The Rice OsRad21-4, an Orthologue of Yeast Rec8 Protein,is Required for Efficient Meiosis

In yeast, Rad21/Scc1 and its meiotic variant Rec8 are key players in the establishment and subsequent dissolution of sister chromatid cohesion for mitosis and meiosis, respectively, which are essential for chromosome segregation. Unlike yeast, our identification revealed that the rice genome has 4 RAD21-like genes that share lower than 21% identity at polypeptide levels, and each is present as a single copy in this genome. Here we describe our analysis of the function of OsRAD21-4 by RNAi. Western blot analyses indicated that the protein was most abundant in young flowers and less in leaves and buds but absent in roots. In flowers, the expression was further defined to premeiotic pollen mother cells (PMCs) and meiotic PMCs of anthers. Meiotic chromosome behaviors were monitored from male meiocytes of OsRAD21-4-deficient lines mediated by RNAi. The male meiocytes showed multiple aberrant events at meiotic prophase I, including over-condensation of chromosomes, precocious segregation of homologues and chromosome fragmentation. Fluorescence in situ hybridization experiments revealed that the deficient lines were defective in homologous pairing and cohesion at sister chromatid arms. These defects resulted in unequal chromosome segregation and aberrant spore generation. These observations suggest that OsRad21-4 is essential for efficient meiosis.     

Meiosis-Specific Cohesin Component,Stag3 Is Essential for Maintaining Centromere Chromatid Cohesion,and Required for DNA Repair and Synapsis between Homologous Chromosomes

Cohesins are important for chromosome structure and chromosome segregation during mitosis and meiosis. Cohesins are composed of two structural maintenance of chromosomes (SMC1-SMC3) proteins that form a V-shaped heterodimer structure, which is bridged by a α-kleisin protein and a stromal antigen (STAG) protein. Previous studies in mouse have shown that there is one SMC1 protein (SMC1β), two α-kleisins (RAD21L and REC8) and one STAG protein (STAG3) that are meiosis-specific. During meiosis, homologous chromosomes must recombine with one another in the context of a tripartite structure known as the synaptonemal complex (SC). From interaction studies, it has been shown that there are at least four meiosis-specific forms of cohesin, which together with the mitotic cohesin complex, are lateral components of the SC. STAG3 is the only meiosis-specific subunit that is represented within all four meiosis-specific cohesin complexes. In Stag3 mutant germ cells, the protein level of other meiosis-specific cohesin subunits (SMC1β, RAD21L and REC8) is reduced, and their localization to chromosome axes is disrupted. In contrast, the mitotic cohesin complex remains intact and localizes robustly to the meiotic chromosome axes. The instability of meiosis-specific cohesins observed in Stag3 mutants results in aberrant DNA repair processes, and disruption of synapsis between homologous chromosomes. Furthermore, mutation of Stag3 results in perturbation of pericentromeric heterochromatin clustering, and disruption of centromere cohesion between sister chromatids during meiotic prophase. These defects result in early prophase I arrest and apoptosis in both male and female germ cells. The meiotic defects observed in Stag3 mutants are more severe when compared to single mutants for Smc1β, Rec8 and Rad21l, however they are not as severe as the Rec8, Rad21l double mutants. Taken together, our study demonstrates that STAG3 is required for the stability of all meiosis-specific cohesin complexes. Furthermore, our data suggests that STAG3 is required for structural changes of chromosomes that mediate chromosome pairing and synapsis, DNA repair and progression of meiosis.     

The Cohesin Subunit Rad21 Is Required for Synaptonemal Complex Maintenance,but Not Sister Chromatid Cohesion,during Drosophila Female Meiosis

Replicated sister chromatids are held in close association from the time of their synthesis until their separation during the next mitosis. This association is mediated by the ring-shaped cohesin complex that appears to embrace the sister chromatids. Upon proteolytic cleavage of the α-kleisin cohesin subunit at the metaphase-to-anaphase transition by separase, sister chromatids are separated and segregated onto the daughter nuclei. The more complex segregation of chromosomes during meiosis is thought to depend on the replacement of the mitotic α-kleisin cohesin subunit Rad21/Scc1/Mcd1 by the meiotic paralog Rec8. In Drosophila, however, no clear Rec8 homolog has been identified so far. Therefore, we have analyzed the role of the mitotic Drosophila α-kleisin Rad21 during female meiosis. Inactivation of an engineered Rad21 variant by premature, ectopic cleavage during oogenesis results not only in loss of cohesin from meiotic chromatin, but also in precocious disassembly of the synaptonemal complex (SC). We demonstrate that the lateral SC component C(2)M can interact directly with Rad21, potentially explaining why Rad21 is required for SC maintenance. Intriguingly, the experimentally induced premature Rad21 elimination, as well as the expression of a Rad21 variant with destroyed separase consensus cleavage sites, do not interfere with chromosome segregation during meiosis, while successful mitotic divisions are completely prevented. Thus, chromatid cohesion during female meiosis does not depend on Rad21-containing cohesin.     

Proteasome Activity is Required for Centromere Separation Independently of Securin Degradation in Human Cells

Loss of centromere cohesion during anaphase in human cells is regulated by the spindle assembly checkpoint and is thought to depend on a ubiquitin ligase, the Anaphase Promoting Complex/Cyclosome (APC). APC-Cdc20 adds ubiquitin chains to securin inducing its destruction by the proteasome and these events correlate with the loss of sister chromatid cohesion and the onset of anaphase. But whether securin destruction is necessary and sufficient for anaphase initiation is not clear. Therefore, we asked if proteasome activity is needed for anaphase onset in human cells that lack securin. We find that even in the absence of securin, a metaphase block with cohered sister centromeres can be enforced in the absence of proteasome activity. Therefore, other targets of the proteasome must be degraded to allow anaphase onset.     

Separase Is Required for Homolog and Sister Disjunction during Drosophila melanogaster Male Meiosis,but Not for Biorientation of Sister Centromeres

Spatially controlled release of sister chromatid cohesion during progression through the meiotic divisions is of paramount importance for error-free chromosome segregation during meiosis. Cohesion is mediated by the cohesin protein complex and cleavage of one of its subunits by the endoprotease separase removes cohesin first from chromosome arms during exit from meiosis I and later from the pericentromeric region during exit from meiosis II. At the onset of the meiotic divisions, cohesin has also been proposed to be present within the centromeric region for the unification of sister centromeres into a single functional entity, allowing bipolar orientation of paired homologs within the meiosis I spindle. Separase-mediated removal of centromeric cohesin during exit from meiosis I might explain sister centromere individualization which is essential for subsequent biorientation of sister centromeres during meiosis II. To characterize a potential involvement of separase in sister centromere individualization before meiosis II, we have studied meiosis in Drosophila melanogaster males where homologs are not paired in the canonical manner. Meiosis does not include meiotic recombination and synaptonemal complex formation in these males. Instead, an alternative homolog conjunction system keeps homologous chromosomes in pairs. Using independent strategies for spermatocyte-specific depletion of separase complex subunits in combination with time-lapse imaging, we demonstrate that separase is required for the inactivation of this alternative conjunction at anaphase I onset. Mutations that abolish alternative homolog conjunction therefore result in random segregation of univalents during meiosis I also after separase depletion. Interestingly, these univalents become bioriented during meiosis II, suggesting that sister centromere individualization before meiosis II does not require separase.     

The Rec8 Gene of Schizosaccharomyces Pombe Is Involved in Linear Element Formation, Chromosome Pairing and Sister-Chromatid Cohesion during Meiosis

The fission yeast Schizosaccharomyces pombe does not form tripartite synaptonemal complexes during meiotic prophase, but axial core-like structures (linear elements). To probe the relationship between meiotic recombination and the structure, pairing, and segregation of meiotic chromosomes, we genetically and cytologically characterized the rec8-110 mutant, which is partially deficient in meiotic recombination. The pattern of spore viability indicates that chromosome segregation is affected in the mutant. A detailed segregational analysis in the rec8-110 mutant revealed more spores disomic for chromosome III than in a wild-type strain. Aberrant segregations are caused by precocious segregation of sister chromatids at meiosis I, rather than by nondisjunction as a consequence of lack of crossovers. In situ hybridization further showed that the sister chromatids are separated prematurely during meiotic prophase. Moreover, the mutant forms aberrant linear elements and shows a shortened meiotic prophase. Meiotic chromosome pairing in interstitial and centromeric regions is strongly impaired in rec8-110, whereas the chromosome ends are less deficient in pairing. We propose that the rec8 gene encodes a protein required for linear element formation and that the different phenotypes of rec8-110 reflect direct and indirect consequences of the absence of regular linear elements.     

Mek1 Suppression of Meiotic Double-Strand Break Repair Is Specific to Sister Chromatids,Chromosome Autonomous and Independent of Rec8 Cohesin Complexes

During meiosis, recombination is directed to occur between homologous chromosomes to create connections necessary for proper segregation at meiosis I. Partner choice is determined at the time of strand invasion and is mediated by two recombinases: Rad51 and the meiosis-specific Dmc1. In budding yeast, interhomolog bias is created in part by the activity of a meiosis-specific kinase, Mek1, which is localized to the protein cores of condensed sister chromatids. Analysis of meiotic double-strand break (DSB) repair in haploid and disomic haploid strains reveals that Mek1 suppresses meiotic intersister DSB repair by working directly on sister chromatids. Rec8 cohesin complexes are not required, however, either for suppression of intersister DSB repair or for the repair itself. Regulation of DSB repair in meiosis is chromosome autonomous such that unrepaired breaks on haploid chromosomes do not prevent interhomolog repair between disomic homologs. The pattern of DSB repair in haploids containing Dmc1 and/or Rad51 indicates that Mek1 acts on Rad51-specific recombination processes.IN eukaryotes, meiosis is a specialized type of cell division that produces the gametes required for sexual reproduction. In meiosis, one round of DNA replication is followed by two rounds of chromosome segregation, termed meiosis I and II. As a result of the two divisions, four haploid cells are produced, each containing half the number of chromosomes as the diploid parent. Proper segregation at meiosis I requires connections between homologous chromosomes that are created by a combination of sister chromatid cohesion and recombination (Petronczki et al. 2003). In vegetative cells, cohesion is mediated by multisubunit ring-shaped complexes that are removed by proteolysis of the kleisin subunit, Mcd1/Scc1 (Onn et al. 2008). In meiotic cells, introduction of a meiosis-specific kleisin subunit, Rec8, allows for a two-step removal of cohesion with loss of arm cohesion at anaphase I and centromere cohesion at anaphase II (Klein et al. 1999). Missegregation of chromosomes during meiosis causes abnormal chromosome numbers in gametes that may lead to infertility and genetic disorders such as trisomy 21 or Down''s syndrome.In mitotically dividing budding yeast cells, recombination is mediated by an evolutionarily conserved RecA-like recombinase, Rad51, and occurs preferentially between sister chromatids (Kadyk and Hartwell 1992). In contrast, recombination during meiosis is initiated by the deliberate formation of double-strand breaks (DSBs) by an evolutionarily conserved, topoisomerase-like protein, Spo11, and occurs preferentially between homologous chromosomes (Jackson and Fink 1985; Schwacha and Kleckner 1997; Keeney 2001). After DSB formation, the 5′ ends on either side of the breaks are resected, resulting in 3′ single stranded (ss) tails. Rad51, and the meiosis-specific recombinase Dmc1, bind to the 3′ ssDNA tails to form protein/DNA filaments that promote strand invasion of homologous chromosomes. DNA synthesis and ligation result in the formation of double Holliday junctions, which are then preferentially resolved into crossovers (Allers and Lichten 2001; Hunter 2007).The precise roles that the Rad51 and Dmc1 recombinase activities play in meiotic recombination have been unclear because experiments have indicated both overlapping and distinct functions for the two proteins (Sheridan and Bishop 2006; Hunter 2007). While both rad51Δ and dmc1Δ mutants reduce interhomolog recombination, other studies suggest that Rad51, in complex with the accessory protein Rad54, is involved primarily in intersister DSB repair. In contrast, Dmc1, in conjunction with the accessory protein Rdh54/Tid1 (a paralog of Rad54), effects DSB repair in meiotic cells by invasion of nonsister chromatids (Dresser et al. 1997; Schwacha and Kleckner 1997; Shinohara et al. 1997a,b; Arbel et al. 1999; Bishop et al. 1999; Hayase et al. 2004; Sheridan and Bishop 2006).The preference for recombination to occur between homologous chromosomes during meiosis is created in part by Dmc1. DSBs accumulate in dmc1Δ diploids due to a failure in strand invasion (Bishop et al. 1992; Hunter and Kleckner 2001). In the efficiently sporulating SK1 strain background, these unrepaired breaks trigger the meiotic recombination checkpoint, resulting in prophase arrest (Lydall et al. 1996; Roeder and Bailis 2000). In dmc1Δ mutants, Rad51 is present at DSBs, yet there is no strand invasion of sister chromatids (Bishop 1994; Shinohara et al. 1997a). These results suggest that in addition to Dmc1 promoting interhomolog strand invasion, Rad51 activity must also be suppressed.Recent studies have shown that during meiosis Rad51 recombinase activity is inhibited by two different mechanisms that decrease the formation of Rad51/Rad54 complexes: (1) binding of the meiosis-specific Hed1 protein to Rad51, thereby excluding interaction with Rad54, and (2) reduction in the affinity of Rad54 for Rad51 due to phosphorylation of Rad54 by Mek1 (Tsubouchi and Roeder 2006; Busygina et al. 2008; Niu et al. 2009). Mek1 is a meiosis-specific kinase that is activated in response to DSBs (Niu et al. 2005, 2007; Carballo et al. 2008). In addition to phosphorylating Rad54, Mek1 phosphorylation of an as yet undetermined substrate is required to suppress Rad51/Rad54-mediated strand invasion of sister chromatids (Niu et al. 2009).To dissect the mechanism by which Mek1 suppresses meiotic intersister DSB repair, we took advantage of the ability of yeast cells to undergo haploid meiosis. The lack of homologous chromosomes in haploid cells makes it possible to examine sister-chromatid-specific events in the absence of interhomolog recombination. De Massy et al. (1994) previously observed a delay in DSB repair in haploid cells and proposed that this delay was due to a constraint in using sister chromatids. We have shown that this delay is dependent on MEK1 and utilized the haploid system to determine various biological parameters required to suppress meiotic intersister DSB repair. Our results indicate that Rad51 and Dmc1 recombinase activities have distinct roles during meiosis and that interhomolog bias is established specifically on sister chromatids through regulation of Rad51, not Dmc1. rec8Δ diploids exhibit defects in meiotic DSB repair (Klein et al. 1999; Brar et al. 2009). Given that cohesin complexes are specific for sister chromatids, we investigated the role of REC8 in intersister DSB repair and found it is required neither for suppressing intersister DSB repair during meiosis nor for the repair itself.     

A Simplified Procedure for the Observation In Situ Of Chromosome Aberrations or Sister Chromatid Exchanges and the Estimation of the Mitotic Index in Mammalian Monolayer Cell Cultures

Chinese hamster V-79 cells are widely used in short term screening for potential physical or chemical mutagens of the environment. A simplified version of the standard Giemsa protocol of Moorhead and the Feulgen plus Giemsa protocol of Wolff and Perry is given which permits the observations in situ of chromosome aberrations or sister chromatid exchanges and the estimation of the mitotic index in the Petri dishes used for the culture of the V-79 cells.     

The Essential Function of SETDB1 in Homologous Chromosome Pairing and Synapsis during Meiosis

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Mammalian E-type Cyclins Control Chromosome Pairing,Telomere Stability and CDK2 Localization in Male Meiosis

Loss of function of cyclin E1 or E2, important regulators of the mitotic cell cycle, yields viable mice, but E2-deficient males display reduced fertility. To elucidate the role of E-type cyclins during spermatogenesis, we characterized their expression patterns and produced additional deletions of Ccne1 and Ccne2 alleles in the germline, revealing unexpected meiotic functions. While Ccne2 mRNA and protein are abundantly expressed in spermatocytes, Ccne1 mRNA is present but its protein is detected only at low levels. However, abundant levels of cyclin E1 protein are detected in spermatocytes deficient in cyclin E2 protein. Additional depletion of E-type cyclins in the germline resulted in increasingly enhanced spermatogenic abnormalities and corresponding decreased fertility and loss of germ cells by apoptosis. Profound meiotic defects were observed in spermatocytes, including abnormal pairing and synapsis of homologous chromosomes, heterologous chromosome associations, unrepaired double-strand DNA breaks, disruptions in telomeric structure and defects in cyclin-dependent-kinase 2 localization. These results highlight a new role for E-type cyclins as important regulators of male meiosis.     

水稻第十染色体短臂的等臂四体及其在着丝点定位中的应用

为了确定连锁图上着丝点的位置,常常需要一些特殊的遗传材料,如端部着丝点染色体和等臂染色体材料等等。在水稻上,为了获得这类遗传材料,从中籼３０３７同源三倍体后代产生的非整倍体中,在筛选初级三体的同时,进行了等臂染色体变异材料的筛选。其中在三体１０的后代中发现了１株形态与三体１０相似的变异株,经细胞学检查,证明它是第十染色体短臂的等臂四体。利用该等臂四体及其自交后代中分离出的次级三体,以及相应染色体的初级三体和正常二倍体为材料,通过ＲＦＬＰ不同分子标记在这些材料上所表现剂量效应的不同,确定Ｇ１１２５、Ｇ３３３、Ｌ１６９等３个分子标记位于第十染色体短臂上,Ｇ１０８４及其它１６个分子标记位于第十染色体长臂上。参照Ｋｕｒａｔａ等的分子图谱,将第十染色体的着丝点定位在分子标记Ｇ１１２５和Ｇ１０８４之间,两标记相距３．２ｃＭ［８］。     

Investigation of Homologous Crossing over and Sister Chromatid Exchange in the Wheat Nor-B2 Locus Coding for Rrna and Gli-B2 Locus Coding for Gliadins

Recombination was investigated within the Nor-B2 locus of wheat chromosome 6B that contains several thousand of the 18S-5.8S-26S rRNA (rDNA) repeated units. Additionally, recombination was assessed for several chromosome regions, in arm 6Bq between the centromere and the B2 locus (awn suppressor) and in arm 6Bp between the centromere and Nor-B2, between Nor-B2 and a distal C-band and between Nor-B2 and Gli-B2 coding for gliadins. The experimental design permitted the distinction between crossing over between homologous chromosomes and exchange between sister chromatids. No homologous crossing over within the Nor-B2 locus was found in a sample of 446 chromosomes, but one exchange with the attributes of unequal sister chromatid exchange was identified. The molecular characteristics of this presumed sister chromatid exchange indicate that the spacer variants present in the Nor-B2 locus are clustered. No homologous recombination was detected within the distal Gli-B2 locus containing repeated genes coding for gliadin seed-storage proteins. Both arms of chromosome 6B showed low crossing-over frequency in the proximal regions. The distance from the centromere to Nor-B2 was only from 0.3 to 2.2 cM although it accounts for about two-thirds of the metaphase chromosome arm, which shows a great distortion of the metaphase map of the arm. The level of homologous recombination within the Nor-B2 locus is lower than in the chromosome region immediately distal to it. Whether it is comparable to that in the chromosome region proximal to it could not be determined. Recombination frequencies of different pairs of chromosome 6B in all but one interval paralleled the frequencies of their metaphase I pairing: Lower pairing at metaphase I was paralleled by lower crossing-over frequency. This relationship indicated that reduced metaphase I pairing between 6B chromosomes from different populations is due to impaired crossing-over and not due to precocious chiasma terminalization.     

Mouse HFM1/Mer3 Is Required for Crossover Formation and Complete Synapsis of Homologous Chromosomes during Meiosis

Faithful chromosome segregation during meiosis requires that homologous chromosomes associate and recombine. Chiasmata, the cytological manifestation of recombination, provide the physical link that holds the homologs together as a pair, facilitating their orientation on the spindle at meiosis I. Formation of most crossover (CO) events requires the assistance of a group of proteins collectively known as ZMM. HFM1/Mer3 is in this group of proteins and is required for normal progression of homologous recombination and proper synapsis between homologous chromosomes in a number of model organisms. Our work is the first study in mammals showing the in vivo function of mouse HFM1. Cytological observations suggest that initial steps of recombination are largely normal in a majority of Hfm1^−/− spermatocytes. Intermediate and late stages of recombination appear aberrant, as chromosomal localization of MSH4 is altered and formation of MLH1foci is drastically reduced. In agreement, chiasma formation is reduced, and cells arrest with subsequent apoptosis at diakinesis. Our results indicate that deletion of Hfm1 leads to the elimination of a major fraction but not all COs. Formation of chromosome axial elements and homologous pairing is apparently normal, and Hfm1^−/− spermatocytes progress to the end of prophase I without apparent developmental delay or apoptosis. However, synapsis is altered with components of the central region of the synaptonemal complex frequently failing to extend the full length of the chromosome axes. We propose that initial steps of recombination are sufficient to support homology recognition, pairing, and initial chromosome synapsis and that HFM1 is required to form normal numbers of COs and to complete synapsis.     

Drosophila Centrosomin Protein is Required for Male Meiosis and Assembly of the Flagellar Axoneme

Centrosomes and microtubules play crucial roles during cell division and differentiation. Spermatogenesis is a useful system for studying centrosomal function since it involves both mitosis and meiosis, and also transformation of the centriole into the sperm basal body. Centrosomin is a protein localized to the mitotic centrosomes in Drosophila melanogaster. We have found a novel isoform of centrosomin expressed during spermatogenesis. Additionally, an anticentrosomin antibody labels both the mitotic and meiotic centrosomes as well as the basal body. Mutational analysis shows that centrosomin is required for spindle organization during meiosis and for organization of the sperm axoneme. These results suggest that centrosomin is a necessary component of the meiotic centrosomes and the spermatid basal body.     

The Kinesin AtPSS1 Promotes Synapsis and is Required for Proper Crossover Distribution in Meiosis

Meiotic crossovers (COs) shape genetic diversity by mixing homologous chromosomes at each generation. CO distribution is a highly regulated process. CO assurance forces the occurrence of at least one obligatory CO per chromosome pair, CO homeostasis smoothes out the number of COs when faced with variation in precursor number and CO interference keeps multiple COs away from each other along a chromosome. In several organisms, it has been shown that cytoskeleton forces are transduced to the meiotic nucleus via KASH- and SUN-domain proteins, to promote chromosome synapsis and recombination. Here we show that the Arabidopsis kinesin AtPSS1 plays a major role in chromosome synapsis and regulation of CO distribution. In Atpss1 meiotic cells, chromosome axes and DNA double strand breaks (DSBs) appear to form normally but only a variable portion of the genome synapses and is competent for CO formation. Some chromosomes fail to form the obligatory CO, while there is an increased CO density in competent regions. However, the total number of COs per cell is unaffected. We further show that the kinesin motor domain of AtPSS1 is required for its meiotic function, and that AtPSS1 interacts directly with WIP1 and WIP2, two KASH-domain proteins. Finally, meiocytes missing AtPSS1 and/or SUN proteins show similar meiotic defects suggesting that AtPSS1 and SUNs act in the same pathway. This suggests that forces produced by the AtPSS1 kinesin and transduced by WIPs/SUNs, are required to authorize complete synapsis and regulate maturation of recombination intermediates into COs. We suggest that a form of homeostasis applies, which maintains the total number of COs per cell even if only a part of the genome is competent for CO formation.     

Kinesin‐14 is Important for Chromosome Segregation During Mitosis and Meiosis in the Ciliate Tetrahymena thermophila

Ciliates such as Tetrahymena thermophila have two distinct nuclei within one cell: the micronucleus that undergoes mitosis and meiosis and the macronucleus that undergoes amitosis, a type of nuclear division that does not involve a bipolar spindle, but still relies on intranuclear microtubules. Ciliates provide an opportunity for the discovery of factors that specifically contribute to chromosome segregation based on a bipolar spindle, by identification of factors that affect the micronuclear but not the macronuclear division. Kinesin‐14 is a conserved minus‐end directed microtubule motor that cross‐links microtubules and contributes to the bipolar spindle sizing and organization. Here, we use homologous DNA recombination to knock out genes that encode kinesin‐14 orthologues (KIN141, KIN142) in Tetrahymena. A loss of KIN141 led to severe defects in the chromosome segregation during both mitosis and meiosis but did not affect amitosis. A loss of KIN141 altered the shape of the meiotic spindle in a way consistent with the KIN141's contribution to the organization of the spindle poles. EGFP‐tagged KIN141 preferentially accumulated at the spindle poles during the meiotic prophase and metaphase I. Thus, in ciliates, kinesin‐14 is important for nuclear divisions that involve a bipolar spindle.