Apoptosis in Drosophila

Holometabolous insects, such as the dipteran Drosophila, exhibit two distinct life forms and undergo a complete metamorphosis. The Drosophila embryo develops over the course of one day and hatches into a motile and feeding first instar larva, which grows and subsequently moults twice (Figure 1). The first and second larval instars each last about a day, while the third instar lasts approximately three days. At the end of the third larval instar, the animal ceases to feed, climbs up a suitable substrate, and initiates pupariation. The pupal stage lasts approximately four days, during which time the larva metamorphoses into the adult fly. The ease of rearing Drosophila, their fecundity, and short life cycle have all contributed to their use as a genetic model system (see Ashburner, 1989). In addition, the Drosophila genome is relatively small (∼1.6 X 108 base pairs per haploid genome), and is organised into only four chromosomes which are all linked by a common chromocentre. The large polytene chromosomes of the larval salivary gland greatly facilitate the cytological mapping of chromosomal rearrangements as well as the localisation of cloned genes via in situ hybridisation. In addition, the use of balancer chromosomes, which repress recombination and provide dominant phenotypic markers, greatly facilitates genetic screens and the stable maintenance of homozygous lethal chromosomes.