In this approach, two notable methods are pollen typing ( Drouaud et al., 2013) and the use of fluorescent markers in seed ( Melamed-Bessudo et al., 2005) and in pollen tetrads ( Preuss et al., 1994 Francis et al., 2006, 2007 Berchowitz and Copenhaver, 2008 Sun et al., 2012 Yelina et al., 2013). One approach is high-throughput screening of many recombination events in a defined region. Owing to the augmentation of mapped genetic markers, studies have benefited from an increasing degree of resolution in the mapping of CO events. Understanding the regulation and the landscape of CO and non-crossover events has been a major endeavor in genetic research, as early as a century ago with studies on genetic linkage in sweet pea ( Lathyrus odoratus Bateson et al., 1906), followed by Thomas Hunt Morgan's ( Morgan, 1911) work in Drosophila melanogaster, determining the rate of meiotic CO between linked loci. ![]() It should be noted that only a minority of the DSBs induced at the onset of meiosis turn into CO events ( Youds and Boulton, 2011). This can result in heterozygosity loss or gene conversion ( San Filippo et al., 2008 Baudat et al., 2013). When the ends of a broken DNA invade homologous sequences on the homologous chromosome, a heteroduplex intermediate is formed and its resolution gives rise to a crossover ( CO) event, namely, the reciprocal exchange of large homologous chromosomal segments and/or to a non-crossover event ( San Filippo et al., 2008), namely, a nonreciprocal exchange of short DNA sequences found in the heteroduplex intermediate. The process of meiotic recombination is initiated by DNA double-strand break ( DSB) induction. In summary, the crossover motifs are associated with epigenetic landscapes corresponding to open chromatin and contributing to the nonuniformity of crossovers in Arabidopsis. This landscape was conserved in the decreased DNA methylation1 mutant. Cytosine methylation levels showed a gradual decrease within ∼2 kb of the three motifs, being lowest at sites where crossover occurred. For example, we show that there is a peak of nucleosome occupancy and of H3K4me3 around the CCN and CTT repeat motifs while nucleosome occupancy was lowest around the A-rich motif. Analysis of epigenetic modifications around the motifs showed, in most cases, a specific epigenetic architecture. The A-rich motif was preferentially associated with promoters, while the CCN repeat and the CTT repeat motifs were preferentially associated with genes. One of these motifs, the CCN repeat, was previously unknown in plants. Three DNA motifs enriched in crossover regions and less abundant in crossover-poor pericentric regions were identified. Crossovers were more frequent than expected in promoters. To better understand the factors regulating this variable landscape, we performed a detailed genetic and epigenetic analysis of 737 crossover events in Arabidopsis thaliana. ![]() The rate of crossover, the reciprocal exchanges of homologous chromosomal segments, is not uniform along chromosomes differing between male and female meiocytes.
0 Comments
Leave a Reply. |