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 th ese initial studies, we observed that decreases in NOR size are far mo re common than increases. Here, we have investigated the inheritance o f NOR size in a strain with an unusually small NOR. We call this strai n SNO for small nuclelous organizer. We found that progeny that inheri t their rDNA from SNO receive either an NOR that is larger than that o f 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 fr om the other parent in the cross. This argues against the idea that th e 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 unco nscious bias by the experimenter against isolating such spores. These results can most easily be explained by a combination of unequal siste r chromatid exchanges in the rDNA, or sister chromatid conversion, cou pled with selection against nuclei harboring small NORs during the pre meiotic phase of the Neurospora life cycle. Other, less conventional, explanations are also possible, such as ''directed'' increase in the t arget NOR without corresponding loss at some other NOR.