Use of Tetratype Frequencies for estimating Spindle Overlapping at the Second Division of Meiosis in ‘Ordered’ Tetrads

Abstract

WITH ordered tetrads, such as occur in the asci of Neurospora crassa, the second-division meiotic spindles do not overlap (Fig. 1a) and the subsequent mitotic spindles are also well separated, with the result that analysis of the sequence of spores within the ascus allows a distinction to be made between first- and second-division segregation for a particular pair of alleles. With unordered tetrads, second-division segregation frequencies can be calculated from the frequencies of tetratype tetrads (for example, AB, ab, Ab, aB) for three independent loci, A/a, B/b and C/c, considered in pairs (A and B, B and C, C and A)1. When there is uncertainty whether the tetrads of a particular organism are ordered or not, use can be made of tetratype frequencies to estimate the frequencies of partial and complete spindle overlapping at the second division of meiosis. It is important to recognize such overlapping, since it may lead to inaccurate mapping of the centromeres, and, moreover, partial spindle overlap may mimic the effects of two-strand double cross-overs across the centromere. Thus the excess of such cross-overs found by Lindegren and Lindegren2 in N. crassa has been attributed3 to spindle overlapping, although there is good genetic evidence that such overlapping is not responsible4.