Developmental behaviour of fragments of symmetrical and asymmetrical imaginal discs of Drosophila melanogaster (Diptera).

Earlier experiments have demonstrated a very high 'regulative' ability in the genital disc of Drosophila melanogaster. Medially sectioned discs, even from mature larvae, are able to form a fully differentiated normal genital apparatus if the fragments are cultured in vivo in a larval host for a sufficient length of time before metamorphosis. Even a quarter of a disc has this 'regulative' ability. However, it should be emphasized that the fragment as a unit is not capable of 'regulation', but only its single anlage elements (Hadorn, Bertani & Gallera, 1949; Hadorn, 1963; Ursprung, 1959). These findings appeared to contradict the prevailing belief that Drosophila exhibits a mosaic development. This view was favoured by Geigy (1931), who showed that an anlage pattern of presumptive imaginal structures was already present at an early stage of embryogenesis. Similarly, mosaic development in the leg disc was postulated by Bodenstein (1941), who found, for example, that when half of a leg disc formed the sex comb the complementary half did not. The unpaired genital disc is a bilaterally symmetrical anlage whose left and right halves contain the same presumptive material (Text-fig, la). In contrast, in the paired leg discs, the anlage material is distributed asymmetrically (Textfig. 2). Thus, medial sectioning of the genital disc results in two equal halves, while any two halves of the leg disc are unequal with respect to their anlage material. These facts have led us to ask the following questions. (1) Is it possible to induce regulation and/or regeneration in fragments of young leg discs when transplanted into young larval hosts? (2) What is the behaviour of half of a genital disc which has been made experimentally asymmetrical with respect to one of its anlagen when it has sufficient time for 'regulative' growth?