Corresponding-site interference, synaptinemal complex structure, and 8+:0mand 7+:1moctads fromwild-type×mutantcrosses ofAscobolus immersus

Abstract

SUMMARY‘Wider ratio’ octads (8:0, 0:8, 7:1 and 1:7) regularly occurred inwild-type(+) ×whiteascospore(w) crosses of the Pasadena strains ofAscobolus. Control crosses showed that phenocopies and false octad clusters were absent or rare; no reversion fromwto + occurred, but mutation from + towwas found at a number of loci, with nearly all 0+:8wand many 2+:6woctads in + ×wcrosses arising from mutation, not conversion. Nearly all 8+:0w, 7+:1wand 6+:2woctads appeared to arise by conversion.The finding of genuine wider ratio octads implies hybrid-DNA formation at corresponding sites inbothpairs of non-sister chromatids in the same bivalent, which conflicts with models of the synaptinemal complex requiring that only two of the four chromatids pair intimately at any point. Octad types arising from hybrid-DNA formation at corresponding sites in both pairs of non-sister chromatids were described and formulae were derived for their frequencies. The lack of genuine wider ratio octads in several otherAscobolusstudies was shown to be explicable quantitatively in terms of their conversion frequencies.‘Corresponding-site interference’ is defined as interference between the two pairs of non-sister chromatids of a bivalent in hybrid-DNA formation at exactly corresponding sites. Formulae based on observed octad frequencies were derived for calculating coincidence values for this kind of interference. Corresponding-site interference was found to be weak, with coincidence values differing between crosses with high and with low conversion frequencies.