Fig. 4.

Prophase chromosomal events in diploid meiosis. a “Fill-in-the-holes” model for CO position selection (see also Wang et al. 2015). The array of early total DSB-mediated recombinational interactions (e.g., bridges; b–d (below)) is acted upon by a CO designation process. Each CO designation (red star) sets up an inhibitory zone of “CO interference” (blue arrows) via a signal that spreads outwards in both directions, dissipating with distance. This signal prevents bridge interactions in the affected region from undergoing CO designation (indicated by bridges changed to yellow). Subsequent CO designations occur in regions away from previously established interference zones, ultimately filling in the holes between previous CO sites. CO designation is very efficient, thus ensuring that all homolog pairs acquire at least one (first, obligatory) CO. b–f Homolog coalignment (“pairing”) is mediated by inter-axis bridges that comprise DSB-mediated recombinational interactions and followed by SC formation (“synapsis”). b, e Human male meiotic prophase chromosomes visualized by immunofluorescence illumination. c, d, f Allium cepa axes and associated “zygotene” recombination nodules (ZNs) or bridges (corresponding approximately to many/all DSB-mediated interactions) visualized by electron microscopy of PTA-stained spread preparations (from Albini and Jones 1987). b Leptotene/zygotene nucleus illustrates bridges containing single-strand binding protein RPA (white arrows), a direct player in Rad51/Dmc1-mediated strand exchange for recombination, with accompanying onset of synapsis. Green indicates SMC3 cohesin axis, blue centromeres (blue), and red RPA protein (from Oliver-Bonet et al. 2007). c, d Bridge configurations and incipient synapsis corresponding to the stage in b. e Pachytene synaptic configurations with SYCP3 axes of the SC (red), centromeres (blue), and Mlh1 foci marking sites of COs (green) (from Gruhn et al. 2013). f Two bivalents showing, respectively, extensive synapsis in progress and coalignment. In c, d, f, black arrows indicate examples of “nodules” or bridges of five types: (a) associated with SCs, (b) with association sites, (c) midway between axial cores in close alignment, (d) paired structures at matching sites on axial cores, and (e) apparently bridging the space between two converging axial cores. Blue arrows and text indicate positions of forming/formed SC