Dynamics of meiotic chromosomes and CO during meiosis. (A) Changes in meiotic chromosomes for recombination during meiosis. At the leptotene stage, chromosomes form axis elements and DSBs are generated in excess. A subset of DSBs are repaired as COs. In the zygotene stage, assembly of the SC is initiated at designated CO sites. In the pachytene stage, SC assembly is complete. ZMM MSH4/5, HEI10 and MLH1/3 localize to CO sites. In the diplotene stage, SC disassembles and homologs remain linked by COs. In the diakinesis stage, chromosomes condense and COs can be detected by acid spreading. In metaphase I, chromosomes are highly compacted and bivalents align at the metaphase I plate, linked by COs called as chiasmata. (B) Model of meiotic recombination. SPO11 and its accessory proteins catalyze the formation of meiotic DSBs. DSB ends are resected to produce 3′ single-strand tails which, with the help of RAD51 and DMC1, invade non-sister chromatids to form D-loops. The majority of D-loops are resolved into non-COs while approximately 5% of D-loops mature into COs via two pathways. Class I COs are interference sensitive and depend on ZMMs and MLH1/3, representing approximately 85%-90% of total COs. Class II COs are non-interfering, formed by MUS81, and restricted by anti-CO factors such as FANCM.|@|~(^,^)~|@|Approaches to detect COs. (A) Arabidopsis developmental stages for analyzing meiotic recombination. Closed buds (0.3-0.5 mm in size) are used for cytological analysis. Mature pollen and seeds are analyzed using pollen and seed FTLs, respectively. Leaves are analyzed by GBS. Scale bar = 1 mm. (B) Cytological analysis of COs. DAPI staining (white) of chromosomes at metaphase I (left image). MLH1 (green) immunostaining at the diakinesis stage (right image). Scale bars = 0.5 μm. (C) Seed FTLs. Segregation of fluorescent reporters is shown during meiosis as non-recombinants and recombinants. NR, non-recombinant; R, recombinant. Fluorescent proteins in seeds from self-fertilized hemizygous plants (GR/++) that contain two T-DNAs (GFP, RFP) on the same chromosome are analyzed by CellProfiler. Scale bar = 1 mm. (D) Three-color pollen FTLs in qrt mutant background. DeepTetrad software accurately analyzes images of fluorescent pollen tetrads in a high throughput manner. Scale bar = 0.25 μm. Pollen FTLs produce 12 types of tetrads according to the location and number of CO. NR, non-recombinant; SCO, single crossover; DCO, double crossover. (E) Genome-wide CO map by GBS. Two different accessions (Col and Ler) are crossed and F₁ plants are self-fertilized. Sequencing and GBS libraries are constructed for F₂ individuals to precisely map COs on the genome.

© Mol. Cells