Gene conversion alone accounts for more than 90% of recombination events at the am locus of Neurospora crassa

We have used closely flanking molecular markers located ~4 kb distal and 6 kb proximal of the am locus to investigate the incidence of crossover events associated with the generation of prototrophic recombinants in a cross heteroallelic am¹ am⁶. Ninety-three percent of prototrophs were generated by events that did not recombine the molecular markers, indication that simple conversion accounts for the formation of most prototrophs and that associated crossovers are much less frequent (β0.07) than estimated previously using more distant flanking markers. This suggests that conversion and crossing over result from alternate modes of resolution of Holliday junctions then, at least for the am locus of Neurospora, the mode of resolution is strongly biased in favor of retaining the parental association of flanking sequences. Because estimates of the association of conversion and crossing over based on more distant gene markers are similar for yeast and Neurospora (β0.35), our observation may have general significance.