Expansion and contraction of the nucleolus organizer region of Neurospora: changes originate in both proximal and distal segments.

Previously we have shown that the nucleolus organizer region (NOR) of Neurospora crassa changes size frequently during the premeiotic portion of the sexual phase. Here, we have investigated whether these changes in size originate only in specific regions of the NOR, or are distributed throughout the NOR. In two special strains of Neurospora, the NOR was divided into proximal and distal segments. In the first, the NOR was divided by a translocation breakpoint and, in the second, the NOR was divided by a meiotic crossover point. The two strains were crossed individually to normal sequence tester strains and the sizes of the proximal and distal segments were followed by pulsed-field gel electrophoresis. The analysis of progeny from both crosses indicates that the events affecting NOR size are not limited to a specific region of the NOR. Additionally, we have obtained evidence that the rDNA of N. crassa can undergo unequal sister chromatid exchange.