ECTOPIC MITOTIC RECOMBINATION IN DROSOPHILA PROBED WITH BACTERIAL BETA-GALACTOSIDASE GENE-BASED REPORTER TRANSGENES

Plasmids were constructed to investigate homologous mitotic recombinat ion in Drosophila cells. Heteroalleles containing truncated but overla pping segments of the bacterial beta-galactosidase gene (lacZ) were po sitioned either on separate plasmids or as direct repeats on the same chromosome. Recombination reconstituted a functional lacZ gene leading to expression of LacZ(+) activity detectable by histochemical stainin g. High extrachromosomal recombination (ECR) frequencies between unlin ked heteroalleles were observed upon transient co-transfection into Dr osophila melanogaster Schneider line 2 (S2) cells. Stably transfected cells containing the lacZ heteroalleles linked on a chromosome exhibit ed intrachromosomal recombination (ICR) frequencies two orders of magn itude lower than ECR frequencies. Recombination was inducible by expos ing the cells to ethyl methanesulphonate or mitomycin C. Recombination products were characterized by multiplex PCR analysis and unequal sis ter chromatid recombination was found as the predominant mechanism rec onstituting the lacZ gene. To investigate recombination in vivo imagin al disc cells from transgenic larvae carrying the reporter gene on the X chromosome were isolated and stained for LacZ(+) activity. The pres ence of a few LacZ(+) clones indicated that mitotic recombination even ts occurred at frequencies two orders of magnitude lower than the corr esponding event in cultured cells and late during larval development.