Whole Genome Sequencing of Meiotic Tetrads Reveals the Nature of (non-) Crossover Events in Arabidopsis

Meiotic recombination, a fundamental process of sexual reproduction, introduces genetic diversity by shuffling parental allele combinations. Meiotic recombination starts with double strand breaks, which serve as assembly point for repair machineries, which can be resolved into either crossing overs (CO) or non-crossing overs (NCO). Irrespective of the repair mechanism, double-strand repair can lead to unidirectional transfer of genetic material between homologous chromosomes. This exchange of information is known as gene conversion. Gene conversions can be classified into crossing over gene conversions and non-crossing over gene conversions (NCO). Gene conversions were observed in yeast for the first time and thereafter yeast became the model to study gene conversions, supported by the possibility to extract all four products; tetrads, of single meiosis. Recently in Arabidopsis, the identification of quartet1 mutant that prevents the separation of pollen after meiosis facilitates to study tetrads.

We studied both types of gene conversion in Arabidopsis with a single base pair resolution by re-sequencing the whole genomes of 13 complete tetrads and 10 double haploid lines of a cross of Arabidopsis thaliana Col-0 and Ler. This allowed us to examine number of recombinations and gene conversions per meiosis, genomic characteristic of gene conversions, precise make-up and interferences to genomic features. On an average, we find 10.8 recombinations per meiosis, reconfirming recombination rate from previous studies. However, the track lengths of NCO are comparatively less as compared to CO gene conversion. Majority of CO and NCO gene conversion tracks feature a (CTT)_n cis-element motif, which is known to be bound by a transcription factor. Majority of recombination sites were accompanied by nucleosome-exclusion motifs, suggesting that there may be a preference for double-strand breaks in open-chromatids. Differently to earlier reports about ancestral recombination hotspot we could not identify an enrichment of CO locations in transposable elements.