Site specific induction of gene conversion: The effects of homozygosity of the ade6 mutant M26 of Schizosaccharomyces pombe on meiotic gene conversion

Summary

The ade6 mutant M26 of the yeast Schizosaccharomyces pombe uniquely influences both intragenic recombination and gene conversion of markers within its locus. To determine whether the marker effect of M26 is triggered by informational heterozygosity at that site, tetrads from the cross mat1-P mat2-P ade6-M26 Sup9 ura5xmat1-M mat2-O ade6-M26-M210 Sup9 ura5⁺ were analyzed and the conversion of the distal marker M210 scored. In a sample of 1005 tetrads 14 conversion tetrads were observed 8 of which were 3M26: 1M26-M210, 5 of which were 1M26:3M26-M210 and one of which was 2M26:1M26/M26-M210:1M26-M210. Since the 15.5 aberrant asci expected (frequency of an event at M26 X the probability that the M210 site will be included or (0.031+0.031) (0.25) (1005)=15.5) are not different from the 14 observed, the two doses of M26 act in an additive fashion in this cross. These results are consistent with an interpretation that conversion results from a sequence of events involving site specific breakage at M26, degradation of the broken strand, and the availability of an appropriate template which introduces new information as synthesis proceeds. This is congruent with the parity that is observed reflecting the fact intragenic recombination is signaled with equal probability on each of the strands due to the homozygosity of M26 in this cross. In addition, the single p.m.s. tetrad may be explained on the basis that hybrid DNA is formed, although its production is secondary and not a direct consequence of the processes leading to gene conversion. Given these results, it is possible to conclude that this marker effect is a result of the M26 leision itself and is not signaled as a consequence of specialized interactions involving the corresponding wild type nucleotide sequence.