Evidence for meiotic recombination in Ascobolus involving only one member of a tetrad.

HE models for genetic recombination which were designed by WHITEHOUSE T(1963) and HOLLIDAY (1 964) differ in some details, but both include a stage at which two chromatids of the tetrad carry corresponding segments of hybrid DNA (a region in which the two complementary strands come from different chromatids). The models were designed to account for both reciprocal and nonreciprocal recombination in crosses between allelic mutants. (Reciprocal recombination is defined here as intragenic recombination in which all marked sites in the involved gene segregate 2:2; if one or more sites shows 3:l segregation, it is called nonreciprocal recombination.) The models’ predictions about relative frequencies of these two results, among those tetrads which produced nonmutant recombinants, depended on the distance separating the mutant sites in comparison with the length of a hybrid DNA segment. The maximum proportion of nonreciprocal events should have been observed when the two mutant sites were close together. If they were close enough so that one or both usually fell in the hybrid DNA segment when recombination occurred between them, the models predicted similar frequencies of reciprocal and nonreciprocal events (see Figure 1 ) . However, a proportion of nonreciprocal recombinants much higher than this prediction has been observed in tetrad analyses in Neurospora (STADLER and TOWE 1963), yeast (FOGEL and HURST 1967), and Ascobolus (KRUSZEWSKA and GAJEWSKI 1967). The analysis of intragenic recombination at the his, locus in yeast by FOGEL and HURST (1967) yielded a second observation which was difficult to explain by the hybrid DNA models. The products of reciprocal and nonreciprocal recombination differed in their outside markers (Table 1). Virtually all the reciprocal recombination products carried the same nonparental combination of markers, while the nonreciprocal recombination products might have any of the four possible marker combinations. The models offer no basis for any difference in markers accompanying reciprocal as opposed to nonreciprocal recombination between sites which are close together (as the his, mutants must be, in view of the high proportion of nonreciprocal recombination). WHITEHOUSE (1 967) has attempted to account for this kind of observation by “secondary crossing over”an event which would result from a restriction of the length of the hybrid DVA4 region by certain segregating sites.