FACTIBILIDAD DE CONSTRUCCIÓN

The structural development of the Drosophila head capsule has been extensively studied. An appreciation of this research was essential to my investigation because the development of the stalk-eyed fly was largely unknown.

Drosophila imaginai discs

The larva of the holometabolus insect, D. melanogaster, undergoes complete

metamorphosis during pupation to form the adult fly or imago, which bears little resemblance to the larva. The adult structures of D. melanogaster are derived from sets of cells known as imaginai discs, which are set-aside during embryogenesis. The legs, wings, halteres, genitaha and head region of the imago are all derived from imaginai discs. The epidermis of the adult head is formed by the fusion of the clypeolabral (Gehring and Seippel 1967), labial (Wildermuth and Hadom 1965) and eye-antennal discs (Haynie and Bryant 1986). The dorsal head region is derived from the eye-antennal disc and the description, which follows therefore centres on this disc (Haynie and Bryant 1986).

Embryonic origins of the Drosophila eye-antennal disc

Enhancer-trap hnes, with p-galactosidase expression in the presumptive imaginai discs (1(2)4B7, B itxeta l. 1989, Hartenstein and Jan 1992) were used to track the movement of the disc primordia during Drosophila embryogenesis (Younossi-Hartenstein et al. 1993). Initially, the primordia of the eye-antennal discs make up two long and narrow stripes of

70-80 cells along a depression in the dorsal embryonic head region, which eventually forms the dorsal pouch. Late in embryogenesis, the dorsal pouch shortens and the eye- antennal disc primordia condense into two small clusters of cells, which invaginate from the embryonic epidermis to form a pair of imaginai discs.

Morphology of the Drosophila eye-antennal disc

The eye-antennal discs grow mitotically throughout larval development (reviewed Postlewaite 1978). As they develop, the discs increase in size and fold into a morphologically complex three-dimensional structure. One day old larvae have oval­ shaped discs but by the second day of development distinct anterior and posterior regions can be distinguished either side of a constriction within the disc (Bodenstein 1950) (Fig. 1.6). The mature disc has two sides: the disc proper, which is a thick and folded columnar epithehum, and the peripodal membrane on the opposite side of the disc, which is a thin and unfolded layer of squamous or cuboidal cells (Milner et al. 1983). This epithelial sack surrounds a cavity known as the disc lumen. The narrow anterior disc region is attached to the cephalopharyngeal skeleton and the broader posterior disc region is linked to the brain. The discs are also joined to one other via a thin layer of squamous cells known as the

interantennal connection (Madavan and Schneidermann 1977, Milner et al. 1984).

Fate map of the Drosophila eye-antennal disc

The primordia of adult structures are located in fixed positions within imaginai discs and consequently a map of adult precursors may be derived. A detailed fate map of the mature eye-antennal disc showing the location of the primordia, which will form adult head structures, was established via in vivo culture (Haynie and Bryant 1986) (Fig. 1.6). The disc was cut into multiple overlapping sections by a combination of anterior-posterior, medial-lateral and circular incisions. This donor tissue was cultured in host larvae and examined for the presence of cuticle and cuticular structures following metamorphosis, which would identify the adult head region it had formedjThese data were then used to deduce a fate map with the precursors of adult structures located in the disc proper.

peripodial membrane and disc folds (Fig. 1.6), The adult head is derived from a pair of eye-antennal discs therefore only one half of the adult structures were formed from a single disc.

In summary, the anterior region of the disc proper contains the primordia of the antenna and palpus (Fig. 1.6). Nearly concentric rings of tissue within this region give rise to each of the antennal segments with the primordia of the most distal structure located centrally. The posterior region of the disc proper includes the eye and head capsule primordia. The primordia of the ventral head were located in the posterior ventral flap whilst precursors of the dorsal head were found dorsally in the posterior portion. It was originally believed that the peripodial membrane did not contribute to the formation of adult structures and instead disintegrated during pupation (Poodry and Schneiderman 1970). However, Haynie and Bryant (1986) showed that the membrane derives elements of the head capsule. The spatial relationship between adjacent adult structures, rather than direct observation, was used to designate primordia to the peripodial membrane. Cross sections of the disc also revealed that there were sufficient cells in these areas to give rise to head capsule structures, due to a thickening of the peripodial membrane.

The fate map derived by Haynie and Bryant (1986) was extended during a detailed study of the dorsal head vertex (Royet and Finkelstein 1995). Lines with P-galactosidase expression in sensory precursor cells (A 101, Huang et al. 1991) and the primordia of photoreceptors (LI, Mozer and Benzer 1993) were used to identify the precise location of ocelli and bristle precursors in the dorsal head primordium and confirmed the crude map positions produced by Haynie and Bryant (1986) (Royet and Finkelstein 1995). Neither the physical nor the molecular map provided direct evidence for the location of the frons precursors. Instead, they were assumed to lie in between the precursors of the ocelli and orbital bristles, in the same way that they are arranged in the adult head (Royet and Finkelstein 1995).

The enhancer-trap line, which marks the ocelh precursors (LI, Mozer and Benzer 1993), was also used to track the morphogenetic movements of the eye-antennal disc during larval development (Royet and Finkelstein 1995). In early to mid-third instar larvae, ocelh precursors were present at the edge of the dorsal side of the posterior disc portion. At mid- third instar the dorsal side of the posterior portion folded inwards forming the dorsal lateral flap and the ocelh precursors were consequently displaced with the flap edge. By the late third instar, the flap unfolded and the ocelh primordia were relocated to their original position.

The stalk-eyed fly eye-antennal disc

Prior to my investigations, the embryonic origins and fate map of the stalk-eyed fly eye- antennal disc were unknown. However, the morphology of the stalk-eyed fly eye-antennal disc was recently described during a structural comparison of Drosophila and C. whitei using sections of staged pupae (Buschbeck et al. 2001). This study primarily focused on changes in neural development as a consequence of the evolution of eyestalks. However, the stalk-eyed fly eye-antennal disc was briefly described prior to eversion. These authors reported that the posterior disc portion of the stalk-eyed fly is larger than the same region in Drosophila, which probably relates to the greater number of ommatidia in stalk-eyed flies (Buschbeck et al. 2001). Furthermore, it was also observed that the anterior disc portion lies closer to the posterior disc portion in C. whitei than D. melanogaster (Buschbeck et al. 2001).