Here we probe the relationships between assembly of the synaptonemal complex (SC) and progression of recombination between homologous chromosomes during Caenorhabditis elegans meiosis. We identify SYP-2 as a structural component of the SC central region and show that central region assembly depends on proper morphogenesis of chromosome axes. We find that the SC central region is dispensable for initiation of recombination and for loading of DNA strand-exchange protein RAD-51, despite the fact that extensive RAD-51 loading normally occurs in the context of assembled SC. Further, persistence of RAD-51 foci and absence of crossover products in meiotic mutants suggests that SC central region components and recombination proteins MSH-4 and MSH-5 are required to promote conversion of resected double-strand breaks into stable post-strand exchange intermediates. Our data also suggest that early prophase barriers to utilization of sister chromatids as repair templates do not depend on central region assembly.

Synaptonemal complex (SC) assembly must occur between correctly paired homologous chromosomes to promote formation of chiasmata. Here, we identify the Caenorhabditis elegans HORMA-domain protein HTP-1 as a key player in coordinating establishment of homolog pairing and synapsis in C. elegans and provide evidence that checkpoint-like mechanisms couple these early meiotic prophase events. htp-1 mutants are defective in the establishment of pairing, but in contrast with the pairing-defective chk-2 mutant, SC assembly is not inhibited and generalized nonhomologous synapsis occurs. Extensive nonhomologous synapsis in htp-1; chk-2 double mutants indicates that HTP-1 is required for the inhibition of SC assembly observed in chk-2 gonads. htp-1 mutants show a decreased abundance of nuclei exhibiting a polarized organization that normally accompanies establishment of pairing; analysis of htp-1; syp-2 double mutants suggests that HTP-1 is needed to prevent premature exit from this polarized nuclear organization and that this exit stops homology search. Further, based on experiments monitoring the formation of recombination intermediates and crossover products, we suggest that htp-1 mutants are defective in preventing the use of sister chromatids as recombination partners. We propose a model in which HTP-1 functions to establish or maintain multiple constraints that operate to ensure coordination of events leading to chiasma formation.

Fanconi anemia (FA) is a complex cancer susceptibility disorder associated with DNA repair defects and infertility, yet the precise function of the FA proteins in genome maintenance remains unclear. Here we report that C. elegans FANCD2 (fcd-2) is dispensable for normal meiotic recombination but is required in crossover defective mutants to prevent illegitimate repair of meiotic breaks by nonhomologous end joining (NHEJ). In mitotic cells, we show that DNA repair defects of C. elegans fcd-2 mutants and FA-deficient human cells are significantly suppressed by eliminating NHEJ. Moreover, NHEJ factors are inappropriately recruited to sites of replication stress in the absence of FANCD2. Our findings are consistent with the interpretation that FA results from the promiscuous action of NHEJ during DNA repair. We propose that a critical function of the FA pathway is to channel lesions into accurate, as opposed to error-prone, repair pathways.

Segregation of homologous chromosomes during meiosis depends on linkages (chiasmata) created by crossovers and on selective release of a subset of sister chromatid cohesion at anaphase I. DuringCaenorhabditis elegans meiosis, each chromosome pair forms a single crossover, and the position of this event determines which chromosomal regions will undergo cohesion release at anaphase I. Here we provide insight into the basis of this coupling by uncovering a large-scale regional change in chromosome axis composition that is triggered by crossovers. We show that axial element components HTP-1 and HTP-2 are removed during late pachytene, in a crossover-dependent manner, from the regions that will later be targeted for anaphase I cohesion release. We demonstrate correspondence in position and number between chiasmata and HTP-1/2-depleted regions and provide evidence that HTP-1/2 depletion boundaries mark crossover sites. In htp-1 mutants, diakinesis bivalents lack normal asymmetrical features, and sister chromatid cohesion is prematurely lost during the meiotic divisions. We conclude that HTP-1 is central to the mechanism linking crossovers with late-prophase bivalent differentiation and defines the domains where cohesion will be protected until meiosis II. Further, we discuss parallels between the pattern of HTP-1/2 removal in response to crossovers and the phenomenon of crossover interference.[Keywords: Meiosis; chromosome axes; crossover; sister chromatid cohesion; chromosome remodeling; crossover interference] Supplemental material is available at http://www.genesdev.org. Received May 12, 2008; revised version accepted August 18, 2008. In sexually reproducing organisms, diploid germ cells produce haploid gametes through the specialized cell division program of meiosis. At the onset of meiosis, DNA is replicated and sister chromatid cohesion (SCC) is established (Nasmyth and Schleiffer 2004). In contrast to mitotic cell cycles, this single round of meiotic DNA replication is followed by two rounds of cell division, the first segregating homologous chromosomes (homologs), and the second segregating sister chromatids (Petronczki et al. 2003). This pattern of segregation requires an extended prophase during which chromosomes must assemble meiosis-specific axial structures, locate, and align with their homologs, stabilize this alignment through assembly of the synaptonemal complex (SC), and undergo crossover recombination events between their DNA molecules (Page and Hawley 2003). Crossovers that form in this context play a crucial role in promoting meiotic chromosome segregation, as they collaborate with SCC (on domains flanking the crossover site) to form the basis of chiasmata, cytologically visible connections between the homologs that are revealed upon SC disassembly and structural remodeling of chromosomes during late prophase (Jones 1987). Chiasmata allow homologs to remain connected while orienting away from each other toward opposite poles of the metaphase I spindle. Subsequently, the SCC that maintains the co...

The correct pairing and segregation of chromosomes during meiosis is essential for genetic stability and subsequent fertility. This is more difficult to achieve in polyploid species, such as wheat, because they possess more than one diploid set of similar chromosomes. In wheat, the Ph1 locus ensures correct homologue pairing and recombination. Although clustering of telomeres into a bouquet early in meiosis has been suggested to facilitate homologue pairing, centromeres associate in pairs in polyploid cereals early during floral development. We can now extend this observation to root development. Here we show that the Ph1 locus acts both meiotically and somatically by reducing non-homologous centromere associations. This has the effect of promoting true homologous association when centromeres are induced to associate. In fact, non-homologously associated centromeres separate at the beginning of meiosis in the presence, but not the absence, of Ph1. This permits the correction of homologue association during the telomere-bouquet stage in meiosis. We conclude that the Ph1 locus is not responsible for the induction of centromere association, but rather for its specificity.