The detection, in unordered octads, of 6+:2m and 2+:6m ratios with postmeiotic segregation, and of aberrant 4:4s, and their use in corresponding-site interference studies

Abstract

SUMMARY: Systems for the detection of postmeiotic segregation in unordered octads are described for 4:4(2), 6:2(2), 2:6(2), 5:3(3), 3:5(3) and 4:4(4) segregations, where the number in parentheses is the number of pairs of non-identical sister-spores. Linkage relations, centromere distances and phenotypic detectability of suitable combinations of markers for initial selective identification of such segregations, and subsequent confirmation tests, are discussed; correction formulae are given to allow for non-detection of certain classes in particular circumstances. Using one such system in Ascobolus immersus, we detected 4:4(2), 6:2(2) and 2:6(2) segregations at the w-78 site: 2:6(2)s were previously unknown. Initial visual detection in this system was from octad phenotype ratios for w-78 (white ascospore) and gr-3 (granular ascospore pigmentation), with mating type and parafluorophenylalanine resistance as confirmatory markers after germination. Corresponding-site interference for wider ratio and – for the first time – narrower ratio unique classes was determined: it was moderately positive or moderately negative, depending on the cross and segregation class involved.