The genomic landscape of meiotic crossovers and gene conversions in Arabidopsis thaliana

摘要

Most living organisms package their DNA into bundles called chromosomes. These chromosomes generally form pairs, with each chromosome in the pair containing the same number of genes. The genes also come in the same order, but the exact sequence of DNA bases within the genes can be different. When sex cells—such as egg, sperm or pollen cells—are made, each pair of chromosomes is separated so that the each sex cell contains only half the normal number of chromosomes. However, before they are separated, the pairs swap lengths of DNA via recombination events. These can involve exchanging large chunks of the chromosomes this is called a ‘crossover’. Alternatively, short stretches of one chromosome can be replaced by the corresponding region from the other in the pair. When these ‘non-crossovers’ cause a change in the DNA sequence they are known as gene conversions. Long-standing questions in the field of plant biology include how common are gene conversions? How much DNA is typically exchanged? And where in the chromosomes do these events happen most? Now, Wijnker et al. have addressed these questions by focusing on the accurate detection of recombination events, with a special emphasis on gene conversions, in the plant biologist’s favourite species Arabidopsis. Searching for recombination events is a challenge because, when piecing together an entire genome from lots of shorter stretches of DNA—typically called ‘reads’, it is easy to misplace some of the pieces. However, meticulous examination of these short DNA reads allowed Wijnker et al. to reliably identify gene conversions on a genome-wide scale. In Arabidopsis, gene conversion appears to be unexpectedly rare—with approximately one gene conversion detected per 140–240 non-crossovers. Recombination tends to occur in regions of the chromosomes where the DNA is only loosely packaged, is not heavily modified by the process of ‘DNA methylation’, and also near the start of genes. Furthermore, two specific sequences of DNA bases were identified that marked ‘hot spots’ in the chromosomes, where recombination happens more frequently. Wijnker et al. suggest that the low number of gene conversions detected indicates that non-crossovers tend to exchange very short stretches of DNA. However, future research may point to additional mechanisms that explain the low incidence of gene conversion in Arabidopsis.