Abstract
Many of the functions required for formation of the imaginal discs of Drosophila melanogaster larvae, from which adult structures are derived, are disc-specific and not required for formation of other larval tissues. Mutants blocked in disc-specific functions can produce mature viable larvae, indicating that larval development is not dependent on concomitant disc development. Some of the mutant larvae have no detectable discs (discless mutants), and other have small discs that cannot differentiate into adult structures (small disc mutants). From the results of genetic complementation and mapping experiments with such mutants, it was estimated that the genome of D. melanogaster contains around a thousand complementation groups, or about a fifth of all complementation groups in the genome, that are essential specifically for the development of all the discs. The developmental defects in some of the disc mutants appear to involve autonomous functions of the imaginal cells, and in other mutants functions provided by the larval environment to support disc development. This distinction was made on the basis of two functional tests. The purpose of one of the tests was to detect the presence in young embryos of determined imaginal cells capable of differentiating after transplantation to a wild-type host; the other test measured the capacity of early third-instar larvae to act as hosts for the further development of immature discs transplanted from a wild-type donor. Three discless mutants that were tested in this way appeared to be defective in autonomous functions of the imaginal cells, since the embryos contained no imaginal cells that could be detected in the first test, whereas the larvae, although devoid of discs, showed a normal capacity to support disc development in the second test. One of the small disc mutants tested was defective in a larval rather than an imaginal cell function, since the embryos had a normal complement of functional imaginal cells, whereas the larvae were incapable of supporting disc development. Thus, both types of disc-specific functions are essential for normal disc development.