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

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Abstract
The developmental behaviour of imaginal discs of Drosophila melanogaster was investigated. The discs were medially halved and transplanted into 55 h old larval hosts to allow a sufficient time for cell proliferation. The different anlagen for the male first leg are asymmetrically distributed on the leg disc. Its median and lateral halves did not differentiate any structures of the complementary half even after intensive growth. The proliferation resulted only in an enlargement or duplication of the already present anlagen. In the symmetrical male genital disc certain anlagen could be eliminated by unilateral local irradiation with UV. This technique allowed the production of asymmetrical discs. After cutting, the non-irradiated half formed a complete and symmetrical genital apparatus. The irradiated half differentiated into a symmetrical apparatus which lacked, however, single elements on both sides. A schematic illustration is given. The results are interpreted to mean that the cell divisions initiated by the fragmentation of the disc always repeat the same state of determination. During a culture period of 3-4 days before metamorphosis a replacement of missing anlagen does not occur. The two terms ‘regeneration’ and ‘regulation’ are discussed with respect to imaginal discs. We have proposed the modified term ‘proliferative regulation’ to describe the special behaviour of imaginal disc fragments. This phenomenon consists of two processes : (1) an intensive cell proliferation whereby the state of determination is conserved; (2) a division of the overgrown blastema into areas of a fixed size (homonomous arealization).

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