Quantifying Meiotic Crossover Recombination in Arabidopsis Lines Expressing Fluorescent Reporters in Seeds Using SeedScoring Pipeline for CellProfiler

MSH2 stimulates interfering and inhibits non-interfering crossovers in response to genetic polymorphism

Meiotic crossovers can be formed through the interfering pathway, in which one crossover prevents another from forming nearby, or by an independent non-interfering pathway. In Arabidopsis, local sequence polymorphism between homologs can stimulate interfering crossovers in a MSH2-dependent manner. To understand how MSH2 regulates crossovers formed by the two pathways, we combined Arabidopsis mutants that elevate non-interfering crossovers with msh2 mutants. We demonstrate that MSH2 blocks non-interfering crossovers at polymorphic loci, which is the opposite effect to interfering crossovers. We also observe MSH2-independent crossover inhibition at highly polymorphic sites. We measure recombination along the chromosome arms in lines differing in patterns of heterozygosity and observe a MSH2-dependent crossover increase at the boundaries between heterozygous and homozygous regions. Here, we show that MSH2 is a master regulator of meiotic DSB repair in Arabidopsis, with antagonistic effects on interfering and non-interfering crossovers, which shapes the crossover landscape in relation to interhomolog polymorphism.

The effect of DNA polymorphisms and natural variation on crossover hotspot activity in Arabidopsis hybrids

In hybrid organisms, genetically divergent homologous chromosomes pair and recombine during meiosis; however, the effect of specific types of polymorphisms on crossover is poorly understood. Here, to analyze this in Arabidopsis, we develop the seed-typing method that enables the massively parallel fine-mapping of crossovers by sequencing. We show that structural variants, observed in one of the generated intervals, do not change crossover frequency unless they are located directly within crossover hotspots. Both natural and Cas9-induced deletions result in lower hotspot activity but are not compensated by increases in immediately adjacent hotspots. To examine the effect of single nucleotide polymorphisms on crossover formation, we analyze hotspot activity in mismatch detection-deficient msh2 mutants. Surprisingly, polymorphic hotspots show reduced activity in msh2. In lines where only the hotspot-containing interval is heterozygous, crossover numbers increase above those in the inbred (homozygous). We conclude that MSH2 shapes crossover distribution by stimulating hotspot activity at polymorphic regions.

Short-interval traffic lines: versatile tools for genetic analysis in Arabidopsis thaliana

Traffic lines are transgenic stocks of Arabidopsis thaliana that contain a pair of linked seed-specific eGFP and DsRed markers. These stocks were originally developed for the purpose of studying recombination, but can also be used to follow the inheritance of unmarked chromosomes placed in trans to the marked chromosome. They are particularly useful for this latter purpose if the distance between markers is short, making double recombination within this interval relatively rare. We generated 163 traffic lines that cover the Arabidopsis genome in overlapping intervals of approximately 1.2 Mb (6.9 cM). These stocks make it possible to predict the genotype of a plant based on its seed fluorescence (or lack thereof) and facilitate many experiments in genetic analysis that are difficult, tedious, or expensive to perform using current techniques. Here, we show how these lines enable a phenotypic analysis of alleles with weak or variable phenotypes, genetic mapping of novel mutations, introducing transgenes into a lethal or sterile genetic background, and separating closely linked mutations.