GENETIC AND MOLECULAR ANALYSIS OF TERMINAL DELETIONS OF CHROMOSOME 3ROF DROSOPHILA-MELANOGASTER

Terminal deletions of chromosome 3R are induced at a high frequency (3 .2 x 10(-3)) by irradiating 45-4 Drosophila melanogaster females with a low dose of X-rays. The 45-4 line carries a white transgene inserted at 16 kb from the terminus and is homozygous for the mu-2 mutation, a gene involved in the repair of double-strand DNA breaks. Four of the 51 recovered deleted strains have lost module, the distalmost essentia l gene on chromosome 3R. Breakpoints of 22 deletions have been localis ed in a single hybridisation step, using pulsed-field gel electrophore sis to separate genomic DNA fragments obtained from digestion with a r are-cutter restriction enzyme. Breaks do not occur at random, but are rather clustered in three susceptible chromosomal domains. Backcross e xperiments resulting in transheterozygous (deleted chromosome/45-4) an imals indicate that the activity of the white transgene is enhanced wh en the DNA break has occurred proximal to a critical position. This su ggests that homologous chromosomal pairing distal to the critical posi tion results in the definition of a more compact chromatin structure a nd, due to position effect, in the silencing white,