The Saccharomyces cerevisiae centromere protein Slk19p is required for two successive divisions during meiosis

Saccharomyces cerevisiae deficient in the early anaphase release of Cdc14 can traverse anaphase I without ribosomal DNA disjunction and successfully complete meiosis

Eukaryotic meiosis is a specialized cell cycle of two nuclear divisions that give rise to haploid gametes. The phosphatase Cdc14 is essential for meiosis in the yeast Saccharomyces cerevisiae. Cdc14 is sequestered in the nucleolus, a nuclear domain containing the ribosomal DNA, by its binding partner Net1, and released in two distinct waves, first in early anaphase I, then in anaphase II. Current models posit that the meiosis I release is required for ribosomal DNA disjunction, disassembly of the anaphase spindle, spindle pole re-duplication and counteraction of cyclin-dependent kinase, all of which are essential events. We examined Cdc14 release in net1-6cdk mutant cells, which lack six key Net1 CDK phosphorylation sites. Cdc14 release in early anaphase I was partially inhibited, and disjunction of the rDNA was fully inhibited. Failure to disjoin the rDNA is lethal in mitosis, and we expected the same to be true for meiosis I. However, the cells reliably completed both meiotic divisions to produce four viable spores. Therefore, segregation of the rDNA into all four meiotic products can be postponed until meiosis II without decreasing the fidelity of chromosome inheritance.

Meiosis in budding yeast

Meiosis is a specialized cell division program that is essential for sexual reproduction. The two meiotic divisions reduce chromosome number by half, typically generating haploid genomes that are packaged into gametes. To achieve this ploidy reduction, meiosis relies on highly unusual chromosomal processes including the pairing of homologous chromosomes, assembly of the synaptonemal complex, programmed formation of DNA breaks followed by their processing into crossovers, and the segregation of homologous chromosomes during the first meiotic division. These processes are embedded in a carefully orchestrated cell differentiation program with multiple interdependencies between DNA metabolism, chromosome morphogenesis, and waves of gene expression that together ensure the correct number of chromosomes is delivered to the next generation. Studies in the budding yeast Saccharomyces cerevisiae have established essentially all fundamental paradigms of meiosis-specific chromosome metabolism and have uncovered components and molecular mechanisms that underlie these conserved processes. Here, we provide an overview of all stages of meiosis in this key model system and highlight how basic mechanisms of genome stability, chromosome architecture, and cell cycle control have been adapted to achieve the unique outcome of meiosis.

The Cdc14 Phosphatase Controls Resolution of Recombination Intermediates and Crossover Formation during Meiosis

Meiotic defects derived from incorrect DNA repair during gametogenesis can lead to mutations, aneuploidies and infertility. The coordinated resolution of meiotic recombination intermediates is required for crossover formation, ultimately necessary for the accurate completion of both rounds of chromosome segregation. Numerous master kinases orchestrate the correct assembly and activity of the repair machinery. Although much less is known, the reversal of phosphorylation events in meiosis must also be key to coordinate the timing and functionality of repair enzymes. Cdc14 is a crucial phosphatase required for the dephosphorylation of multiple CDK1 targets in many eukaryotes. Mutations that inactivate this phosphatase lead to meiotic failure, but until now it was unknown if Cdc14 plays a direct role in meiotic recombination. Here, we show that the elimination of Cdc14 leads to severe defects in the processing and resolution of recombination intermediates, causing a drastic depletion in crossovers when other repair pathways are compromised. We also show that Cdc14 is required for the correct activity and localization of the Holliday Junction resolvase Yen1/GEN1. We reveal that Cdc14 regulates Yen1 activity from meiosis I onwards, and this function is essential for crossover resolution in the absence of other repair pathways. We also demonstrate that Cdc14 and Yen1 are required to safeguard sister chromatid segregation during the second meiotic division, a late action that is independent of the earlier role in crossover formation. Thus, this work uncovers previously undescribed functions of the evolutionary conserved Cdc14 phosphatase in the regulation of meiotic recombination.

Meiosis-Specific Functions of Kinesin Motors in Cohesin Removal and Maintenance of Chromosome Integrity in Budding Yeast

Kinesin motors provide the molecular forces at the kinetochore-microtubule interface and along the spindle to control chromosome segregation. During meiosis with two rounds of microtubule assembly-disassembly, the roles of motor proteins remain unexplored. We observed that in contrast to mitosis, Cin8 and Kip3 together are indispensable for meiosis. While examining meiosis in cin8Δ kip3Δ cells, we detected chromosome breakage in the meiosis II cells. The double mutant exhibits a delay in cohesin removal during anaphase I. Consequently, some cells fail to undergo meiosis II and form dyads, while some, as they progress through meiosis II, cause a defect in chromosome integrity. We believe that in the latter cells, an imbalance of spindle-mediated force and the simultaneous persistence of cohesin on chromosomes cause their breakage. We provide evidence that tension generated by Cin8 and Kip3 through microtubule cross-linking is essential for signaling efficient cohesin removal and the maintenance of chromosome integrity during meiosis.

Slk19p of Saccharomyces cerevisiae regulates anaphase spindle dynamics through two independent mechanisms

Slk19p is a member of the Cdc-14 early anaphase release (FEAR) pathway, a signaling network that is responsible for activation of the cell-cycle regulator Cdc14p in Saccharomyces cerevisiae. Disruption of the FEAR pathway results in defects in anaphase, including alterations in the assembly and behavior of the anaphase spindle. Many phenotypes of slk19Δ mutants are consistent with a loss of FEAR signaling, but other phenotypes suggest that Slk19p may have FEAR-independent roles in modulating the behavior of microtubules in anaphase. Here, a series of SLK19 in-frame deletion mutations were used to test whether Slk19p has distinct roles in anaphase that can be ascribed to specific regions of the protein. Separation-of-function alleles were identified that are defective for either FEAR signaling or aspects of anaphase spindle function. The data suggest that in early anaphase one region of Slk19p is essential for FEAR signaling, while later in anaphase another region is critical for maintaining the coordination between spindle elongation and the growth of interpolar microtubules.

Closing mitosis: the functions of the Cdc14 phosphatase and its regulation

Completion of the cell cycle requires the temporal and spatial coordination of chromosome segregation with mitotic spindle disassembly and cytokinesis. In budding yeast, the protein phosphatase Cdc14 is a key regulator of these late mitotic events. Here, we review the functions of Cdc14 and how this phosphatase is regulated to accomplish the coupling of mitotic processes. We also discuss the function and regulation of Cdc14 in other eukaryotes, emphasizing conserved features.

Kinetochore rearrangement in meiosis II requires attachment to the spindle

The distinctive behaviors of chromosomes in mitosis and meiosis depend upon differences in kinetochore position. Kinetochore position is well established except for a critical transition between meiosis I and meiosis II. We examined kinetochore position during the transition and compared it with the position of kinetochores in mitosis. Immunofluorescence staining using the 3F3/2 antibody showed that in mitosis in grasshopper cells, as in other organisms, kinetochores are positioned on opposite sides of the two sister chromatids. In meiosis I, sister kinetochores are positioned side by side. At nuclear envelope breakdown in meiosis II, sister kinetochores are still side by side, but are separated by the time all chromosomes have fully attached in metaphase II. Micromanipulation experiments reveal that this switch from side-by-side to separated sister kinetochores requires attachment to the spindle. Moreover, it is irreversible, as chromosomes detached from a metaphase II spindle retain separate kinetochores. How this critical separation of sister kinetochores occurs in meiosis is uncertain, but clearly it is not built into the chromosome before nuclear envelope breakdown, as it is in mitosis.

At the interface between signaling and executing anaphase--Cdc14 and the FEAR network

Anaphase is the stage of the cell cycle when the duplicated genome is separated to opposite poles of the cell. The irreversible nature of this event confers a unique burden on the cell and it is therefore not surprising that the regulation of this cell cycle stage is complex. In budding yeast, a signaling network known as the Cdc fourteen early anaphase release (FEAR) network and its effector, the protein phosphatase Cdc14, play a key role in the coordination of the multiple events that occur during anaphase, such as partitioning of the DNA, regulation of spindle stability, activation of microtubule forces, and initiation of mitotic exit. These functions of the FEAR network contribute to genomic stability by coordinating the completion of anaphase and the execution of mitotic exit.

The Iml3 protein of the budding yeast is required for the prevention of precocious sister chromatid separation in meiosis I and for sister chromatid disjunction in meiosis II

The mitotic kinetochore of the budding yeast contains a number of proteins which are required for chromosome transmission but are non-essential for vegetative growth. We show that one such protein, Iml3, is essential for meiosis, in that the absence of this protein results in reduced spore viability, precocious sister chromatid segregation of artificial and natural chromosomes in meiosis I and chromosome non-disjunction in meiosis II.

TheDUETgene is necessary for chromosome organization and progression during male meiosis inArabidopsisand encodes a PHD finger protein

Progression through the meiotic cell cycle is an essential part of the developmental program of sporogenesis in plants. The duet mutant of Arabidopsis was identified as a male sterile mutant that lacked pollen and underwent an aberrant male meiosis. Male meiocyte division resulted in the formation of two cells instead of a normal tetrad. In wild type, male meiosis extends across two successive bud positions in an inflorescence whereas in duet, meiotic stages covered three to five bud positions indicating defective progression. Normal microspores were absent in the mutant and the products of the aberrant meiosis were uni- to tri-nucleate cells that later degenerated, resulting in anthers containing largely empty locules. Defects in male meiotic chromosome organization were observed starting from diplotene and extending to subsequent stages of meiosis. There was an accumulation of meiotic structures at metaphase 1, suggesting an arrest in cell cycle progression. Double mutant analysis revealed interaction with dyad, a mutation causing chromosome cohesion during female meiosis. Cloning and molecular analysis of DUET indicated that it potentially encodes a PHD-finger protein and shows specific expression in male meiocytes. Taken together these data suggest that DUET is required for male meiotic chromosome organization and progression.

Meiosis: polo, FEAR and the art of dividing reductionally

Recent studies on the regulation of meiosis have uncovered new roles for old acquaintances: the polo-like kinase Cdc5 has been found to dictate proper kinetochore orientation during meiosis I, while the FEAR pathway is essential for some, but not all, aspects of meiosis I exit.

The Cdc14 phosphatase and the FEAR network control meiotic spindle disassembly and chromosome segregation

During meiosis, DNA replication is followed by two consecutive rounds of chromosome segregation. Cells lacking the protein phosphatase CDC14 or its regulators, SPO12 and SLK19, undergo only a single meiotic division, with some chromosomes segregating reductionally and others equationally. We find that this abnormal chromosome behavior is due to an uncoupling of meiotic events. Anaphase I spindle disassembly is delayed in cdc14-1, slk19Delta, or spo12Delta mutants, but the chromosome segregation cycle continues, so that both meiotic chromosome segregation phases take place on the persisting meiosis I spindle. Our results show that Cdc14, Slk19, and Spo12 are not only required for meiosis I spindle disassembly but also play a pivotal role in establishing two consecutive chromosome segregation phases, a key feature of the meiotic cell cycle.

FEARless in meiosis

Degradation of mitotic cyclins is critical for exit from mitosis. Recent studies in budding yeast address the role of cyclin degradation in meiosis. Cyclin stabilization in meiosis I interferes with anaphase I spindle disassembly but, surprisingly, does not halt progression into meiosis II.

Division of the nucleolus and its release of CDC14 during anaphase of meiosis I depends on separase, SPO12, and SLK19

Disjunction of maternal and paternal centromeres during meiosis I requires crossing over between homologous chromatids, which creates chiasmata that hold homologs together. It also depends on a mechanism ensuring that maternal and paternal sister kinetochore pairs attach to oppositely oriented microtubules. Proteolytic cleavage of cohesin's Rec8 subunit by separase destroys cohesion between sister chromatid arms at anaphase I and thereby resolves chiasmata. The Spo12 and Slk19 proteins have been implicated in regulating meiosis I kinetochore orientation and/or in preventing cleavage of Rec8 at centromeres. We show here that the role of these proteins is instead to promote nucleolar segregation, including release of the Cdc14 phosphatase required for Cdk1 inactivation and disassembly of the anaphase I spindle. Separase is also required but surprisingly not its protease activity. It has two mechanistically different roles during meiosis I. Loss of the protease-independent function alone results in a second meiotic division occurring on anaphase I spindles in spo12delta and slk19delta mutants.

The yeast kinetochore protein Slk19 is required to prevent aberrant chromosome segregation in meiosis and mitosis

BACKGROUND

Slk19 is a coiled-coil protein, which locates to the kinetochores of S. cerevisiae. Most cells lacking Slk19 undergo incomplete meiosis and form dyads during sporulation. Endogenous chromosomes appeared to be predominantly divided in an equational manner during single-division meiosis of slk19 null mutants.

RESULTS

We have monitored the segregation of artificial chromosomes (YACs) in slk19 null mutants during both single-division meiosis and complete meiosis. In contrast to the results obtained with endogenous chromosomes, YACs only rarely undergo equational segregation during single division meiosis, although high rates of aberrant segregation were detected. This accounts for the high frequency of lethal spores among dyads of slk19 delta null mutants. The fraction of slk19 delta cells that were able to form tetrads solely exhibited YAC segregation defects in meiosis II, whereas the segregation of YACs in meiosis I was normal in these cells. This result might indicate that correct chromosome division in meiosis I is a prerequisite for tetrad formation. slk19 null mutants also showed YAC instability in mitosis and reduced survival after the induction of mitotic spindle damage.

CONCLUSION

Slk19 is required to avoid aberrant segregation of chromosomes in meiosis I and II and in mitosis. We suggest that the absence of Slk19 leads to uncoupling of chromosome movement from completion of microtubule attachment and resolution of chromosome cohesion.

Identification and analysis ofDYAD: a gene required for meiotic chromosome organisation and female meiotic progression inArabidopsis

The dyad mutant of Arabidopsis was previously identified as being defective in female meiosis. We report here the analysis of the DYAD gene. In ovules and anthers DYAD RNA is detected specifically in female and male meiocytes respectively, in premeiotic interphase/meiotic prophase. Analysis of chromosome spreads in female meiocytes showed that in the mutant, chromosomes did not undergo synapsis and formed ten univalents instead of five bivalents. Unlike mutations in AtDMC1 and AtSPO11 which also affect bivalent formation as the univalent chromosomes segregate randomly, the dyad univalents formed an ordered metaphase plate and underwent an equational division. This suggests a requirement for DYAD for chromosome synapsis and centromere configuration in female meiosis. The dyad mutant showed increased and persistent expression of a meiosis-specific marker, pAtDMC1::GUS during female meiosis, indicative of defective meiotic progression. The sequence of the putative protein encoded by DYAD did not reveal strong similarity to other proteins. DYAD is therefore likely to encode a novel protein required for meiotic chromosome organisation and female meiotic progression.

Spo13 protects meiotic cohesin at centromeres in meiosis I

In the absence of Spo13, budding yeast cells complete a single meiotic division during which sister chromatids often separate. We investigated the function of Spo13 by following chromosomes tagged with green fluorescent protein. The occurrence of a single division in spo13Delta homozygous diploids depends on the spindle checkpoint. Eliminating the checkpoint accelerates meiosis I in spo13Delta cells and allows them to undergo two divisions in which sister chromatids often separate in meiosis I and segregate randomly in meiosis II. Overexpression of Spo13 and the meiosis-specific cohesin Rec8 in mitotic cells prevents separation of sister chromatids despite destruction of Pds1 and activation of Esp1. This phenotype depends on the combined overexpression of both proteins and mimics one aspect of meiosis I chromosome behavior. Overexpressing the mitotic cohesin, Scc1/Mcd1, does not substitute for Rec8, suggesting that the combined actions of Spo13 and Rec8 are important for preventing sister centromere separation in meiosis I.

Only connect: linking meiotic DNA replication to chromosome dynamics

The process of meiosis reduces a diploid cell to four haploid gametes and is accompanied by extensive recombination. Thus, chromosome dynamics in meiosis are significantly different than in mitotic cells. This review analyzes unique features of meiotic DNA replication and describes how it affects subsequent recombination and chromosome segregation.

The molecular basis of sister-chromatid cohesion

The replicated copies of each chromosome, the sister chromatids, are attached prior to their segregation in mitosis and meiosis. This association or cohesion is critical for each sister chromatid to bind to microtubules from opposite spindle poles and thus segregate away from each other at anaphase of mitosis or meiosis II. The cohesin protein complex is essential for cohesion in both mitosis and meiosis, and cleavage of one of the subunits is sufficient for loss of cohesion at anaphase. The localization of the cohesin complex and other cohesion proteins permits evaluation of the positions of sister-chromatid associations within the chromosome structure, as well as the relationship between cohesion and condensation. Recently, two key riddles in the mechanism of meiotic chromosome segregation have yielded to molecular answers. First, analysis of the cohesin complex in meiosis provides molecular support for the long-standing hypothesis that sister-chromatid cohesion links homologs in meiosis I by stabilizing chiasmata. Second, the isolation of the monopolin protein that controls kinetochore behavior in meiosis I defines a functional basis by which sister kinetochores are directed toward the same pole in meiosis I.

Meiosis: how to create a specialized cell cycle

During the meiotic cell cycle, a single round of DNA replication precedes two nuclear divisions. Recent work has shown that the proteins controlling the mitotic cell cycle are either replaced by homologous proteins only expressed during the meiotic cell cycle or modulated by meiosis-specific factors to bring about this specialized cell cycle.

The Kar3-interacting protein Cik1p plays a critical role in passage through meiosis I in Saccharomyces cerevisiae

Meiosis I in Saccharomyces cerevisiae is dependent upon the motor protein Kar3. Absence of Kar3p in meiosis results in an arrest in prophase I. Cik1p and Vik1p are kinesin-associated proteins known to modulate the function of Kar3p in the microtubule-dependent processes of karyogamy and mitosis. Experiments were performed to determine whether Cik1p and Vik1p are also important for the function of Kar3p during meiosis. The meiotic phenotypes of a cik1 mutant were found to be similar to those of kar3 mutants. Cells without Cik1p exhibit a meiotic defect in homologous recombination and synaptonemal complex formation. Most cik1 mutant cells, like kar3 mutants, arrest in meiotic prophase; however, in cik1 mutants this arrest is less severe. These data are consistent with the model that Cik1p is necessary for some, but not all, of the roles of Kar3p in meiosis I. vik1 mutants sporulate at wild-type levels, but have reduced spore viability. This loss in viability is partially attributable to vegetative chromosome loss in vik1 diploids. Cellular localization experiments reveal that Kar3p, Cik1p, and Vik1p are present throughout meiosis and are consistent with Cik1p and Vik1p having different meiotic roles.

SWITCH1 (SWI1): a novel protein required for the establishment of sister chromatid cohesion and for bivalent formation at meiosis

We have characterized a new gene, SWI1, involved in sister chromatid cohesion during both male and female meiosis in Arabidopsis thaliana. A first allele, swi1.1, was obtained as a T-DNA tagged mutant and was described previously as abnormal exclusively in female meiosis. We have isolated a new allele, swi1.2, which is defective for both male and female meiosis. In swi1.2 male meiosis, the classical steps of prophase were not observed, especially because homologs do not synapse. Chromatid arms and centromeres lost their cohesion in a stepwise manner before metaphase I, and 20 chromatids instead of five bivalents were seen at the metaphase plate, which was followed by an aberrant segregation. In contrast, swi1.2 female meiocytes performed a mitotic-like division instead of meiosis, indicating a distinct role for SWI1 or a different effect of the loss of SWI1 function in both processes. The SWI1 gene was cloned; the putative SWI1 protein did not show strong similarity to any known protein. Plants transformed with a SWI1-GFP fusion indicated that SWI1 protein is present in meiocyte nuclei, before meiosis and at a very early stage of prophase. Thus, SWI1 appears to be a novel protein involved in chromatid cohesion establishment and in chromosome structure during meiosis, but with clear differences between male and female meiosis.

Functional genomics identifies monopolin: a kinetochore protein required for segregation of homologs during meiosis i

The orderly reduction in chromosome number that occurs during meiosis depends on two aspects of chromosome behavior specific to the first meiotic division. These are the retention of cohesion between sister centromeres and their attachment to microtubules that extend to the same pole (monopolar attachment). By deleting genes that are upregulated during meiosis, we identified in Saccharomyces cerevisiae a kinetochore associated protein, Mam1 (Monopolin), which is essential for monopolar attachment. We also show that the meiosis-specific cohesin, Rec8, is essential for maintaining cohesion between sister centromeres but not for monopolar attachment. We conclude that monopolar attachment during meiosis I requires at least one meiosis-specific protein and is independent of the process that protects sister centromere cohesion.

Sister chromatid separation: falling apart at the seams

Cohesion between sister chromatids must be dissolved at the time of chromosome segregation. Recent studies reveal that the principles of cohesion dissolution in mitosis and meiosis are the same, but that there are important differences that stem from the distinct natures of these two processes.

Slk19p is necessary to prevent separation of sister chromatids in meiosis I

BACKGROUND

A fundamental difference between meiotic and mitotic chromosome segregation is that in meiosis I, sister chromatids remain joined, moving as a unit to one pole of the spindle rather than separating as they do in mitosis. It has long been known that the sustained linkage of sister chromatids through meiotic anaphase I is accomplished by association of the chromatids at the centromere region. The localization of the cohesin Rec8p to the centromeres is essential for maintenance of sister chromatid cohesion through meiosis I, but the molecular basis for the regulation of Rec8p and sister kinetochores in meiosis remains a mystery.

RESULTS

We show that the SLK19 gene product from Saccharomyces cerevisiae is essential for proper chromosome segregation during meiosis I. When slk19 mutants were induced to sporulate they completed events characteristic of meiotic prophase I, but at the first meiotic division they segregated their sister chromatids to opposite poles at high frequencies. The vast majority of these cells did not perform a second meiotic division and proceeded to form dyads (asci containing two spores). Slk19p was found to localize to centromere regions of chromosomes during meiotic prophase where it remained until anaphase I. In the absence of Slk19p, Rec8p was not maintained at the centromere region through anaphase I as it is in wild-type cells. Finally, we demonstrate that Slk19p appears to function downstream of the meiosis-specific protein Spo13p in control of sister chromatid behavior during meiosis I.

CONCLUSIONS

Our results suggest that Slk19p is essential at the centromere of meiotic chromosomes to prevent the premature separation of sister chromatids at meiosis I.