Sister chromatid cohesion ensures the faithful segregation of chromosomes in mitosis and in both meiotic divisions. Meiosis-specific components of the cohesin complex, including the recently described SMC1 isoform SMC1 beta, were suggested to be required for meiotic sister chromatid cohesion and DNA recombination. Here we show that SMC1 beta-deficient mice of both sexes are sterile. Male meiosis is blocked in pachytene; female meiosis is highly error-prone but continues until metaphase II. Prophase axial elements (AEs) are markedly shortened, chromatin extends further from the AEs, chromosome synapsis is incomplete, and sister chromatid cohesion in chromosome arms and at centromeres is lost prematurely. In addition, crossover-associated recombination foci are absent or reduced, and meiosis-specific perinuclear telomere arrangements are impaired. Thus, SMC1 beta has a key role in meiotic cohesion, the assembly of AEs, synapsis, recombination, and chromosome movements.

During the extended prophase to the meiosis I division, chromosomes assemble axial elements (AE) along replicated sister chromatids whose ends attach to the inner nuclear membrane (NM) via a specialized conical thickening. Here, we show at the EM level that in Sycp3 ؊/؊ spermatocyte chromosomes lack the AE and the conical end thickening, but still they attach their telomeres to the inner NM with an electron-dense plate that contains T 2 AG 3 repeats. Immunofluorescence detected telomere proteins, SCP2, and the meiosis-specific cohesin STAG3 at the Sycp3 ؊/؊ telomere. Bouquet stage spermatocytes were approximately threefold enriched, and the number of telomere but not centromere signals was reduced to the haploid in advanced Sycp3 ؊/؊ spermatocytes, which indicates a special mode of homolog pairing at the mammalian telomere. Fluorescence in situ hybridization with mouse chromosome 8-and 12-specific subsatellite probes uncovered reduced levels of regional homolog pairing, whereas painting of chromosomes 13 revealed partial or complete juxtapositioning of homologs; however, condensation of Sycp3 ؊/؊ bivalents was defective. Electron microscopic analysis of AE-deficient spermatocytes revealed that transverse filaments formed short structures reminiscent of the synaptonemal complex central region, which likely mediate stable homolog pairing. It appears that the AE is required for chromosome condensation, rapid exit from the bouquet stage, and fine-tuning of homolog pairing. INTRODUCTIONMeiosis is essential for sexual reproduction in that it enables the independent assortment of homologous chromosomes (homologs) and the reduction of chromosome number to the haploid. To this end, homologs pair and establish stable connections during an extended prophase that precedes the reductional meiosis I division (MI). A subsequent second division (MII) separates sister chromatids, leading to formation of haploid gametes or spores. Before pairing of homologs at prophase I, these extend and attach their ends (telomeres) to the nuclear envelope. Concomitantly with this change in nuclear architecture, protein axes (axial elements; AEs) are formed along replicated sister chromatids (leptotene). Subsequently, chromosomes move and initiate stable pairing between them (zygotene) by assembly of proteinaceous transverse filaments between AEs (now called lateral elements; LEs), which forms the synaptonemal complex (SC) that connects all homologs at pachytene (von Wettstein et al., 1984;Hunter, 2003).AEs contain SCP3 and SCP2 proteins that assemble on cohesin cores that are laid down during premeiotic S phase (Prieto et al., 2001;Eijpe et al., 2003). Resolution of sister chromatid cohesion, first between chromatids during MI and subsequently between sister centromeres during the meiosis II division, allows chromosome reduction (Buonomo et al., 2000; for reviews, see Lee and Orr-Weaver, 2001;Petronczki et al., 2003). These processes have evolved by addition of meiosis-specific components to the cohesin complex. In mammals, for instance, STAG3...

Telomeres fail to attach to the nuclear envelope and lose structural integrity in cells lacking SMC1β.

Besides the established central role of poly(ADP-ribose) polymerase-1 (Parp-1) and Parp-2 in the maintenance of genomic integrity, accumulating evidence indicates that poly(ADP-ribosyl)ation may modulate epigenetic modifications under physiological conditions. Here, we provide in vivo evidence for the pleiotropic involvement of Parp-2 in both meiotic and postmeiotic processes. We show that Parp-2-deficient mice exhibit severely impaired spermatogenesis, with a defect in prophase of meiosis I characterized by massive apoptosis at pachytene and metaphase I stages. Although Parp-2 ؊/؊ spermatocytes exhibit normal telomere dynamics and normal chromosome synapsis, they display defective meiotic sex chromosome inactivation associated with derailed regulation of histone acetylation and methylation and up-regulated X-and Y-linked gene expression. Furthermore, a drastically reduced number of crossover-associated Mlh1 foci are associated with chromosome missegregation at metaphase I. Moreover, Parp-2 ؊/؊ spermatids are severely compromised in differentiation and exhibit a marked delay in nuclear elongation. Altogether, our findings indicate that, in addition to its well known role in DNA repair, Parp-2 exerts essential functions during meiosis I and haploid gamete differentiation.

Chromosome segregation errors are a significant cause of aneuploidy among human neonates and often result from errors in female meiosis that occur during fetal life. For the latter reason, little is known about chromosome dynamics during female prophase I. Here, we analyzed chromosome reorganization, and centromere and telomere dynamics in meiosis in the human female by immunofluorescent staining of the SYCP3 and SYCP1 synaptonemal complex proteins and the course of recombinational DNA repair by IF of phospho-histone H2A.X (gamma-H2AX), RPA and MLH1 recombination proteins. We found that SYCP3, but not SYCP1, aggregates appear in the preleptotene nucleus and some persist up to pachytene. Telomere clustering (bouquet stage) in oocytes lasted from late-leptotene to early pachytene-significantly longer than in the male. Leptotene and zygotene oocytes and spermatocytes showed strong gamma-H2AX labeling, while gamma-H2AX patches, which colocalized with RPA, were present on SYCP1-tagged pachytene SCs. This was rarely seen in the male and may suggest that synapsis installs faster with respect to progression of recombinational double-strand break repair or that the latter is slower in the female. It is speculated that the presence of gamma-H2AX into pachytene highlights female-specific peculiarities of recombination, chromosome behavior and checkpoint control that may contribute to female susceptibility for aneuploidy.