STAG2 and Rad21 mammalian mitotic cohesins are implicated in meiosis

STAG/SA proteins are specific cohesin complex subunits that maintain sister chromatid cohesion in mitosis and meiosis. Two members of this family, STAG1/SA1 and STAG2/SA2,double dagger are classified as mitotic cohesins, as they are found in human somatic cells and in Xenopus laevis as components of the cohesin(SA1) and cohesin(SA2) complexes, in which the shared subunits are Rad21/SCC1, SMC1 and SMC3 proteins. A recently reported third family member, STAG3, is germinal cell-specific and is a subunit of the meiotic cohesin complex. To date, the meiosis-specific cohesin complex has been considered to be responsible for sister chromatid cohesion during meiosis. We studied replacement of the mitotic by the meiotic cohesin complex during mouse germinal cell maturation, and we show that mammalian STAG2 and Rad21 are also involved in several meiosis stages. Immunofluorescence results suggest that a cohesin complex containing Rad21 and STAG2 cooperates with a STAG3-specific complex to maintain sister chromatid cohesion during the diplotene stage of meiosis.     

Sequential loading of cohesin subunits during the first meiotic prophase of grasshoppers

The cohesin complexes play a key role in chromosome segregation during both mitosis and meiosis. They establish sister chromatid cohesion between duplicating DNA molecules during S-phase, but they also have an important role during postreplicative double-strand break repair in mitosis, as well as during recombination between homologous chromosomes in meiosis. An additional function in meiosis is related to the sister kinetochore cohesion, so they can be pulled by microtubules to the same pole at anaphase I. Data about the dynamics of cohesin subunits during meiosis are scarce; therefore, it is of great interest to characterize how the formation of the cohesin complexes is achieved in order to understand the roles of the different subunits within them. We have investigated the spatio-temporal distribution of three different cohesin subunits in prophase I grasshopper spermatocytes. We found that structural maintenance of chromosome protein 3 (SMC3) appears as early as preleptotene, and its localization resembles the location of the unsynapsed axial elements, whereas radiation-sensitive mutant 21 (RAD21) (sister chromatid cohesion protein 1, SCC1) and stromal antigen protein 1 (SA1) (sister chromatid cohesion protein 3, SCC3) are not visualized until zygotene, since they are located in the synapsed regions of the bivalents. During pachytene, the distribution of the three cohesin subunits is very similar and all appear along the trajectories of the lateral elements of the autosomal synaptonemal complexes. However, whereas SMC3 also appears over the single and unsynapsed X chromosome, RAD21 and SA1 do not. We conclude that the loading of SMC3 and the non-SMC subunits, RAD21 and SA1, occurs in different steps throughout prophase I grasshopper meiosis. These results strongly suggest the participation of SMC3 in the initial cohesin axis formation as early as preleptotene, thus contributing to sister chromatid cohesion, with a later association of both RAD21 and SA1 subunits at zygotene to reinforce and stabilize the bivalent structure. Therefore, we speculate that more than one cohesin complex participates in the sister chromatid cohesion at prophase I.     

The Cohesion Protein SOLO Associates with SMC1 and Is Required for Synapsis,Recombination, Homolog Bias and Cohesion and Pairing of Centromeres in Drosophila Meiosis

Cohesion between sister chromatids is mediated by cohesin and is essential for proper meiotic segregation of both sister chromatids and homologs. solo encodes a Drosophila meiosis-specific cohesion protein with no apparent sequence homology to cohesins that is required in male meiosis for centromere cohesion, proper orientation of sister centromeres and centromere enrichment of the cohesin subunit SMC1. In this study, we show that solo is involved in multiple aspects of meiosis in female Drosophila. Null mutations in solo caused the following phenotypes: 1) high frequencies of homolog and sister chromatid nondisjunction (NDJ) and sharply reduced frequencies of homolog exchange; 2) reduced transmission of a ring-X chromosome, an indicator of elevated frequencies of sister chromatid exchange (SCE); 3) premature loss of centromere pairing and cohesion during prophase I, as indicated by elevated foci counts of the centromere protein CID; 4) instability of the lateral elements (LE)s and central regions of synaptonemal complexes (SCs), as indicated by fragmented and spotty staining of the chromosome core/LE component SMC1 and the transverse filament protein C(3)G, respectively, at all stages of pachytene. SOLO and SMC1 are both enriched on centromeres throughout prophase I, co-align along the lateral elements of SCs and reciprocally co-immunoprecipitate from ovarian protein extracts. Our studies demonstrate that SOLO is closely associated with meiotic cohesin and required both for enrichment of cohesin on centromeres and stable assembly of cohesin into chromosome cores. These events underlie and are required for stable cohesion of centromeres, synapsis of homologous chromosomes, and a recombination mechanism that suppresses SCE to preferentially generate homolog crossovers (homolog bias). We propose that SOLO is a subunit of a specialized meiotic cohesin complex that mediates both centromeric and axial arm cohesion and promotes homolog bias as a component of chromosome cores.     

A new role for the mitotic RAD21/SCC1 cohesin in meiotic chromosome cohesion and segregation in the mouse

The evolutionarily conserved cohesin complex is required for the establishment and maintenance of sister chromatid cohesion, in turn essential for proper chromosome segregation. RAD21/SCC1 is a regulatory subunit of the mitotic cohesin complex, as it links together all other subunits of the complex. The destruction of RAD21/SCC1 along chromosomal arms and later at centromeres results in the dissociation of the cohesin complex, facilitating chromosome segregation. Here, we report for the first time that mammalian RAD21/SCC1 associates with the axial/lateral elements of the synaptonemal complex along chromosome arms and on centromeres of mouse spermatocytes. Importantly, RAD21/SCC1 is lost from chromosome arms in late prophase I but persists on centromeres. The loss of centromeric RAD21/SCC1 coincides with the separation of sister chromatids at anaphase II. These findings support a role for mammalian RAD21/SCC1 in maintaining sister chromatid cohesion in meiosis.     

Meiotic cohesin STAG3 is required for chromosome axis formation and sister chromatid cohesion

The cohesin complex is essential for mitosis and meiosis. The specific meiotic roles of individual cohesin proteins are incompletely understood. We report in vivo functions of the only meiosis‐specific STAG component of cohesin, STAG3. Newly generated STAG3‐deficient mice of both sexes are sterile with meiotic arrest. In these mice, meiotic chromosome architecture is severely disrupted as no bona fide axial elements (AE) form and homologous chromosomes do not synapse. Axial element protein SYCP3 forms dot‐like structures, many partially overlapping with centromeres. Asynapsis marker HORMAD1 is diffusely distributed throughout the chromatin, and SYCP1, which normally marks synapsed axes, is largely absent. Centromeric and telomeric sister chromatid cohesion are impaired. Centromere and telomere clustering occurs in the absence of STAG3, and telomere structure is not severely affected. Other cohesin proteins are present, localize throughout the STAG3‐devoid chromatin, and form complexes with cohesin SMC1β. No other deficiency in a single meiosis‐specific cohesin causes a phenotype as drastic as STAG3 deficiency. STAG3 emerges as the key STAG cohesin involved in major functions of meiotic cohesin.     

Sisters Unbound Is Required for Meiotic Centromeric Cohesion in Drosophila melanogaster

Regular meiotic chromosome segregation requires sister centromeres to mono-orient (orient to the same pole) during the first meiotic division (meiosis I) when homologous chromosomes segregate, and to bi-orient (orient to opposite poles) during the second meiotic division (meiosis II) when sister chromatids segregate. Both orientation patterns require cohesion between sister centromeres, which is established during meiotic DNA replication and persists until anaphase of meiosis II. Meiotic cohesion is mediated by a conserved four-protein complex called cohesin that includes two structural maintenance of chromosomes (SMC) subunits (SMC1 and SMC3) and two non-SMC subunits. In Drosophila melanogaster, however, the meiotic cohesion apparatus has not been fully characterized and the non-SMC subunits have not been identified. We have identified a novel Drosophila gene called sisters unbound (sunn), which is required for stable sister chromatid cohesion throughout meiosis. sunn mutations disrupt centromere cohesion during prophase I and cause high frequencies of non-disjunction (NDJ) at both meiotic divisions in both sexes. SUNN co-localizes at centromeres with the cohesion proteins SMC1 and SOLO in both sexes and is necessary for the recruitment of both proteins to centromeres. Although SUNN lacks sequence homology to cohesins, bioinformatic analysis indicates that SUNN may be a structural homolog of the non-SMC cohesin subunit stromalin (SA), suggesting that SUNN may serve as a meiosis-specific cohesin subunit. In conclusion, our data show that SUNN is an essential meiosis-specific Drosophila cohesion protein.     

Changes in the expression and localization of cohesin subunits during meiosis in a non-mammalian vertebrate, the medaka fish

Until the onset of anaphase, sister chromatids are bound to each other by a multi-subunit protein complex called cohesin. Since chromosomes in meiosis behave differently from those in mitosis, the cohesion and separation of homologous chromosomes and sister chromatids in meiosis are thought to be regulated by meiosis-specific cohesin subunits. Actually, several meiosis-specific cohesin subunits, including Rec8, STAG3 and SMC1beta, are known to exist in mammals; however, there are no reports of meiosis-specific cohesin subunits in other vertebrates. To investigate the protein expression and localization of cohesin subunits during meiosis in non-mammalian species, we isolated cDNA clones encoding SMC1alpha, SMC1beta, SMC3 and Rad21 in the medaka and produced antibodies against recombinant proteins. Medaka SMC1beta was expressed solely in gonads, while SMC1alpha, SMC3 and Rad21 were also expressed in other organs and in cultured cells. SMC1beta forms a complex with SMC3 but not with Rad21, in contrast to SMC1alpha, which forms complexes with both SMC3 and Rad21. SMC1alpha and Rad21 were mainly expressed in mitotically dividing cells in the testis (somatic cells and spermatogonia), although their weak expression was detected in pre-leptotene spermatocytes. SMC1beta was expressed in spermatogonia and spermatocytes. SMC1beta was localized along the chromosomal arms as well as on the centromeres in meiotic prophase I, and its existence on the chromosomes persisted up to metaphase II, a situation different from that reported in the mouse, in which SMC1beta is lost from the chromosome arms in late pachytene despite its universal presence in vertebrates.     

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.     

Casein Kinase 1 and Phosphorylation of Cohesin Subunit Rec11 (SA3) Promote Meiotic Recombination through Linear Element Formation

Proper meiotic chromosome segregation, essential for sexual reproduction, requires timely formation and removal of sister chromatid cohesion and crossing-over between homologs. Early in meiosis cohesins hold sisters together and also promote formation of DNA double-strand breaks, obligate precursors to crossovers. Later, cohesin cleavage allows chromosome segregation. We show that in fission yeast redundant casein kinase 1 homologs, Hhp1 and Hhp2, previously shown to regulate segregation via phosphorylation of the Rec8 cohesin subunit, are also required for high-level meiotic DNA breakage and recombination. Unexpectedly, these kinases also mediate phosphorylation of a different meiosis-specific cohesin subunit Rec11. This phosphorylation in turn leads to loading of linear element proteins Rec10 and Rec27, related to synaptonemal complex proteins of other species, and thereby promotes DNA breakage and recombination. Our results provide novel insights into the regulation of chromosomal features required for crossing-over and successful reproduction. The mammalian functional homolog of Rec11 (STAG3) is also phosphorylated during meiosis and appears to be required for fertility, indicating wide conservation of the meiotic events reported here.     

Meiotic cohesin REC8 marks the axial elements of rat synaptonemal complexes before cohesins SMC1beta and SMC3

In meiotic prophase, the sister chromatids of each chromosome develop a common axial element (AE) that is integrated into the synaptonemal complex (SC). We analyzed the incorporation of sister chromatid cohesion proteins (cohesins) and other AE components into AEs. Meiotic cohesin REC8 appeared shortly before premeiotic S phase in the nucleus and formed AE-like structures (REC8-AEs) from premeiotic S phase on. Subsequently, meiotic cohesin SMC1beta, cohesin SMC3, and AE proteins SCP2 and SCP3 formed dots along REC8-AEs, which extended and fused until they lined REC8-AEs along their length. In metaphase I, SMC1beta, SMC3, SCP2, and SCP3 disappeared from the chromosome arms and accumulated around the centromeres, where they stayed until anaphase II. In striking contrast, REC8 persisted along the chromosome arms until anaphase I and near the centromeres until anaphase II. We propose that REC8 provides a basis for AE formation and that the first steps in AE assembly do not require SMC1beta, SMC3, SCP2, and SCP3. Furthermore, SMC1beta, SMC3, SCP2, and SCP3 cannot provide arm cohesion during metaphase I. We propose that REC8 then provides cohesion. RAD51 and/or DMC1 coimmunoprecipitates with REC8, suggesting that REC8 may also provide a basis for assembly of recombination complexes.     

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.     

STAG3‐mediated stabilization of REC8 cohesin complexes promotes chromosome synapsis during meiosis

Cohesion between sister chromatids in mitotic and meiotic cells is promoted by a ring‐shaped protein structure, the cohesin complex. The cohesin core complex is composed of four subunits, including two structural maintenance of chromosome (SMC) proteins, one α‐kleisin protein, and one SA protein. Meiotic cells express both mitotic and meiosis‐specific cohesin core subunits, generating cohesin complexes with different subunit composition and possibly separate meiotic functions. Here, we have analyzed the in vivo function of STAG3, a vertebrate meiosis‐specific SA protein. Mice with a hypomorphic allele of Stag3, which display a severely reduced level of STAG3, are viable but infertile. We show that meiocytes in homozygous mutant Stag3 mice display chromosome axis compaction, aberrant synapsis, impaired recombination and developmental arrest. We find that the three different α‐kleisins present in meiotic cells show different dosage‐dependent requirements for STAG3 and that STAG3‐REC8 cohesin complexes have a critical role in supporting meiotic chromosome structure and functions.     

Identification and molecular characterization of the mammalian α-kleisin RAD21L

Meiosis is a fundamental process that generates new combinations between maternal and paternal genomes and haploid gametes from diploid progenitors. Many of the meiosis-specific events stem from the behavior of the cohesin complex (CC), a proteinaceous ring structure that entraps sister chromatids until the onset of anaphase. CCs ensure chromosome segregation, participate in DNA repair, regulate gene expression, and also contribute to synaptonemal complex (SC) formation at meiosis by keeping long-range distant DNA interactions through its conserved structure. Studies from yeast to humans have led to the assumption that Scc1/RAD21 is the α-kleisin that closes the tripartite CC that entraps two DNA molecules in mitosis, while its paralog REC8 is essential for meiosis. Here we describe the identification of RAD21L, a novel mammalian CC subunit with homology to the RAD21/REC8 α-kleisin subfamily, which is expressed in mouse testis. RAD21L interacts with other cohesin subunits such as SMC1α, SMC1b, SMC3 and with the meiosis-specific STAG3 protein. Thus, our results demonstrate the existence of a new meiotic-specific CC constituted by this α-kleisin and expand the view of REC8 as the only specific meiotic α-kleisin.     

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.     

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.     

The cohesin REC8 prevents illegitimate inter‐sister synaptonemal complex assembly

During meiosis, a specialized chromosome structure is assembled to promote pairing/synapsis of homologous chromosomes and meiotic recombination, a process yielding chiasmata between homologs to ensure accurate segregation. Meiosis‐specific cohesin complexes mediating sister chromatid cohesion play pivotal roles in almost all these events, including synaptonemal complex (SC) formation. In this issue of EMBO Reports, Agostinho and colleagues have examined chromosome axes and SC structures by taking advantage of a hypomorphic Stag3 mutant in which the levels of the cohesin subunit REC8 are partly reduced 6 . Using super‐resolution microscopy, the authors illuminate previously unforeseen chromosome axis structures, showing locally separated axes in regions where REC8 is absent, regardless of RAD21L or RAD21 cohesin localization. Furthermore, they assessed the relationship between sister chromatid cohesion and inter‐sister SC formation, demonstrating that “axial opening” in the REC8‐free region is accompanied by illegitimate SC formation between sister chromatids. This study highlights the physiological importance of REC8 in sister chromatid cohesion and proper SC formation during meiosis, suggesting a new model in which a high density of REC8 deposition along the chromosome prevents illegitimate inter‐sister SC formation.     

Functional links between <Emphasis Type="Italic">Drosophila</Emphasis> Nipped-B and cohesin in somatic and meiotic cells

Drosophila Nipped-B is an essential protein that has multiple functions. It facilitates expression of homeobox genes and is also required for sister chromatid cohesion. Nipped-B is conserved from yeast to man, and its orthologs also play roles in deoxyribonucleic acid repair and meiosis. Mutation of the human ortholog, Nipped-B-Like (NIPBL), causes Cornelia de Lange syndrome (CdLS), associated with multiple developmental defects. The Nipped-B protein family is required for the cohesin complex that mediates sister chromatid cohesion to bind to chromosomes. A key question, therefore, is whether the Nipped-B family regulates gene expression, meiosis, and development by controlling cohesin. To gain insights into Nipped-B’s functions, we compared the effects of several Nipped-B mutations on gene expression, sister chromatid cohesion, and meiosis. We also examined association of Nipped-B and cohesin with somatic and meiotic chromosomes by immunostaining. Missense Nipped-B alleles affecting the same HEAT repeat motifs as CdLS-causing NIPBL mutations have intermediate effects on both gene expression and mitotic chromatid cohesion, linking these two functions and the role of NIPBL in human development. Nipped-B colocalizes extensively with cohesin on chromosomes in both somatic and meiotic cells and is present in soluble complexes with cohesin subunits in nuclear extracts. In meiosis, Nipped-B also colocalizes with the synaptonemal complex and contributes to maintenance of meiotic chromosome cores. These results support the idea that direct regulation of cohesin function underlies the diverse functions of Nipped-B and its orthologs. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Gause, Webber, and Misulovin provided equal contributions.     

Pds5 is required for homologue pairing and inhibits synapsis of sister chromatids during yeast meiosis

During meiosis, homologues become juxtaposed and synapsed along their entire length. Mutations in the cohesin complex disrupt not only sister chromatid cohesion but also homologue pairing and synaptonemal complex formation. In this study, we report that Pds5, a cohesin-associated protein known to regulate sister chromatid cohesion, is required for homologue pairing and synapsis in budding yeast. Pds5 colocalizes with cohesin along the length of meiotic chromosomes. In the absence of Pds5, the meiotic cohesin subunit Rec8 remains bound to chromosomes with only minor defects in sister chromatid cohesion, but sister chromatids synapse instead of homologues. Double-strand breaks (DSBs) are formed but are not repaired efficiently. In addition, meiotic chromosomes undergo hypercondensation. When the mitotic cohesin subunit Mcd1 is substituted for Rec8 in Pds5-depleted cells, chromosomes still hypercondense, but synapsis of sister chromatids is abolished. These data suggest that Pds5 modulates the Rec8 activity to facilitate chromosome morphological changes required for homologue synapsis, DSB repair, and meiotic chromosome segregation.     

Dynamics of cohesin subunits in grasshopper meiotic divisions

The cohesin complex plays a key role for the maintenance of sister chromatid cohesion and faithful chromosome segregation in both mitosis and meiosis. This complex is formed by two structural maintenance of chromosomes protein family (SMC) subunits and two non-SMC subunits: an α-kleisin subunit SCC1/RAD21/REC8 and an SCC3-like protein. Several studies carried out in different species have revealed that the distribution of the cohesin subunits along the chromosomes during meiotic prophase I is not regular and that some subunits are distinctly incorporated at different cell stages. However, the accurate distribution of the different cohesin subunits in condensed meiotic chromosomes is still controversial. Here, we describe the dynamics of the cohesin subunits SMC1α, SMC3, RAD21 and SA1 during both meiotic divisions in grasshoppers. Although these subunits show a similar patched labelling at the interchromatid domain of metaphase I bivalents, SMCs and non-SMCs subunits do not always colocalise. Indeed, SA1 is the only cohesin subunit accumulated at the centromeric region of all metaphase I chromosomes. Additionally, non-SMC subunits do not appear at the interchromatid domain in either single X or B chromosomes. These data suggest the existence of several cohesin complexes during metaphase I. The cohesin subunits analysed are released from chromosomes at the beginning of anaphase I, with the exception of SA1 which can be detected at the centromeres until telophase II. These observations indicate that the cohesin components may be differentially loaded and released from meiotic chromosomes during the first and second meiotic divisions. The roles of these cohesin complexes for the maintenance of chromosome structure and their involvement in homologous segregation at first meiotic division are proposed and discussed.     

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.