Two mechanisms for directional gene conversion.

G234 is a large silent deletion located in the middle of gene b2, which controls spore pigmentation in Ascobolus immersus. Its gene conversion directionality was studied in asci, which show evidence of heteroduplex DNA at flanking markers, and was compared to the behavior of closely linked single-base-pair insertions or deletions. We found that with the G234 deletion, the genotype of the donor strand in the heteroduplex is preferentially recovered, irrespective of its G234 or wild-type nature, whereas with single-base-pair insertions or deletions, the direction of conversion favors one genotype, whether it was the donor or the recipient strand. We conclude that there exists two mechanisms for directional gene conversion, the "donor-directed" conversion mechanism being epistatic to the "genotype-directed" one. We discuss these data with regard to models for mismatch repair.