Evidence for the coincident initiation of homolog pairing and synapsis during the telomere-clustering (bouquet) stage of meiotic prophase

To improve knowledge of the prerequisites for meiotic chromosome segregatio n in higher eukaryotes, we analyzed the spatial distribution of a pair of h omologs before and during early meiotic prophase, Three-dimensional images of fluorescence in situ hybridization (FISH) were used to localize a single pair of homologs in diploid nuclei of a chromosome-addition line of oat, o at-maize9b, The system provided a robust assay for pairing based on cytolog ical colocalization of FISH signals. Using a triple labeling scheme for sim ultaneous imaging of chromatin, telomeres and the homolog pair, we determin ed the timing of pairing in relation to the onset of three sequential hallm arks of early meiotic prophase: chromatin condensation (the leptotene stage ), meiotic telomere clustering (the bouquet stage) and the initiation of sy napsis (the zygotene stage), We found that the two homologs were mostly unp aired up through middle leptotene, at which point their spherical cloud-lik e domains began to transform into elongated and stretched-out domains. At l ate leptotene, the homologs had completely reorganized into long extended f ibers, and the beginning of the bouquet stage was conspicuously marked by t he de novo clustering of telomeres at the nuclear periphery. The homologs p aired and synapsed during the bouquet stage, consistent with the timing of pairing observed for several oat 5S rDNA loci. In summary, results from ana lysis of more than 100 intact nuclei lead us to conclude that pairing and s ynapsis of homologous chromosomes are largely coincident processes, ruling out a role for premeiotic pairing in this system. These findings suggest th at the genome-wide remodeling of chromatin and telomere-mediated nuclear re organization are prerequisite steps to the DNA sequence-based homology-sear ch process in higher eukaryotes.