Experimental Studies On Imaginal Discs

The biology of imaginal discs has been described in a number of species but has been experimentally analyzed in only a few. As with other metamorphosing organs, there have been studies on the endocrine control of imaginal disc eversion, differentiation, and cuticle secretion. Other types of experiment, such as ablation or transplantation of portions of imaginal discs, have been used to study aspects of developmental patterning, such as the development of pigmentation patterns in the wings of butterflies and moths. And in a sense, all studies of the evolution of adult Holometabola reflect on the histoblast cells and discs from which portions of those adults are formed.

However, the most extensively studied imaginal discs are those of the fruit fly, Drosophila melanogaster. This species, which has been the subject of intense genetic studies for a century, has become a model for the study of a variety of biological problems. Gaining an understanding of the imaginal discs in this species has been particularly critical because the entire adult epidermis is derived from imaginal discs or disclike structures. Thus, any mutation that alters the external form of the adult D. melanogaster does so by altering the development of these imaginal structures. Thus a large literature has appeared on the genetics, development, and cell biology of these structures and, via comparisons with other drosophilids, a number of evolutionary studies have been published, as well.

The imaginal discs of D. melanogaster are at one extreme in the spectrum of imaginal tissues, and it is not clear to what extent the mechanisms underlying this apparently derived state are shared by other Holometabola. As in other cyclorrhaphan Diptera, the metamorphosis of D. melanogaster is unusual in several respects. First, the cuticle of the last instar is not shed during the formation of the pupa (pupariation), as occurs in most Holometabola. Rather, the larval cuticle is retained and converted into an outer covering, the pupal case. Second, in D. melanogaster the nondisc cells of the larval epidermis are polyploid (containing more than the diploid number of chromosomes), and these die during the early stages of metamorphosis. In many other Holometabola, large portions of the larval epidermis and appendages are retained in the adult. Finally, in D. melanogaster the imaginal disc primordia are formed during embryonic development, rather than during the last instar as they are in some other Holometabola.

In D. melanogaster embryos each imaginal disc primordium contains 10 to 40 cells, which divide during the three instars to form as many as 50,000 cells by late third instar. The arrangement of imaginal discs in the late third instar is shown in Fig. 2. The adult head is derived from a pair of fused eye—antennal discs, as well as pairs of proboscis (labial) and labral (cibarial or clypeolabral) discs. The adult thorax is derived from the three pairs of ventral leg discs and, dorsally, pairs of prothoracic (humeral), wing, and haltere discs. Each of these contributes to the body wall, and also forms the appendage for which it is named. Each segment of the adult abdomen is formed from four pairs of small "histoblast nests" and the genitals from the genital disc.

At the late third instar each imaginal disc consists of a simple epithelial sac, invaginated from the larval epithelium (Fig. 1). One surface of each sac forms the thickened and folded disc epithelium, from which most of the adult structures are derived. The other surface of the sac is the thinner peripodial membrane, and each disc remains connected to the larval epithelium by a long, narrow stalk. During the first few hours of pupariation each disc everts through the stalk, expands, and eventually sutures together with adjacent discs. The peripodial membrane is lost during this process, and the polyploid cells of the larval epidermis are histolysed and replaced. The portions of each disc epithelium that are fated to form the appendages unfold and lengthen: the prospective legs and antennae form long tubes, whereas the prospective

Genital-

Labial Humeral

Clypeolabral

Antenna + eye

Wing

Haltere hedgehog induces decapentaplegic

Genital-

FIGURE 2 Imaginal discs in late third instar D. melanogaster. [From Blair, S. S. (1999). Drosophila imaginal disc development: Patterning the adult fly. In "Development-Genetics, Epigenetics, and Environmental Regulation" (V. E. A. Russo, D. Cove, L. Edgar, R. Jaenisch, and F. Salamini, eds.), p. 348. © Springer-Verlag GmbH & Co. KG, Heidelberg.]

dorsal and ventral surfaces of the wing lengthen and flatten together. The disc cells then secrete the pupal cuticle, which is separated from the epithelial surface shortly thereafter. After a delay during which there is further morphogenesis and differentiation, the adult cuticle is secreted.

Many aspects of the biology of imaginal discs of D. melanogaster have been examined over the years and continue to be the subjects of intense research. These include the hormonal control of disc development and cuticle secretion, the molecular genetics of cell division and cell growth, and various problems in cell biology. Perhaps the most striking advances, however, have been made in developmental patterning by examining for the most part the large wing, leg, and eye—antennal discs. Space constraints prevent the discussion of many fascinating and important research areas, such as the patterning of the compound eye, the formation of sensory organs, and the planar polarity of the disc epithelium. Instead, the following discussion briefly reviews some early developmental events that help establish the identities and axes of the discs.

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