Regeneration and duplication in imaginal discs

When two complementary fragments of a Drosophila imaginal disc are cultured in adult abdomens before transfer to host larvae for metamorphosis, the usual result is that one of the two fragments regenerates the missing parts while the other fragment duplicates the anlagen already present. To account for this it is proposed that in the disc there is a gradient of developmental capacity, by which new positional information can be generated from a cut surface only in the downward direction in the gradient, irrespective of the physical direction faced by the cut surface. The same kind of behaviour is shown by other epimorphic systems, such as the regenerating appendages of amphibians and of hemmimetabolous insects, and the regenerating body segments of annelids. The regeneration-duplication rule has been shown to apply for thirteen different cuts in the wing disc, some across the proximodistal and others across the anteroposterior axis. In both axes there is a reversal in the direction of regenerative ability at the approximate centre of the disc. As an alternative to the hypothesis of orthogonal double gradients, it is proposed that the wing disc has multiple gradients of developmental capacity which radiate from the centre of the disc. The location of this centre is known, and its properties are being studied.