A Natural Meiotic DNA Break Site in Schizosaccharomyces pombe Is a Hotspot of Gene Conversion, Highly Associated With Crossing Over

Abstract

In Schizosaccharomyces pombe, meiosis-specific DNA breaks that initiate recombination are observed at prominent but widely separated sites. We investigated the relationship between breakage and recombination at one of these sites, the mbs1 locus on chromosome I. Breaks corresponding to 10% of chromatids were mapped to four clusters spread over a 2.1-kb region. Gene conversion of markers within the clusters occurred in 11% of tetrads (3% of meiotic chromatids), making mbs1 a conversion hotspot when compared to other fission yeast markers. Approximately 80% of these conversions were associated with crossing over of flanking markers, suggesting a strong bias in meiotic break repair toward the generation of crossovers. This bias was observed in conversion events at three other loci, ade6, ade7, and ura1. A total of 50-80% of all crossovers seen in a 90-kb region flanking mbs1 occurred in a 4.8-kb interval containing the break sites. Thus, mbs1 is also a hotspot of crossing over, with breakage at mbs1 generating most of the crossovers in the 90-kb interval. Neither Rec12 (Spo11 ortholog) nor I-SceI-induced breakage at mbs1 was significantly associated with crossing over in an apparently break-free interval Ͼ25 kb away. Possible mechanisms for generating crossovers in such break-free intervals are discussed. I N eukaryotic cells, DNA double-strand breaks (DSBs) ferred sites at widely separated physical distances ‫-05ف(‬ 100 kb on average). In contrast, crossovers are observed are a dangerous form of DNA damage and yet are also crucial for the process of meiosis. During vegetative at a relatively constant frequency per unit of physical distance (Young et al. 2002); i.e., crossovers are not growth, DSBs are a threat because, left unrepaired, they cause chromosomal fragmentation. In addition, repair consistently more frequent in intervals where strong breaks are observed than in apparently break-free interof DSBs can promote genomic instability, including DNA rearrangements resulting from homologous revals. It was proposed that the DSB and crossover patterns could be reconciled if meiotic DSBs are able to generate combination. In contrast, the deliberate generation of DSBs appears to be a conserved feature of meiosis precrossovers over large distances (tens of kilobases) around their positions (Young et al. 2002). cisely because of their ability to promote homologous recombination and generate crossovers. Crossovers, in Two factors control how the frequency of DSBs relates to the frequency of crossovers. First, DSBs can be return, are critical for correct meiotic chromosome segregation and the promotion of genetic diversity among paired using either sister chromatids or homologous chromosomes (homologs), but genetically observable the products of meiosis. The importance of DSB formation in generating meiotic crossovers has been well escrossovers can arise only from interhomolog events. The relative frequency of intersister and interhomolog tablished in Saccharomyces cerevisiae and Schizosaccharomyces pombe and in...

Linear Element-Independent Meiotic Recombination in Schizosaccharomyces pombe

“…If conversions at ade6 occurred without high levels of associated crossing over, this observation could be accounted for; i.e., rec10-155 is defective in the conversions at ade6 that have little associated crossing over. However, Cromie et al (2005) recently demonstrated that there are high levels of crossovers associated with conversions at ade6, dismissing the proposal that there is a greater reduction in conversions at ade6 than crossovers in the rec10-155 mutant because there is limited crossing over associated with ade6 gene conversions. A similar, but less dramatic, observation has also been made for mutants defective in the meiosis-specific cyclin Rem1, where a reduction in gene conversions is observed ($4-fold) at ade6, but no reduction in crossing over in the adjacent ade6-arg1 interval (Malapeira et al 2005).…”

A Combination of cis and trans Control Can Solve the Hotspot Conversion Paradox

“…Hotspots in yeast (Malone et al 1994;Wu and Lichten 1995;Petes 2001;Cromie et al 2005), mice (Guillon and De Massy 2002;Kelmenson et al 2005;Shifman et al 2006;Baudat and De Massy 2007), and humans ( Jeffreys et al 1998( Jeffreys et al , 2000( Jeffreys et al , 2001Crawford et al 2004;McVean et al 2004;Myers et al 2005;Conrad et al 2006;International HapMap Consortium 2007) have now been well characterized, and evidence suggests that hotspots also exist in chimpanzees (Ptak et al 2005) and several plants (Dooner and Martinez-Ferez 1997;Okagaki and Weil 1997;Yao et al 2002;Drouaud et al 2006). While some other well-studied eukaryotes (e.g., Drosophila melanogaster and Caenorhabditis elegans) show no evidence of hotspots (Hey 2004), the phenomenon is widespread enough across the tree of life to merit substantial study.…”

“…One striking characteristic of hotspots is that they are subject to a form of meiotic drive: when a DSB occurs at a hotspot heterozygous for an active (''hot'') and an inactive (''cold'') hotspot allele, the cold allele tends to appear in a higher proportion of the offspring than does the hot allele (often in an $3:1 ratio rather than the expected 2:2) (Catcheside 1975;Nicolas et al 1989;Grimm et al 1991;Malone et al 1994;Guillon and De Massy 2002;Jeffreys and Neumann 2002;Jeffreys and May 2004;Cromie et al 2005). This is likely the result of the mechanism by which recombination is thought to be initiated: a doublestrand break (DSB) forms on one chromatid; this break extends a variable distance in the 59 direction on each strand; and the sequence of the nonsister chromatid is used as a template to repair the gap.…”

The Meiotic Bouquet Promotes Homolog Interactions and Restricts Ectopic Recombination inSchizosaccharomyces pombe

“…Additionally, to explain the negative interference between ade6 1 convertants and tps16-arg1 crossovers (Table 2), the frequency of conversion at tps16 would have to be at least 17%, but only among ade6 1 spores. Gene conversion in S. pombe meiosis is frequently associated with crossing over between flanking markers (Grimm et al 1994;Cromie et al 2005). Negative interference between a conversion and a crossover could be explained if the conversion and crossover were not separate events.…”

“…In meiosis, unlike mitosis, gene conversion is frequently associated with crossing over (Grimm et al 1994;Virgin et al 2001;Cromie et al 2005 and references therein). Consequently, the recombination events that occasionally occur between dispersed repetitive DNA, such as transposons, genes for tRNAs, subtelomeric sequences, and multigene families, may frequently involve crossovers.…”