Genetic collection of meiotic mutants of rye Secale cereale L

Genetic collection of meiotic mutants of winter rye Secale cereale L. (2n = 14) was created. Mutations were detected in inbred F2 generations after self-fertilization of the F1 hybrids, obtained by individual crossing of rye plants (cultivar Vyatka) or weedy rye with plants from autofertile lines. The mutations cause partial or complete plant sterility and are maintained in collection in a heterozygous state. Genetic analysis accompanied by cytogenetic study of meiosis has revealed six mutation types. (1) Nonallelic asynaptic mutations sy1 and sy9 caused the formation of only axial chromosome elements in prophase and anaphase. The synaptonemal complexes (SCs) were absent, the formation of the chromosome "bouquet" was impaired, and all chromosomes were univalent in meiotic metaphase I in 96% (sy1) and 67% (sy2) of cells. (2) Weak asynaptic mutation sy3, which hindered complete termination of synapsis in prophase II. Subterminal asynaptic segments were always observed in the SC, and at least one pair of univalents was present in metaphase I, but the number of cells with univalents did not exceed 2%. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19, which caused partially nonhomologous synapsis: change in pairing partners and fold-back chromosome synapsis in prophase I. In metaphase I, the number of univalents varied and multivalents were observed. (4) Mutation mei6, which causes the formation of ultrastructural protrusions on the lateral SC elements, gaps and branching of these elements. (5) Allelic mutations mei8 and mei10, which caused irregular chromatin condensation along chromosomes in prophase I, sticking and fragmentation of chromosomes in metaphase I. (6) Allelic mutations mei5 and mei10, which caused chromosome hypercondensation, defects of the division spindle formation, and random arrest of cells at different meiotic stages. However, these mutations did not affect the formation of microspore envelopes even around the cells, whose development was blocked at prophase I. Analysis of cytological pictures of meiosis in double rye mutants reveled epistatic interaction in the mutation series sy9 > sy1 > sy3 > sy19, which reflects the order of switching these genes in the course of meiosis. The expression of genes sy2 and sy19 was shown to be controlled by modifier genes. Most meiotic mutations found in rye have analogs in other plant species.     

The pachytene checkpoint in S. cerevisiae depends on Swe1-mediated phosphorylation of the cyclin-dependent kinase Cdc28

Mutants defective in meiotic recombination and synaptonemal complex formation undergo checkpoint-mediated arrest in mid-meiotic prophase. In S. cerevisiae, this checkpoint requires Swe1, which phosphorylates and inactivates the cyclin-dependent kinase Cdc28. A swe1 deletion allows mutants that normally arrest in meiotic prophase to sporulate at wild-type levels, though sporulation is delayed. This delay is eliminated by overproducing Clb1, the major cyclin required for meiosis I. The Swe1 protein accumulates and is hyperphosphorylated in checkpoint-arrested cells. Our results suggest that meiotic arrest is mediated both by increasing Swe1 activity and limiting cyclin production, with Swe1 being the primary downstream target of checkpoint control. The requirement for Swe1 distinguishes the pachytene checkpoint from the DNA damage checkpoints operating in vegetative cells.     

α-endosulfine (ENSA) regulates exit from prophase I arrest in mouse oocytes

Mammalian oocytes in ovarian follicles are arrested in meiosis at prophase I. This arrest is maintained until ovulation, upon which the oocyte exits from this arrest, progresses through meiosis I and to metaphase of meiosis II. The progression from prophase I to metaphase II, known as meiotic maturation, is mediated by signals that coordinate these transitions in the life of the oocyte. ENSA (α-endosulfine) and ARPP19 (cAMP-regulated phosphoprotein-19) have emerged as regulators of M-phase, with function in inhibition of protein phosphatase 2A (PP2A) activity. Inhibition of PP2A maintains the phosphorylated state of CDK1 substrates, thus allowing progression into and/or maintenance of an M-phase state. We show here ENSA in mouse oocytes plays a key role in the progression from prophase I arrest into M-phase of meiosis I. The majority of ENSA-deficient oocytes fail to exit from prophase I arrest. This function of ENSA in oocytes is dependent on PP2A, and specifically on the regulatory subunit PPP2R2D (also known as B55δ). Treatment of ENSA-deficient oocytes with Okadaic acid to inhibit PP2A rescues the defect in meiotic progression, with Okadaic acid-treated, ENSA-deficient oocytes being able to exit from prophase I arrest. Similarly, oocytes deficient in both ENSA and PPP2R2D are able to exit from prophase I arrest to an extent similar to wild-type oocytes. These data are evidence of a role for ENSA in regulating meiotic maturation in mammalian oocytes, and also have potential relevance to human oocyte biology, as mouse and human have genes encoding both Arpp19 and Ensa.     

Zinc maintains prophase I arrest in mouse oocytes through regulation of the MOS-MAPK pathway

Meiosis in mammalian females is marked by two arrest points, at prophase I and metaphase II, which must be tightly regulated in order to produce a haploid gamete at the time of fertilization. The transition metal zinc has emerged as a necessary and dynamic regulator of the establishment, maintenance, and exit from metaphase II arrest, but the roles of zinc during prophase I arrest are largely unknown. In this study, we investigate the mechanisms of zinc regulation during the first meiotic arrest. Disrupting zinc availability in the prophase I arrested oocyte by treatment with the heavy metal chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)-ethylenediamine (TPEN) causes meiotic resumption even in the presence of pharmacological inhibitors of meiosis. We further show that the MOS-MAPK pathway mediates zinc-dependent prophase I arrest, as the pathway prematurely activates during TPEN-induced meiotic resumption. Conversely, inhibition of the MOS-MAPK pathway maintains prophase I arrest. While prolonged zinc insufficiency ultimately results in telophase I arrest, early and transient exposure of oocytes to TPEN is sufficient to induce meiotic resumption and bypass the telophase I block, allowing the formation of developmentally competent eggs upon parthenogenetic activation. These results establish zinc as a crucial regulator of meiosis throughout the entirety of oocyte maturation, including the maintenance of and release from the first and second meiotic arrest points.     

alpha-Endosulfine is a conserved protein required for oocyte meiotic maturation in Drosophila

Meiosis is coupled to gamete development and must be well regulated to prevent aneuploidy. During meiotic maturation, Drosophila oocytes progress from prophase I to metaphase I. The molecular factors controlling meiotic maturation timing, however, are poorly understood. We show that Drosophila alpha-endosulfine (endos) plays a key role in this process. endos mutant oocytes have a prolonged prophase I and fail to progress to metaphase I. This phenotype is similar to that of mutants of cdc2 (synonymous with cdk1) and of twine, the meiotic homolog of cdc25, which is required for Cdk1 activation. We found that Twine and Polo kinase levels are reduced in endos mutants, and identified Early girl (Elgi), a predicted E3 ubiquitin ligase, as a strong Endos-binding protein. In elgi mutant oocytes, the transition into metaphase I occurs prematurely, but Polo and Twine levels are unaffected. These results suggest that Endos controls meiotic maturation by regulating Twine and Polo levels, and, independently, by antagonizing Elgi. Finally, germline-specific expression of the human alpha-endosulfine ENSA rescues the endos mutant meiotic defects and infertility, and alpha-endosulfine is expressed in mouse oocytes, suggesting potential conservation of its meiotic function.     

Ascus development in two temperature-sensitive four-spore mutants of Neurospora crassa

Two nonallelic Four-spore mutants are known in which ascospore walls enclose the four immediate products of meiosis rather than the normal eight products of a postmeiotic mitosis. Expression depends on temperature. The Four-spore phenotype is expressed when the developing asci are subjected either to high temperatures (25-30 degrees C) for Fsp-1 or to low temperatures (15-20 degrees C) for Fsp-2. Heterozygous Fsp-1 X Fsp-1+ crosses make eight-spored asci at 15-20 degrees C but produce many four-spored asci at 25 degrees C and mostly four-spored asci at 30 degrees C. Homozygous Fsp-1 X Fsp-1 crosses respond similarly to increasing temperature but make 40-50% four-spored asci even at 20 degrees C. Heterozygous Fsp-2 X Fsp-2+ crosses produce almost exclusively four-spored asci at 15 degrees C but a mixture of four- and eight-spored asci at 20, 25, and 30 degrees C. Homozygous Fsp-2 X Fsp-2 crosses make all four-spored asci at 15 and 20 degrees C and a mixture of four- and eight-spored asci at 25 and 30 degrees C. When both Fsp-1 and Fsp-2 are present in a cross, either homozygous or heterozygous, no asci contain more than four ascospores at any temperature. Limited temperature-shift experiments with Fsp-1 and Fsp-2 show that the sensitive period for Four-spore expression is sometime after meiotic prophase, possibly at interphase II.     

Mechanisms of chromosome orientation revealed by two meiotic mutants in Drosophila melanogaster

Two disjunction defective meiotic mutants, ord and mei-S332, each of which disrupts meiosis in both male and female Drosophila melanogaster, were analyzed cytologically and genetically in the male germ-line. It was observed that sister-chromatids are frequently associated abnormally during prophase I and metaphase I in ord. Sister chromatid associations in mei-S332 are generally normal during prophase I and metaphase I. By telophase I, sister chromatids have frequently precociously separated in both mutants. During the first division sister chromatids disjoin from one another frequently in ord and rarely in mei-S332. It is argued that the simplest interpretation of the observations is that each mutant is defective in sister chromatid cohesiveness and that the defect in ord manifests itself earlier than does the defect in mei-S332. In addition, based on these mutant effects, several conclusions regarding normal meiotic processes are drawn. (1) The phenotype of these mutants support the proposition that the second meiotic metaphase (mitotic-type) position of chromosomes and their equational orientation is a consequence of the equilibrium, at the metaphase plate, of pulling forces acting at the kinetochores and directed towards the poles. (2) Chromosomes which lag during the second meiotic division tend to be lost. (3) Sister chromatid cohesiveness, or some function necessary for sister chromatid cohesiveness, is required for the normal reductional orientation of sister kinetochores during the first meiotic division. (4) The kinetochores of a half-bivalent are double at the time of chromosome orientation during the first meiotic division. Finally, functions which are required throughout meiosis in both sexes must be considered in the pathways of meiotic control.     

The ultrastructural meiotic phenotype of the radiation sensitive mutant rad 6-1 in yeast

An ultrastructural analysis of three yeast rad 6-1/rad 6-1 diploids on sporulation medium for 0, 6, 10, and 24 h shows that arrest occurs at meiotic prophase. Two strains, CL 139 and PU 6, fail to complete chromosome synapsis based on the continued presence of single chromosomal cores in arrested nuclei. A clone derived from CL 139, however, showed complete pairing as evident from the presence of 17 synaptonemal complexes. All three strains underwent spindle pole body duplication but the poles failed to form a proper metaphase I spindle. A revertant Rad 6⁺ isolated from CL 139 showed normal chromosome behaviour and normal kinetic functions. It is concluded that the absence of meiotic recombination in some Rad 6^– strains may result from asynapsis, but that in other strains (e.g., CL 139s) recombination fails in spite of complete synapsis. In all cases the lack of sporulation is adequately explained by failure of the kinetic apparatus to form a metaphase I spindle.     

Comparative analysis of meiotic progression in female mice bearing mutations in genes of the DNA mismatch repair pathway

The DNA mismatch repair (MMR) family functions in a variety of contexts to preserve genome integrity in most eukaryotes. In particular, members of the MMR family are involved in the process of meiotic recombination in germ cells. MMR gene mutations in mice result in meiotic disruption during prophase I, but the extent of this disruption often differs between male and female meiocytes. To address the role of MMR proteins specifically in female meiosis, we explored the progression of oocytes through prophase I and the meiotic divisions in mice harboring deletions in members of the MMR pathway (Mlh1, Mlh3, Exo1, and an ATPase-deficient variant of Mlh1, Mlh1(G67R)). The colocalization of MLH1 and MLH3, key proteins involved in stabilization of nascent crossovers, was dependent on intact heterodimer formation and was highly correlated with the ability of oocytes to progress through to metaphase II. The exception was Exo1(-/-) oocytes, in which normal MLH1/MLH3 localization was observed followed by failure to proceed to metaphase II. All mutant oocytes were able to resume meiosis after dictyate arrest, but they showed a dramatic decline in chiasmata (to less than 25% of normal), accompanied by varied progression through metaphase I. Taken together, these results demonstrate that MMR function is required for the formation and stabilization of crossovers in mammalian oocytes and that, in the absence of a functional MMR system, the failure to maintain chiasmata results in a reduced ability to proceed normally through the first and second meiotic divisions, despite near-normal levels of meiotic resumption after dictyate arrest.     

Mutations at the smo genetic locus affect the shape of diverse cell types in the rice blast fungus

Teflon film surfaces are highly conducive to the formation of infection structures (appressoria) in the plant pathogenic fungus, Magnaporthe grisea. We have utilized Teflon films to screen and select for mutants of M. grisea that are defective in appressorium formation. This approach and several others yielded a group of 14 mutants with a similar phenotype. All the mutant strains make abnormally shaped conidia and appressoria. When two mutant strains are crossed, abnormally shaped asci are formed. Ascus shape is normal when a mutant strain is crossed with a wild-type strain. Despite dramatic alterations in cell shape these strains otherwise grow, form conidia, undergo meiosis, and infect plants normally. This mutant phenotype, which we have termed Smo(-), for abnormal spore morphology, segregates in simple Mendelian fashion in crosses with wild-type strains. Some ascospore lethality is associated with smo mutations. In genetic crosses between mutants, smo mutations fail to recombine and do not demonstrate complementation of the abnormal ascus shape phenotype. We conclude that the smo mutations are alleles of a single genetic locus and are recessive with regard to the the ascus shape defect. Mutations at the SMO locus also permit germinating M. grisea conidia to differentiate appressoria on surfaces that are not normally conducive to infection structure formation. A number of spontaneous smo mutations have been recovered. The frequent occurrence of this mutation suggests that the SMO locus may be highly mutable.     

White-cap mutants and meiotic apoptosis in the basidiomycete Coprinus cinereus

Among many white-cap mutants of Coprinus cinereus, four distinct classes have been identified cytologically. Mutants of one class progress through meiosis normally but fail to sporulate; the defect is post-meiotic and it triggers apoptosis in the tetrad stage. Mutants of the other three classes have defects in meiotic prophase and these are: (1) those that assemble synaptonemal complexes (SCs) normally; (2) those that assemble axial elements (AEs) but not SCs; and (3) those that assemble neither AEs nor SCs even though the chromosomes are condensed and also paired. All three meiotic mutant classes arrest at meiotic metaphase I and the arrest triggers meiosis-specific apoptosis showing characteristic chromatin condensation, DNA fragmentation as shown by the TUNEL assay, cytoplasmic shrinkage, and finally total DNA degradation. Apoptosis is very cell-type specific; it occurs only in the basidia while the neighboring somatic cells are perfectly healthy and the mushroom continues to develop and mature with very few basidiospores produced. The meiotic apoptosis in C. cinereus is under strict cell cycle control rather than at any time after defect; apoptosis is triggered only after entry to meiotic metaphase. It is intriguing to note that C. cinereus has two checkpoints for arrest and entry to apoptosis: one is meiotic at the metaphase I spindle checkpoint regardless of the time of defects, and one is post-meiotic at the tetrad stage. This is in striking contrast to multiple checkpoint arrests and entries to meiotic apoptosis found in the mouse.     

Gld-1, a Tumor Suppressor Gene Required for Oocyte Development in Caenorhabditis Elegans

We have characterized 31 mutations in the gld-1 (defective in germline development) gene of Caenorhabditis elegans. In gld-1(null) hermaphrodites, oogenesis is abolished and a germline tumor forms where oocyte development would normally occur. By contrast, gld-1(null) males are unaffected. The hermaphrodite germline tumor appears to derive from germ cells that enter the meiotic pathway normally but then exit pachytene and return to the mitotic cycle. Certain gld-1 partial loss-of-function mutations also abolish oogenesis, but germ cells arrest in pachytene rather than returning to mitosis. Our results indicate that gld-1 is a tumor suppressor gene required for oocyte development. The tumorous phenotype suggests that gld-1(+) may function to negatively regulate proliferation during meiotic prophase and/or act to direct progression through meiotic prophase. We also show that gld-1(+) has an additional nonessential role in germline sex determination: promotion of hermaphrodite spermatogenesis. This function of gld-1 is inferred from a haplo-insufficient phenotype and from the properties of gain-of-function gld-1 mutations that cause alterations in the sexual identity of germ cells.     

Mutations in Saccharomyces Cerevisiae That Block Meiotic Prophase Chromosome Metabolism and Confer Cell Cycle Arrest at Pachytene Identify Two New Meiosis-Specific Genes Sae1 and Sae3

Two new meiosis-specific genes, SAE1 and SAE3, have been identified in a screen for mutations that confer an intermediate block in meiotic prophase. Such mutations confer a block to spore formation that is circumvented by addition of a mutation that eliminates meiotic recombination initiation and other aspects of chromosome metabolism, i.e., spo11. We show that sae1-1 and sae3-1 mutations each confer a distinct defect in meiotic recombination. sae1-1 produces recombinants but very slowly and ultimately to less than half the wild-type level; sae3-1 makes persistent hyper-resected meiotic double-strand breaks and has a severe defect in formation of recombinants. Both mutants arrest at the pachytene stage of meiotic prophase, sae1-1 temporarily and sae3-1 permanently. The phenotypes conferred by sae3-1 are similar to those conferred by mutation of the yeast RecA homologue DMC1, suggesting that SAE3 and DMC1 act at the same step(s) of chromosome metabolism. These results provide further evidence that intermediate blocks to prophase chromosome metabolism cause cell-cycle arrest. SAE1 encodes a 208-residue protein homologous to vertebrate mRNA cap-binding protein 20. SAE3 corresponds to a meiosis-specific RNA encoding an unusually short open reading frame of 50 codons.     

Cell cycle control by daf-21/Hsp90 at the first meiotic prophase/metaphase boundary during oogenesis in Caenorhabditis elegans

DAF-21, a Caenorhabditis elegans homologue of Hsp90, is expressed primarily in germline cells. Although mutations in the daf-21 gene affect animal fertility, its cellular roles have remained elusive. To phenocopy daf-21 mutations, we impaired the daf-21 function by RNA interference (RNAi), and found that oocytes skipped the diakinesis arrest and displayed a defective diakinesis arrest, which led to the production of endomitotic oocytes with polyploid chromosomes (Emo phenotype). The same Emo phenotype was also observed with RNAi against wee-1.3. To identify a cause for Emo, we examined the CDK-1 (Cdc2) phosphorylation status in Emo animals, since CDK-1 is a key regulator of the prophase/metaphase transition and is kept inactivated by WEE-1.3 kinase during prophase. We immunostained both daf-21(RNAi) and wee-1.3(RNAi) animals with anti-phosphorylated-CDK-1 antibody and observed no detectable phosphates on CDK-1 in either of the animals. We also examined WEE-1.3 expression in daf-21(RNAi) and found a significant reduction of WEE-1.3. These results indicate that CDK-1 was not phosphorylated in either daf-21(RNAi) or wee-1.3(RNAi) animals, and suggest that daf-21 was necessary for producing functional WEE-1.3. Thus, all together, we propose that DAF-21 indirectly regulates the meiotic prophase/metaphase transition during oocyte development by ensuring the normal function of WEE-1.3.     

Completion of meiosis in male zebrafish (<Emphasis Type="Italic">Danio rerio</Emphasis>) despite lack of DNA mismatch repair gene <Emphasis Type="Italic">mlh1</Emphasis>

Mlh1 is a member of DNA mismatch repair (MMR) machinery and is also essential for the stabilization of crossovers during the first meiotic division. Recently, we have shown that zebrafish mlh1 mutant males are completely infertile because of a block in metaphase I, whereas females are fertile but have aneuploid progeny. When studying fertility in males in a two-fold more inbred background, we have however observed low numbers of fertilized eggs (approximately 0.4%). Histological examination of the testis has revealed that all spermatogenic stages prior to spermatids (spermatogonia, primary spermatocytes, and secondary spermatocytes) are significantly increased in the mutant, whereas the total weight of spermatids and spermatozoa is highly decreased (1.8 mg in wild-type vs. 0.1 mg in mutants), a result clearly different from our previous study in which outbred males lack secondary spermatocytes or postmeiotic cells. Thus, a delay of both meiotic divisions occurs rather than complete arrest during meiosis I in these males. Eggs fertilized with mutant sperm develop as malformed embryos and are aneuploid making this male phenotype much more similar to that previously described in the mutant females. Therefore, crossovers are still essential for proper meiosis, but meiotic cell divisions can progress without it, suggesting that this mutant is a suitable model for studying the cellular mechanisms of completing meiosis without crossover stabilization. Marcelo C. Leal and Harma Feitsma contributed equally to this work. This work was supported by the Brazilian Foundation CAPES, the Cancer Genomics Center (Nationaal Regie Orgaan Genomics), the European Union-funded FP6 Integrated Project ZF-MODELS, and Utrecht University.     

Mutation in mouse hei10, an e3 ubiquitin ligase, disrupts meiotic crossing over

Crossing over during meiotic prophase I is required for sexual reproduction in mice and contributes to genome-wide genetic diversity. Here we report on the characterization of an N-ethyl-N-nitrosourea-induced, recessive allele called mei4, which causes sterility in both sexes owing to meiotic defects. In mutant spermatocytes, chromosomes fail to congress properly at the metaphase plate, leading to arrest and apoptosis before the first meiotic division. Mutant oocytes have a similar chromosomal phenotype but in vitro can undergo meiotic divisions and fertilization before arresting. During late meiotic prophase in mei4 mutant males, absence of cyclin dependent kinase 2 and mismatch repair protein association from chromosome cores is correlated with the premature separation of bivalents at diplonema owing to lack of chiasmata. We have identified the causative mutation, a transversion in the 5′ splice donor site of exon 1 in the mouse ortholog of Human Enhancer of Invasion 10 (Hei10; also known as Gm288 in mouse and CCNB1IP1 in human), a putative B-type cyclin E3 ubiquitin ligase. Importantly, orthologs of Hei10 are found exclusively in deuterostomes and not in more ancestral protostomes such as yeast, worms, or flies. The cloning and characterization of the mei4 allele of Hei10 demonstrates a novel link between cell cycle regulation and mismatch repair during prophase I.     

Genetic Collection of Meiotic Mutants of Rye <Emphasis Type="Italic">Secale cereale</Emphasis> L.

Genetic collection of meiotic mutants of winter rye Secale cereale L. (2n = 14) was created. Mutations were detected in inbred F₂ generations after self-fertilization of the F₁ hybrids, obtained by individual crossing of rye plants (cultivar Vyatka) or weedy rye with plants from autofertile lines. The mutations cause partial or complete plant sterility and are maintained in collection in a heterozygous state. Genetic analysis accompanied by cytogenetic study of meiosis has revealed six mutation types. (1) Nonallelic asynaptic mutations sy1 and sy9 caused the formation of only axial chromosome elements in prophase and anaphase. The synaptonemal complexes (SCs) were absent, the formation of the chromosome “bouquet” was impaired, and all chromosomes were univalent in meiotic metaphase I in 96.8% (sy1) and 67% (sy2) of cells. (2) Weak asynaptic mutation sy3, which hindered complete termination of synapsis in prophase I. Subterminal asynaptic segments were always observed in the SC, and at least one pair of univalents was present in metaphase I, but the number of cells with 14 univalents did not exceed 2%. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19, which caused partially nonhomologous synapsis: change in pairing partners and fold-back chromosome synapsis in prophase I. In metaphase I, the number of univalents varied and multivalents were observed. (4) Mutation mei6, which causes the formation of ultrastructural protrusions on the lateral SC elements, gaps and branching of these elements. (5) Allelic mutations mei8 and mei8-10, which caused irregular chromatin condensation along chromosomes in prophase I, sticking and fragmentation of chromosomes in metaphase I. (6) Allelic mutations mei5 and mei10, which caused chromosome hypercondensation, defects of the division spindle formation, and random arrest of cells at different meiotic stages. However, these mutations did not affect the formation of microspore envelopes even around the cells, whose development was blocked at prophase I. Analysis of cytological pictures of meiosis in double rye mutants reveled epistatic interaction in the mutation series sy9 > sy1 > sy3 > sy19, which reflects the order of switching these genes in the course of meiosis. The expression of genes sy2 and sy19 was shown to be controlled by modifier genes. Most meiotic mutations found in rye have analogs in other plant species.     

One class of mutants with disturbed centromere cleavage and chromosome pairing in Sordaria macrospora

Summary Two recessive mutants spo76 and spo77, altered in U.V. sensitivity, protoplast regeneration and meiotic recombination were isolated in Sordaria macrospora. The suppression of the spo76 phenotype by spo77 suggests that they are involved in the same pathway. The asynaptic spo77 exhibits a rare synaptonemal complex (SC) with abnormally thick and double lateral elements (LE). In spo76, an early centromere cleavage leads to a meiotic arrest after metaphase I; SC are formed, but their discontinuous LE appear to be either unique or split into two thin LE. It is suggested that the corresponding wild-type functions are required for the sister chromatid cohesiveness.     

Bypass of a meiotic checkpoint by overproduction of meiotic chromosomal proteins

The Saccharomyces cerevisiae zip1 mutant, which exhibits defects in synaptonemal complex formation and meiotic recombination, triggers a checkpoint that causes cells to arrest at the pachytene stage of meiotic prophase. Overproduction of either the meiotic chromosomal protein Red1 or the meiotic kinase Mek1 bypasses this checkpoint, allowing zip1 cells to sporulate. Red1 or Mek1 overproduction also promotes sporulation of other mutants (zip2, dmc1, hop2) that undergo checkpoint-mediated arrest at pachytene. In addition, Red1 overproduction antagonizes interhomolog interactions in the zip1 mutant, substantially decreasing double-strand break formation, meiotic recombination, and homologous chromosome pairing. Mek1 overproduction, in contrast, suppresses checkpoint-induced arrest without significantly decreasing meiotic recombination. Cooverproduction of Red1 and Mek1 fails to bypass the checkpoint; moreover, overproduction of the meiotic chromosomal protein Hop1 blocks the Red1 and Mek1 overproduction phenotypes. These results suggest that meiotic chromosomal proteins function in the signaling of meiotic prophase defects and that the correct stoichiometry of Red1, Mek1, and Hop1 is needed to achieve checkpoint-mediated cell cycle arrest at pachytene.