Synthesis-Dependent Strand Annealing in Meiosis

Open Access
Publisher Website
Google Scholar
Abstract
Recent studies led to the proposal that meiotic gene conversion can result after transient engagement of the donor chromatid and subsequent DNA synthesis-dependent strand annealing (SDSA). Double Holliday junction (dHJ) intermediates were previously proposed to form both reciprocal crossover recombinants (COs) and noncrossover recombinants (NCOs); however, dHJs are now thought to give rise mainly to COs, with SDSA forming most or all NCOs. To test this model in Saccharomyces cerevisiae, we constructed a random spore system in which it is possible to identify a subset of NCO recombinants that can readily be accounted for by SDSA, but not by dHJ-mediated recombination. The diagnostic class of recombinants is one in which two markers on opposite sides of a double-strand break site are converted, without conversion of an intervening heterologous insertion located on the donor chromatid. This diagnostic class represents 26% of selected NCO recombinants. Tetrad analysis using the same markers provided additional evidence that SDSA is a major pathway for NCO gene conversion in meiosis. In organisms that reproduce sexually, sex cells (gametes) are produced by the specialized cell division called meiosis, which halves the number of chromosomes from two sets (diploid) to one (haploid). During meiosis, homologous DNA molecules exchange genetic material in a process called homologous recombination, thereby contributing to genetic diversity. In addition, a subset of recombinants, called crossovers, creates connections between chromosomes that are required for those chromosomes to be accurately segregated. Accurate segregation ensures that gametes contain one and only one copy of each chromosome. Recombination is initiated by chromosome breakage. A regulatory process then selects a subset of breaks to be healed by a mechanism that forms crossover recombinants. Many of the remaining breaks are healed to form so-called “noncrossover” recombinants (also referred to as “gene conversions”). Until recently, it was thought that crossovers and noncrossovers were formed by nearly identical pathways; which type of recombinant arose was thought to depend on how the last enzyme in the pathway attacked the last DNA intermediate. However, more recent observations suggested that noncrossover recombinants might arise by a mechanism involving less-stable intermediates than those required to make crossovers. In the present work, a yeast strain was constructed that allowed the detection of a genetic signature of such unstable recombination intermediates. This strain provided evidence that meiotic crossovers and noncrossovers do indeed form by quite different mechanisms.