Two-spored asci produced by interrupted sporulation: a novel approach to linkage analysis in yeast

Genetical analysis of two-spored asci formed by interrupted sporulation offers a novel procedure for mapping of centromere-linked genes in Saccharomyces cerevisiae. Unlike the two-spored asci encountered under normal sporulation conditions, these asci are produced by a nonrandom mechanism. They fall into three categories (+ +), (+ -) and (- -) with respect to any marker. The percentage of (+ -) asci varies directly as a function of centromere-linkage of a gene. It is observed that almost 100% asci are of the (+ -) type in case of very tightly linked genes like trp-1 and cdc-10, while in case of markers unlinked to the centromere, e.g, trp-5 and met-8, the (+ -) asci constitute 50% of the total number of asci. Other markers with varying degrees of linkage, e.g. ura-3 and lys-1 show corresponding numbers of (+ -) asci between 50% and 100% of the total asci. These findings are in contrast to the results expected from a random abortion of two spores, in which case the (+ -) asci would constitute 67% of the total number of asci irrespective of the degree of centromere linkage of a marker. The linkage-dependent segregation of markers in these new kind of two-spored asci permits a rapid and accurate estimate of centromere linkage of a gene.