The Use of Linear Asci for Chromosome Mapping in Saccharomyces.

N yeast, as in other organisms where tetrad analysis is feasible, it is possible to Idetermine the distance between a given gene and the centromere of its chromosome. This may be accomplished directly if the relationship of the spores in the ascus is known. Two such relationships have been demonstrated. In the four-spored asci of Schizosaccharomyces pombe, the two spores at each end of the ascus are sisters (LEUPOLD 1950). An ascogenous cell of genotype A a produces an ascus in which spores of composition A , A , a, and a are arranged in that order if there has been no recombination between the locus of A and the centromere. If a crossover has occurred in this region, the following sequences, indicative of second-division segregation, are obtained: ( A a a A ) , (a A A a) , and ( A a A a). In Saccharomycodes Zudwigii, the distribution of the nuclei is such that the two spores a t each end of the ascus are nonsisters (WINGE and LAUSTSEN 1939). If the nuclear distribution is random otherwise, the following noncrossover spore arrays would be expected: ( A a a A ) , (a A A a) , and ( A a A a) . If recombination has occurred between A and the centromere, the same three arrays would be expected half the time, and a fourth array ( A A a a ) the other half. An estimate of the frequency of second-division segregation for a gene in this organism would therefore be twice the frequency of ( A A a a ) asci. The consequence of a non-random nuclear distribution in S. Zudwigii will be discussed later. In linkage studies in Saccharomyces hybrids, it has been customary to dissect asci of the oval type, in which spore relationship is not known. Segregation data obtained from these asci have been used in chromosome mapping by LINDEGREN (1949a, 1951). The equations for calculating gene-centromere linkage in unordered tetrads have been given by PERKINS (1949) and WHITEHOUSE (1950). This method is, however, indirect; in order to determine whether or not a given gene exhibits linkage with the centromere, it is necessary to have segregating in the same asci another gene which exhibits centromere linkage and for which the recombination frequency is known. Saccharomyces hybrids also produce asci which are elongated and in which the spores are arranged in a linear series. Hybrids may be selected which yield as many as 40 percent of linear asci. The investigation reported below was undertaken to determine the pattern of spore array in these asci, for the purpose of ascertaining their usefulness in gene-centromere linkage studies.