SHUGOSHINs and PATRONUS protect meiotic centromere cohesion in Arabidopsis thaliana

In meiosis, chromosome cohesion is maintained by the cohesin complex, which is released in a two‐step manner. At meiosis I, the meiosis‐specific cohesin subunit Rec8 is cleaved by the protease Separase along chromosome arms, allowing homologous chromosome segregation. Next, in meiosis II, cleavage of the remaining centromere cohesin results in separation of the sister chromatids. In eukaryotes, protection of centromeric cohesion in meiosis I is mediated by SHUGOSHINs (SGOs). The Arabidopsis genome contains two SGO homologs. Here we demonstrate that Atsgo1 mutants show a premature loss of cohesion of sister chromatid centromeres at anaphase I and that AtSGO2 partially rescues this loss of cohesion. In addition to SGOs, we characterize PATRONUS which is specifically required for the maintenance of cohesion of sister chromatid centromeres in meiosis II. In contrast to the Atsgo1 Atsgo2 double mutant, patronus T‐DNA insertion mutants only display loss of sister chromatid cohesion after meiosis I, and additionally show disorganized spindles, resulting in defects in chromosome segregation in meiosis. This leads to reduced fertility and aneuploid offspring. Furthermore, we detect aneuploidy in sporophytic tissue, indicating a role for PATRONUS in chromosome segregation in somatic cells. Thus, ploidy stability is preserved in Arabidopsis by PATRONUS during both meiosis and mitosis.     

Rec8 cleavage by separase is required for meiotic nuclear divisions in fission yeast

Sister chromatid cohesion in meiosis is established by cohesin complexes, including the Rec8 subunit. During meiosis I, sister chromatid cohesion is destroyed along the chromosome arms to release connections of recombined homologous chromosomes (homologues), whereas centromeric cohesion persists until it is finally destroyed at anaphase II. In fission yeast, as in mammals, distinct cohesin complexes are used depending on the chromosomal region; Rec8 forms a complex with Rec11 (equivalent to SA3) mainly along chromosome arms, while Psc3 (equivalent to SA1 and SA2) forms a complex mainly in the vicinity of the centromeres. Here we show that separase activation and resultant Rec8 cleavage are required for meiotic chromosome segregation in fission yeast. A non-cleavable form of Rec8 blocks disjunction of homologues at meiosis I. However, displacing non-cleavable Rec8 restrictively from the chromosome arm by genetically depleting Rec11 alleviated the blockage of homologue segregation, but not of sister segregation. We propose that the segregation of homologues at meiosis I and of sisters at meiosis II requires the cleavage of Rec8 along chromosome arms and at the centromeres, respectively.     

The aurora B kinase AIR-2 regulates kinetochores during mitosis and is required for separation of homologous Chromosomes during meiosis

BACKGROUND: Mitotic chromosome segregation depends on bi-orientation and capture of sister kinetochores by microtubules emanating from opposite spindle poles and the near synchronous loss of sister chromatid cohesion. During meiosis I, in contrast, sister kinetochores orient to the same pole, and homologous kinetochores are captured by microtubules emanating from opposite spindle poles. Additionally, mechanisms exist that prevent complete loss of cohesion during meiosis I. These features ensure that homologs separate during meiosis I and sister chromatids remain together until meiosis II. The mechanisms responsible for orienting kinetochores in mitosis and for causing asynchronous loss of cohesion during meiosis are not well understood. RESULTS: During mitosis in C. elegans, aurora B kinase, AIR-2, is not required for sister chromatid separation, but it is required for chromosome segregation. Condensin recruitment during metaphase requires AIR-2; however, condensin functions during prometaphase, independent of AIR-2. During metaphase, AIR-2 promotes chromosome congression to the metaphase plate, perhaps by inhibiting attachment of chromatids to both spindle poles. During meiosis in AIR-2-depleted oocytes, congression of bivalents appears normal, but segregation fails. Localization of AIR-2 on meiotic bivalents suggests this kinase promotes separation of homologs by promoting the loss of cohesion distal to the single chiasma. Inactivation of the phosphatase that antagonizes AIR-2 causes premature separation of chromatids during meiosis I, in a separase-dependent reaction. CONCLUSIONS: Aurora B functions to resolve chiasmata during meiosis I and to regulate kinetochore function during mitosis. Condensin mediates chromosome condensation during prophase, and condensin-independent pathways contribute to chromosome condensation during metaphase.     

Sgo1 is required for co-segregation of sister chromatids during achiasmate meiosis I

The reduction of chromosome number during meiosis is achieved by two successive rounds of chromosome segregation, called meiosis I and meiosis II. While meiosis II is similar to mitosis in that sister kinetochores are bi-oriented and segregate to opposite poles, recombined homologous chromosomes segregate during the first meiotic division. Formation of chiasmata, mono-orientation of sister kinetochores and protection of centromeric cohesion are three major features of meiosis I chromosomes which ensure the reductional nature of chromosome segregation. Here we show that sister chromatids frequently segregate to opposite poles during meiosis I in fission yeast cells that lack both chiasmata and the protector of centromeric cohesion Sgo1. Our data are consistent with the notion that sister kinetochores are frequently bi-oriented in the absence of chiasmata and that Sgo1 prevents equational segregation of sister chromatids during achiasmate meiosis I.Key words: meiosis, chromosome segregation, recombination, kinetochore, Sgo1, fission yeast     

Sgo1 is required for co-segregation of sister chromatids during achiasmate meiosis I

The reduction of chromosome number during meiosis is achieved by two successive rounds of chromosome segregation, called meiosis I and meiosis II. While meiosis II is similar to mitosis in that sister kinetochores are bi-oriented and segregate to opposite poles, recombined homologous chromosomes segregate during the first meiotic division. Formation of chiasmata, mono-orientation of sister kinetochores and protection of centromeric cohesion are three major features of meiosis I chromosomes which ensure the reductional nature of chromosome segregation. Here we show that sister chromatids frequently segregate to opposite poles during meiosis I in fission yeast cells that lack both chiasmata and the protector of centromeric cohesion Sgo1. Our data are consistent with the notion that sister kinetochores are frequently bi-oriented in the absence of chiasmata and that Sgo1 prevents equational segregation of sister chromatids during achiasmate meiosis I.     

Sister chromatid cohesion and recombination in meiosis

Sister chromatids are associated from their formation until their disjunction. Cohesion between sister chromatids is provided by protein complexes, of which some components are conserved across the kingdoms and between the mitotic and meiotic cell cycles. Sister chromatid cohesion is intimately linked to other aspects of chromosome behaviour and metabolism, in particular chromosome condensation, recombination and segregation. Recombination, sister chromatid cohesion and the relation between the two processes must be regulated differently in mitosis and meiosis. In meiosis, cohesion and recombination are modified in such a way that reciprocal exchange and reductional segregation of homologous chromosomes are ensured. Received: 11 October 1999; in revised form: 3 December 1999 / Accepted: 6 December 1999     

Novel genes required for meiotic chromosome segregation are identified by a high-throughput knockout screen in fission yeast

Two rounds of chromosome segregation after only a single round of DNA replication enable the production of haploid gametes from diploid precursors during meiosis. To identify genes involved in meiotic chromosome segregation, we developed an efficient strategy to knock out genes in the fission yeast on a large scale. We used this technique to delete 180 functionally uncharacterized genes whose expression is upregulated during meiosis. Deletion of two genes, sgo1 and mde2, caused massive chromosome missegregation. sgo1 is required for retention of centromeric sister-chromatid cohesion after anaphase I. We show here that mde2 is required for formation of the double-strand breaks necessary for meiotic recombination.     

Spo13 maintains centromeric cohesion and kinetochore coorientation during meiosis I

BACKGROUND: The meiotic cell cycle, the cell division cycle that leads to the generation of gametes, is unique in that a single DNA replication phase is followed by two chromosome segregation phases. During meiosis I, homologous chromosomes are segregated, and during meiosis II, as in mitosis, sister chromatids are partitioned. For homolog segregation to occur during meiosis I, physical linkages called chiasmata need to form between homologs, sister chromatid cohesion has to be lost in a stepwise manner, and sister kinetochores must attach to microtubules emanating from the same spindle pole (coorientation). RESULTS: Here we show that the meiosis-specific factor Spo13 functions in two key aspects of meiotic chromosome segregation. In cells lacking SPO13, cohesin, which is the protein complex that holds sister chromatids together, is not protected from removal around kinetochores during meiosis I but is instead lost along the entire length of the chromosomes. We furthermore find that Spo13 promotes sister kinetochore coorientation by maintaining the monopolin complex at kinetochores. In the absence of SPO13, Mam1 and Lrs4 disassociate from kinetochores prematurely during pro-metaphase I and metaphase I, resulting in a partial defect in sister kinetochore coorientation in spo13 Delta cells. CONCLUSIONS: Our results indicate that Spo13 has the ability to regulate both the stepwise loss of sister chromatid cohesion and kinetochore coorientation, two essential features of meiotic chromosome segregation.     

Reinterpreting pericentromeric heterochromatin

In fission yeast, pericentromeric heterochromatin is directly responsible for the sister chromatid cohesion that assures accurate chromosome segregation. In plants, however, heterochromatin and chromosome segregation appear to be largely unrelated: chromosome transmission is impaired by mutations in cohesion but not by mutations that affect heterochromatin formation. We argue that the formation of pericentromeric heterochromatin is primarily a response to constraints on chromosome mechanics that disfavor the transmission of recombination events in pericentromeric regions. This effect allows pericentromeres to expand to enormous sizes by the accumulation of transposons and through large-scale insertions and inversions. Although sister chromatid cohesion is spatially limited to pericentromeric regions at mitosis and meiosis II, the cohesive domains appear to be defined independently of heterochromatin. The available data from plants suggest that sister chromatid cohesion is marked by histone phosphorylation and mediated by Aurora kinases.     

Separase is required for chromosome segregation during meiosis I in Caenorhabditis elegans.

BACKGROUND: Chromosome segregation during mitosis and meiosis is triggered by dissolution of sister chromatid cohesion, which is mediated by the cohesin complex. Mitotic sister chromatid disjunction requires that cohesion be lost along the entire length of chromosomes, whereas homolog segregation at meiosis I only requires loss of cohesion along chromosome arms. During animal cell mitosis, cohesin is lost in two steps. A nonproteolytic mechanism removes cohesin along chromosome arms during prophase, while the proteolytic cleavage of cohesin's Scc1 subunit by separase removes centromeric cohesin at anaphase. In Saccharomyces cerevisiae and Caenorhabditis elegans, meiotic sister chromatid cohesion is mediated by Rec8, a meiosis-specific variant of cohesin's Scc1 subunit. Homolog segregation in S. cerevisiae is triggered by separase-mediated cleavage of Rec8 along chromosome arms. In principle, chiasmata could be resolved proteolytically by separase or nonproteolytically using a mechanism similar to the mitotic "prophase pathway." RESULTS: Inactivation of separase in C. elegans has little or no effect on homolog alignment on the meiosis I spindle but prevents their timely disjunction. It also interferes with chromatid separation during subsequent embryonic mitotic divisions but does not directly affect cytokinesis. Surprisingly, separase inactivation also causes osmosensitive embryos, possibly due to a defect in the extraembryonic structures, referred to as the "eggshell." CONCLUSIONS: Separase is essential for homologous chromosome disjunction during meiosis I. Proteolytic cleavage, presumably of Rec8, might be a common trigger for the first meiotic division in eukaryotic cells. Cleavage of proteins other than REC-8 might be necessary to render the eggshell impermeable to solutes.     

Molecular insights into mechanisms regulating faithful chromosome separation in female meiosis

The faithful segregation of chromosomes into daughter cells in meiosis is crucial to produce healthy progeny. In gametogenesis, two consecutive rounds of chromosome separation occur with only one round of DNA replication, and the chromosome number is reduced to half to produce haploid gametes. Here, we discuss the molecular mechanisms underlying faithful chromosome separation in meiosis from three aspects: Spindle checkpoint, two-step releases of cohesion, and the specific space-time protection of cohesin.     

Cohesin SMC1 beta is required for meiotic chromosome dynamics, sister chromatid cohesion and DNA recombination

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.     

Genetic interactions between mei-S332 and ord in the control of sister-chromatid cohesion.

The Drosophila mei-S332 and ord gene products are essential for proper sister-chromatid cohesion during meiosis in both males and females. We have constructed flies that contain null mutations for both genes. Double-mutant flies are viable and fertile. Therefore, the lack of an essential role for either gene in mitotic cohesion cannot be explained by compensatory activity of the two proteins during mitotic divisions. Analysis of sex chromosome segregation in the double mutant indicates that ord is epistatic to mei-S332. We demonstrate that ord is not required for MEI-S332 protein to localize to meiotic centromeres. Although overexpression of either protein in a wild-type background does not interfere with normal meiotic chromosome segregation, extra ORD+ protein in mei-S332 mutant males enhances nondisjunction at meiosis II. Our results suggest that a balance between the activity of mei-S332 and ord is required for proper regulation of meiotic cohesion and demonstrate that additional proteins must be functioning to ensure mitotic sister-chromatid cohesion.     

A REC8-dependent plant Shugoshin is required for maintenance of centromeric cohesion during meiosis and has no mitotic functions

During meiosis, sequential release of sister chromatid cohesion (SSC) during two successive nuclear divisions allows the production of haploid gametes from diploid progenitor cells. Release of SSC along chromosome arms allows first a reductional segregation of homologs, and, subsequently, release of centromeric cohesion at anaphase II allows the segregation of chromatids. The Shugoshin (SGO) protein family plays a major role in the protection of centromeric cohesion in Drosophila and yeast. We have isolated a maize mutant that displays premature loss of centromeric cohesion at anaphase I. We showed that this phenotype is due to the absence of ZmSGO1 protein, a maize shugoshin homolog. We also show that ZmSGO1 is localized to the centromeres. The ZmSGO1 protein is not found on mitotic chromosomes and has no obvious mitotic function. On the basis of these results, we propose that ZmSGO1 specifically maintains centromeric cohesion during meiosis I and therefore suggest that SGO1 core functions during meiosis are conserved across kingdoms and in large-genome species. However, in contrast to other Shugoshins, we observed an early and REC8-dependent recruitment of ZmSGO1 in maize, suggesting that control of SGO1 recruitment to chromosomes is different in plants than in other model organisms.     

Induced chromosomal exchange directs the segregation of recombinant chromatids in mitosis of Drosophila.

In meiosis, the segregation of chromosomes at the reductional division is accomplished by first linking homologs together. Genetic exchange generates the bivalents that direct regular chromosome segregation. We show that genetic exchange in mitosis also generates bivalents and that these bivalents direct mitotic chromosome segregation. After FLP-mediated homologous recombination in G2 of the cell cycle, recombinant chromatids consistently segregate away from each other (x segregation). This pattern of segregation also applies to exchange between heterologs. Most, or all, cases of non-x segregation are the result of exchange in G1. Cytological evidence is presented that confirms the existence of the bivalents that direct this pattern of segregation. Our results implicate sister chromatid cohesion in maintenance of the bivalent. The pattern of chromatid segregation can be altered by providing an additional FRT at a more proximal site on one chromosome. We propose that sister chromatid exchange occurs at the more proximal site, allowing the recombinant chromatids to segregate together. This also allowed the recovery of reciprocal translocations following FLP-mediated heterologous recombination. The observation that exchange can generate a bivalent in mitotic divisions provides support for a simple evolutionary relationship between mitosis and meiosis.     

Holocentric plant meiosis: first sisters,then homologues

Meiosis is a crucial process of sexual reproduction by forming haploid gametes from diploid precursor cells. It involves 2 subsequent divisions (meiosis I and meiosis II) after one initial round of DNA replication. Homologous monocentric chromosomes are separated during the first and sister chromatids during the second meiotic division. The faithful segregation of monocentric chromosomes is realized by mono-orientation of fused sister kinetochores at metaphase I and by bi-orientation of sister kinetochores at metaphase II. Conventionally this depends on a 2-step loss of cohesion, along chromosome arms during meiosis I and at sister centromeres during meiosis II.     

Multiple subunits of the Caenorhabditis elegans anaphase-promoting complex are required for chromosome segregation during meiosis I

Two genes, originally identified in genetic screens for Caenorhabditis elegans mutants that arrest in metaphase of meiosis I, prove to encode subunits of the anaphase-promoting complex or cyclosome (APC/C). RNA interference studies reveal that these and other APC/C subunits are essential for the segregation of chromosomal homologs during meiosis I. Further, chromosome segregation during meiosis I requires APC/C functions in addition to the release of sister chromatid cohesion.     

Polo-like kinase Cdc5 promotes chiasmata formation and cosegregation of sister centromeres at meiosis I

During meiosis, two rounds of chromosome segregation occur after a single round of DNA replication, producing haploid progeny from diploid progenitors. Three innovations in chromosome behaviour during meiosis I accomplish this unique division. First, crossovers between maternal and paternal sister chromatids (detected cytologically as chiasmata) bind replicated maternal and paternal chromosomes together. Second, sister kinetochores attach to microtubules from the same pole (mono-polar orientation), causing maternal and paternal centromere pairs (and not sister chromatids) to be separated. Third, sister chromatid cohesion near centromeres is preserved at anaphase I when cohesion along chromosome arms is destroyed. The finding that destruction of mitotic cohesion is regulated by Polo-like kinases prompted us to investigate the meiotic role of the yeast Polo-like kinase Cdc5. We show here that cells lacking Cdc5 synapse homologues and initiate recombination normally, but fail to efficiently resolve recombination intermediates as crossovers. They also fail to properly localize the Lrs4 (ref. 3) and Mam1 (ref. 4) monopolin proteins, resulting in bipolar orientation of sister kinetochores. Cdc5 is thus required both for the formation of chiasmata and for cosegregation of sister centromeres at meiosis I.     

Un ménage à quatre: the molecular biology of chromosome segregation in meiosis

Sexually reproducing organisms rely on the precise reduction of chromosome number during a specialized cell division called meiosis. Whereas mitosis produces diploid daughter cells from diploid cells, meiosis generates haploid gametes from diploid precursors. The molecular mechanisms controlling chromosome transmission during both divisions have started to be delineated. This review focuses on the four fundamental differences between mitotic and meiotic chromosome segregation that allow the ordered reduction of chromosome number in meiosis: (1) reciprocal recombination and formation of chiasmata between homologous chromosomes, (2) suppression of sister kinetochore biorientation, (3) protection of centromeric cohesion, and (4) inhibition of DNA replication between the two meiotic divisions.