MAGNIFICATION IN DROSOPHILA - EVIDENCE FOR AN INDUCIBLE RDNA-SPECIFICRECOMBINATION SYSTEM

An experiment is described that provides evidence for an exchange mech anism to explain the increase in ribosomal gene number that occurs dur ing bobbed magnification. We show that bobbed and bobbed-lethal allele s do not magnify in closed X chromosomes, but that a spontaneous ring opening restores normal magnification. The results provide strong evid ence that the elementary magnifying event is unequal sister chromatid exchange, and can be interpreted in the framework of an inducible rDNA -specific recombination system as the basis of ribosomal gene magnific ation.