A mutant, YS17, at the buff spore colour locus in Sordaria brevicollis, when crossed with wild type, gives rise to aberrant asci with a frequency over 10 times that of other buff mutants. Over 98% of the aberrant asci have 6 wild type and 2 mutant spores. From tests with another buff mutant it is concluded that loss of the mutant spore colour when YS17 shows conversion to wild type is associated with loss of the high frequency conversion, and that both characters are caused by the same mutation. A methionine-requiring mutant (met-1) has been obtained that maps 5 units to the left of buff, and this, together with the nicotinamide-requiring mutant (nic-1) 2 units to the right, has provided flanking markers for buff that can be scored with complete reliability. Crosses between YS17 and 28 other buff mutants have revealed close linkage to three of them which map to its right on the basis of flanking marker behaviour, all the others mapping to its left. The frequency of postmeiotic segregation at the sites of buff mutants near to the site of YS17 is greatly increased in the presence of YS17, and occurs in the chromatid showing conversion to wild type at YS17.
From these and other results, obtained largely by ascus analysis, the following conclusions have been drawn.
(1) The YS17 mutation is probably acting as a recognition site for an endonuclease that initiates recombination, with the result that the frequency of heteroduplex DNA within the buff gene is much increased.
(2) The recombination initiated at YS17 is asymmetric (or at least pre-dominantly so), with the YS17 site acting as a recipient of a nucleotide chain from the other parent, not a donor to it.
(3) The frequency of crossing over associated with conversion at YS17 is variable: about 30% in crosses with most of the buff mutants, about half this value in crosses with wild type, and almost zero in crosses with closely-linked buff mutants.
(4) In about one third of the crossover asci in crosses between YS17 and other buff mutants the crossover is not adjacent to the site of YS17 but separated from it by the site of the allele, which shows normal 4:4 segregation.
(5) It seems necessary to revive the idea of more than one recombination event in proximity, a non-crossover conversion event sometimes leading to a second event – a crossover – in the vicinity. It is tentatively suggested that both might be controlled by a single enzyme aggregate.