Hybrid DNA extension and reciprocal exchanges: alternative issues of an early intermediate during meiotic recombination?

Large heterologies in gene b2 strongly increase the frequencies of reciprocal exchanges on their left border, towards the high conversion end. In a previous study, we observed that heterozygous point mutations located in the high conversion end (region F) stimulate the reciprocal exchanges instigated by the large heterology 138. We have defined some properties of this stimulation. The effect does not depend on the nature of the large heterology used. It is effective only with point mutations located on the left side of the large heterology. It does not depend on the number of heterozygosities accumulated in region F. It is not specific on the location of point mutations in region F: it decreases from region F (left end) to region E (middle part of b2). It is correlated with the mismatch correction efficiencies of the point mutations used. It is not observed in the absence of a large heterology. Point mutation heterozygosities which stimulate reciprocal exchanges also decrease the frequency of HDNA formation in gene b2. We propose a model in which reciprocal exchanges on the one hand and hybrid DNA formation on the other hand correspond to alternative processings of a common recombination intermediate.