Amplification of the nucleolus organizer region during the sexual phase of Neurospora crassa.

Previously we have shown that the nucleolus organizer region (NOR) of Neurospora crassa displays frequent size changes during crosses. In these initial studies, we observed that decreases in NOR size are far more common than increases. Here, we have investigated the inheritance of NOR size in a strain with an unusually small NOR. We call this strain SNO for small nucleolus organizer. We found that progeny that inherit their rDNA from SNO receive either an NOR that is larger than that of SNO or, rarely, the same size, but never an NOR that is smaller than that of SNO. The number of progeny that inherit their NOR from SNO is not significantly different from the number that inherit their NOR from the other parent in the cross. This argues against the idea that the failure to find progeny with NORs smaller than that of SNO is due to inviability of spores carrying such an NOR, or that it is due to unconscious bias by the experimenter against isolating such spores. These results can most easily be explained by a combination of unequal sister chromatid exchanges in the rDNA, or sister chromatid conversion, coupled with selection against nuclei harboring small NORs during the premeiotic phase of the Neurospora life cycle. Other, less conventional, explanations are also possible, such as "directed" increase in the target NOR without corresponding loss at some other NOR.