDHN and pDHN merge to form a single dorsal histoblast nest (DHN). The DHN merges with the VHN and the spiracular anlage between 20 and 28 hours APF. The spiracle, located at the lateral midline, marks the boundary between ventral and dorsal histoblasts, and eventually the boundary between the pleura and the tergite. The fusion of histoblast nests of consecutive segments begins at 28 hours APF and proceeds until 40-42 hours APF, when the formation of a continuous adult epidermis is completed by the fusion of contralateral nests at the dorsal and ventral midlines. Morphological differentiation of the epidermis into sternite, tergite and pleural territories becomes evident shortly thereafter. These regions can be distinguished at 45 hours APF by differences in the shape and arrangement of cells and by the pattern of developing adult muscles (Kopp, 1999).

The division into dorsal tergite, ventral sternite and ventro-lateral pleural cuticle is largely specified during the pupal stage by Wingless, Decapentaplegic and Egf receptor signaling. Expression of wg and dpp is activated at the posterior edge of the anterior compartment by Hedgehog signaling. Within this region, wg and dpp are expressed in domains that are mutually exclusive along the dorso-ventral axis: wg is expressed in the sternite and medio-lateral tergite, whereas dpp expression is confined to the pleura and the dorsal midline. Neither gene is expressed in the lateral tergite. Tergite and sternite cell fates are specified by Wg signaling. Egfr acts synergistically with Wg to promote tergite and sternite identities, and Wg and Egfr activities are opposed by Dpp signaling, which promotes pleural identity. Wg and Dpp interact antagonistically at two levels:(1) their expression is confined to complementary domains by mutual transcriptional repression and (2) Wg and Dpp compete directly with one another by exerting opposite effects on cell fate. Egfr signaling does not affect the expression of wg or dpp, indicating that it interacts with Wg and Dpp at the level of cell fate determination. Within the tergite, the requirements for Wg and Egfr function are roughly complementary: Wg is required mainly in the medial region, whereas Egfr is most important laterally. Dpp signaling at the dorsal midline controls dorso-ventral patterning within the tergite by promoting pigmentation in the medial region (Kopp, 1999).

Ubiquitous expression of a dominant negative form of Egfr (UAS-DN-DER) causes expansion of the pleura at the expense of tergites and sternites. A similar phenotype is caused by a temperature-sensitive combination of DER alleles at 25°C. When DN-DER is coexpressed with DN-dTCF, the entire tergite is transformed to pleura. Conversely, ubiquitous expression of a constitutively active form of Egfr transforms the pleura to tergite and sternite identities. These observations suggest that Egfr signaling functions in concert with Wg to promote tergite and sternite fates, and is particularly important in the lateral tergite region (Kopp, 1999).

The major conclusion of this report is that much of the dorso-ventral (DV) patterning of the adult abdomen is determined by antagonistic interactions between Dpp, which specifies pleural cell fate, and Wg and Egfr signaling, which together specify tergite and sternite fates. Expression of wg and dpp is activated at the posterior edge of the anterior compartment by Hh signaling. Within this zone, wg and dpp are expressed in complementary patterns along the DV axis: wg is expressed in the presumptive sternite and in the medio-lateral region of the tergite, whereas dpp is expressed in the presumptive pleura and at the dorsal midline of the tergite. Although the pattern of Egfr activation in the abdomen has not been determined, Egfr signaling is most important in the lateral tergite, a region where neither wg nor dpp are expressed (Kopp, 1999).

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