Synaptic relations in meiotic gene conversion at the iterated CUP1r locus of S. cerevisiae

This study concerns a comparative molecular analysis of copy number changes in two hybrids that differ in the extent of homologies at the CUP1r locus. Hybrid JW1020 is a diploid wherein each parent contributed nine identical, tandemly arrayed 2.0 kb repeat units. Genomic DNA was isolated from each of the spore colonies in a sample of 200 unselected tetrads. About 15% displayed copy number changes, i.e., increases or decreases of one or more complete units. Changes on a per tetrad basis occurred as often in a single spore colony as changes in each of two spores. Such double changes are rarely reciprocal in character. To account for the observed qualitative and quantitative copy number shifts, we propose a molecular recombination model that posits partial, incomplete synaptic pairing and gene conversion of the unpaired regions with or without associated crossing over. A second contrasting study centers on the copy number alterations and recombinational events uncovered in a molecular analysis of 50 unselected tetrads generated by hybrid EB8. Unlike the hybrid JW1020, the EB8 diploid strain carried a six copy tandem array of 1.1 kb units at the CUP1r locus in one parental homologue and a five copy array of 1.6 kb units at the corresponding chromosome VIII locus. These natural alleles were recovered from industrial yeast strains by conventional genetic procedures and characterized by restrictional analysis. Twelve tetrads exhibit evidence for several different types of recombination events. However,ordinary crossover exchanges are conspicuously absent. We suppose that the repetitious nonhomologies generate DNA configurations sufficient to disrupt the effective synapsis over the entire locus.(ABSTRACT TRUNCATED AT 250 WORDS)