SUMMARY Meiotic chromosome pairing involves not only recognition of homology but also juxtaposition of entire chromosomes in a topologically regular way. Analysis of filamentous fungus Sordaria macrospora reveals that recombination proteins Mer3, Msh4 and Mlh1 play direct roles in all of these aspects, in advance of their known roles in recombination. Absence of Mer3 helicase results in interwoven chromosomes, thereby revealing the existence of features that specifically ensure “entanglement avoidance”. Entanglements that remain at zygotene, i.e. “interlockings”, require Mlh1 for resolution, likely to eliminate constraining recombinational connections. Patterns of Mer3 and Msh4 foci along aligned chromosomes show that the double-strand breaks mediating homologous alignment have spatially separated ends, one localized to each partner axis, and that pairing involves interference among developing interhomolog interactions. We propose that Mer3, Msh4 and Mlh1 execute all of these roles during pairing by modulating the state of nascent double-strand break/partner DNA contacts within axis-associated recombination complexes.

Zipl is a yeast synaptonemal complex (SC) central region component and is required for normal meiotic recombination and crossover interference. Physical analysis of meiotic recombination in a zipi mutant reveals the following: Crossovers appear later than normal and at a reduced level. Noncrossover recombinants, in contrast, seem to appear in two phases: (i) a normal number appear with normal timing and (ii) then additional products appear late, at the same time as crossovers. Also, Holliday junctions are present at unusually late times, presumably as precursors to late-appearing products. Redl is an axial structure component required for formation of cytologically discernible axial elements and SC and maximal levels of recombination. In a redi mutant, crossovers and noncrossovers occur at coordinately reduced levels but with normal timing. If Zipl affected recombination exclusively via SC polymerization, a zip] mutation should confer no recombination defect in a redi strain background. But a red] zip] double mutant exhibits the sum of the two single mutant phenotypes, including the specific deficit of crossovers seen in a zipi strain. We infer that Zipl plays at least one role in recombination that does not involve SC polymerization along the chromosomes. Perhaps some Zipl molecules act first in or around the sites of recombinational interactions to influence the recombination process and thence nucleate SC formation. We propose that a Zipldependent, pre-SC transition early in the recombination reaction is an essential component of meiotic crossover control. A molecular basis for crossover/noncrossover differentiation is also suggested.In meiosis, crossovers ensure the disjunction of homologs at the first division. The number and distribution of crossovers are tightly controlled (1-6). One manifestation of control is crossover interference: the presence of a crossover at one position along a chromosome reduces the probability that a crossover will also be found nearby. Crossover interference may act upon an array of undifferentiated recombinational interactions causing certain ones to mature into crossovers and others to mature into noncrossovers (e.g., refs. 3 and 4).In yeast, meiotic recombination initiates via meiosis-specific double strand breaks (DSBs) (7,8), which occur prior to bulk polymerization of the synaptonemal complex between the structural axes of paired homologs (9). Resected DSBs then invade an intact duplex to form double Holliday junctions; invasion and ensuing steps are approximately concomitant with initiation and progression of SC polymerization, respectively (10). Double Holliday junctions persist throughout much of the period when SC is full-length ("pachytene"). Mature crossover and noncrossover products form an hour or so after Holliday junctions appear, at about the time that SC disappears (9), but not dependent upon SC disassembly (11,12 In this model, all undifferentiated recombinational interactions are placed under "stress"; in addition, each interaction has an intrinsic "...

Chromosomal processes related to formation and function of meiotic chiasmata have been analyzed in Sordaria macrospora. Double-strand breaks (DSBs), programmed or ␥-rays-induced, are found to promote four major events beyond recombination and accompanying synaptonemal complex formation: (1) juxtaposition of homologs from long-distance interactions to close presynaptic coalignment at mid-leptotene; (2) structural destabilization of chromosomes at leptotene/zygotene, including sister axis separation and fracturing, as revealed in a mutant altered in the conserved, axis-associated cohesin-related protein Spo76/Pds5p; (3) exit from the bouquet stage, with accompanying global chromosome movements, at zygotene/pachytene (bouquet stage exit is further found to be a cell-wide regulatory transition and DSB transesterase Spo11p is suggested to have a new noncatalytic role in this transition); (4) normal occurrence of both meiotic divisions, including normal sister separation. Functional interactions between DSBs and the spo76-1 mutation suggest that Spo76/Pds5p opposes local destabilization of axes at developing chiasma sites and raise the possibility of a regulatory mechanism that directly monitors the presence of chiasmata at metaphase I. Local chromosome remodeling at DSB sites appears to trigger an entire cascade of chromosome movements, morphogenetic changes, and regulatory effects that are superimposed upon a foundation of DSB-independent processes. The central unique event of meiosis, reductional segregation of homologs at division I, is mediated by chiasmata. These observable connections between homologs correspond to sites of crossing over at the DNA level and arise during meiotic prophase via a complex series of chromosomal and nuclear changes that extend well beyond the process of DNA recombination and are both intricate and poorly understood.Recombination involves a series of local biochemical changes that begin with programmed double-strand breaks (DSBs) at early prophase and occupy most of prophase (review in Keeney 2001). Formation of chiasmata requires two other types of local changes (e.g., Blat et al. 2002). First, crossing over must occur not only within the DNA, but also between the underlying chromatid axes at corresponding positions. Second, because only one chromatid of each replicated homolog is involved, sister chromatids must be differentiated and separated locally at both the DNA and axis levels. The recombination complexes seen associated with their underlying chromatid axes during early prophase (e.g., Moens et al. 2002) likely mediate spatial, temporal, and functional linkage between events at the DNA and axis levels along the chiasma formation pathway.This progression of local changes occurs in close temporal coordination with a series of global changes in chromosome structure. One obvious meiotic structural feature is the synaptonemal complex (SC), a closepacked array of transverse filaments that links homolog axes at a distance of 100 nm all along their lengths. The SC appears, persists, a...