Brca2 is involved in meiosis in Arabidopsis thaliana as suggested by its interaction with Dmc1

Two BRCA2-like sequences are present in the Arabidopsis genome. Both genes are expressed in flower buds and encode nearly identical proteins, which contain four BRC motifs. In a yeast two-hybrid assay, the Arabidopsis Brca2 proteins interact with Rad51 and Dmc1. RNAi constructs aimed at silencing the BRCA2 genes at meiosis triggered a reproducible sterility phenotype, which was associated with dramatic meiosis alterations. We obtained the same phenotype upon introduction of RNAi constructs aimed at silencing the RAD51 gene at meiosis in dmc1 mutant plants. The meiotic figures we observed strongly suggest that homologous recombination is highly disturbed in these meiotic cells, leaving aberrant recombination events to repair the meiotic double-strand breaks. The 'brca2' meiotic phenotype was eliminated in spo11 mutant plants. Our experiments point to an essential role of Brca2 at meiosis in Arabidopsis. We also propose a role for Rad51 in the dmc1 context.     

DNA topoisomerase II interacts with Lim15/Dmc1 in meiosis

Lim15/Dmc1 is a meiosis specific RecA-like protein. Here we propose its participation in meiotic chromosome pairing-related events along with DNA topoisomerase II. Analysis of protein–protein interactions using in vitro binding assays provided evidence that Coprinus cinereus DNA topoisomerase II (CcTopII) specifically interacts with C.cinereus Lim15/Dmc1 (CcLim15). Co-immunoprecipitation experiments also indicated that the CcLim15 protein interacts with CcTopII in vivo. Furthermore, a significant proportion of CcLim15 and CcTopII could be shown to co-localize on chromosomes from the leptotene to the zygotene stage. Interestingly, CcLim15 can potently activate the relaxation/catenation activity of CcTopII in vitro, and CcTopII suppresses CcLim15-dependent strand transfer activity. On the other hand, while enhancement of CcLim15's DNA-dependent ATPase activity by CcTopII was found in vitro, the same enzyme activity of CcTopII was inhibited by adding CcLim15. The interaction of CcLim15 and CcTopII may facilitate pairing of homologous chromosomes.     

Mitochondrial DNA is synthesized during meiosis and spermiogenesis in the mouse

Following intratesticular injection of [3H]thymidine, spermatozoa were isolated from the caput and cauda epididymis of CD-1 mice. Quantitation of the amount of radiolabelled DNA in whole spermatozoa, sperm heads (nuclei) and sperm midpiece-tails (mitochondria) at time intervals 3-36 days postinjection demonstrated mitochondrial DNA (mtDNA) is synthesized during meiosis and early spermiogenesis. mtDNA synthesis terminates in mid-spermiogenesis suggesting that the mitochondrial genome in spermatozoa has a half-life of at least 15 days.     

Mps1 at kinetochores is essential for female mouse meiosis I

In female meiosis, chromosome missegregations lead to the generation of aneuploid oocytes and can cause the development of trisomies or infertility. Because mammalian female meiosis I is error prone, the full functionality of control mechanisms, such as the spindle assembly checkpoint (SAC), has been put into question. The SAC monitors the correct orientation, microtubule occupancy and tension on proteinaceous structures named kinetochores. Although it has been shown previously that the SAC exists in meiosis I, where attachments are monopolar, the role of microtubule occupancy for silencing the SAC and the importance of certain essential SAC components, such as the kinase Mps1, are unknown in mammalian oocytes. Using a conditional loss-of-function approach, we address the role of Mps1 in meiotic progression and checkpoint control in meiosis I. Our data demonstrate that kinetochore localization of Mps1 is required for the proper timing of prometaphase and is essential for SAC control, chromosome alignment and aurora C localization in meiosis I. The absence of Mps1 from kinetochores severely impairs chromosome segregation in oocyte meiosis I and, therefore, fertility in mice. In addition, we settle a long-standing question in showing that kinetochore-microtubule attachments are present in prometaphase I at a time when most of the SAC protein Mad2 disappears from kinetochores.     

Conversion from mitosis to meiosis: morphology and expression of proliferating cell nuclear antigen (PCNA) and Dmc1 during newt spermatogenesis

The conversion from mitosis to meiosis is a phenomenon specific to the cellular progenitors of gametes; however, the mechanism or mechanisms responsible for this conversion are poorly understood. To this end, some morphological and molecular changes that occur during the initiation of meiosis in newt spermatogenesis are reported in the present paper. In situ morphologic studies revealed that spermatogonial stages comprise two phases: early mitotic generations (G1-G4) and late mitotic generations (G5-G8). Morphologic conversion from secondary spermatogonia to primary spermatocytes occurred during the intermediate stage of premeiotic DNA replication. The expression of proliferating cell nuclear antigen (PCNA), a DNA polymerase-delta auxiliary protein, in spermatogonia was weak in G1, highest during DNA synthesis (S), decreased in G2 and was not detectable in dividing cells. Complementary DNA for newt homologs of DMC1 (disrupted meiotic cDNA), which is an Escherichia coli RecA-like protein specifically active during meiosis, were isolated. The newt Dmc1 mRNA was first expressed significantly during the preleptotene stage and this continued into the spermatid stage. These observations present a basis for investigating the mechanism(s) controlling the conversion of newt spermatogonial cells from mitosis to meiosis.     

The Mei5-Sae3 Protein Complex Mediates Dmc1 Activity in Saccharomyces cerevisiae

During homologous recombination, a number of proteins cooperate to catalyze the loading of recombinases onto single-stranded DNA. Single-stranded DNA-binding proteins stimulate recombination by coating single-stranded DNA and keeping it free of secondary structure; however, in order for recombinases to load on single-stranded-DNA-binding protein-coated DNA, the activity of a class of proteins known as recombination mediators is required. Mediator proteins coordinate the handoff of single-stranded DNA from single-stranded DNA-binding protein to recombinase. Here we show that a complex of Mei5 and Sae3 from Saccharomyces cerevisiae preferentially binds single-stranded DNA and relieves the inhibition of the strand assimilation and DNA binding abilities of the meiotic recombinase Dmc1 imposed by the single-stranded DNA-binding protein replication protein A. Additionally, we demonstrate the physical interaction of Mei5-Sae3 with replication protein A. Our results, together with previous in vivo studies, indicate that Mei5-Sae3 is a mediator of Dmc1 assembly during meiotic recombination in S. cerevisiae.During meiosis, recombination between homologous chromosomes ensures proper segregation into haploid products. Recombination events are initiated by the formation of double strand breaks (DSBs)² in DNA (1). This is followed by resection of free DNA ends to yield 3′ single-stranded tails, upon which recombinase assembles to form nucleoprotein filaments. Following recombinase assembly, the nucleoprotein filament engages a donor chromatid, searches for homologous DNA sequences on that chromatid, and promotes strand exchange to yield a heteroduplex DNA intermediate often referred to as a joint molecule. Although recombinase alone is capable of promoting homology search and strand exchange in vitro, genetic and biochemical studies have demonstrated that normal recombinase function in vivo requires the activity of a number of accessory factors (2). These factors enhance the assembly of nucleoprotein filaments, target capture, homology search, and dissociation of recombinase from duplex DNA.Most eukaryotes possess two recombinases, both homologues of the Escherichia coli recombinase RecA: Rad51, which is the major recombinase in mitotic cells and is also important during meiotic recombination, and Dmc1, which functions only in meiosis. Dmc1 and Rad51 have been shown to assemble at DSBs by immunofluorescence and chromatin immunoprecipitation (3–6), and both proteins oligomerize on single-stranded DNA (ssDNA) to form nucleofilaments that catalyze strand invasion (7–9).A number of biochemical studies have defined the role of accessory factors in stimulating the activity of Rad51 (10–12). Replication protein A (RPA), the yeast ssDNA-binding protein (SSB), removes secondary structure in ssDNA that otherwise prevents formation of fully functional nucleoprotein filaments (13). Both Rad52 protein (11, 12) and the heterodimeric protein Rad55/Rad57 (14) can overcome the inhibitory effect of RPA on Rad51 nucleoprotein filament formation in purified systems, mediating a handoff between RPA and Rad51. It is thought that the mechanism for the mediator activity of Rad52 involves Rad52 recognizing and binding to RPA-coated ssDNA, where it provides nucleation sites for the recruitment of free molecules of Rad51 (15). The tumor suppressor protein BRCA2 also serves as an assembly factor for Rad51 during mitosis in a variety of species that encode orthologues of this protein, including mice (16), corn smut (17), and humans (18).The meiosis-specific recombinase Dmc1 is stimulated by a distinct set of accessory factors. Immunostaining studies suggest that the Rad51 mediators Rad52 and Rad55/Rad57 are not required for assembly of Dmc1 foci in vivo, although Rad51 itself promotes Dmc1 foci (19–21). More recently, immunostaining and chromatin immunoprecipitation experiments demonstrated a role for the Mei5 and Sae3 proteins of Saccharomyces cerevisiae in assembly of Dmc1 at sites of DSBs in vivo (22, 23). Consistent with these observations, mei5 and sae3 mutants display markedly similar meiotic defects as compared with dmc1 mutants, including defects in sporulation, spore viability, crossing over, DSB repair, progression through meiosis, and synaptonemal complex formation (19, 22–24). Finally, the three proteins have been shown to physically interact; Mei5 and Sae3 have been co-purified and co-immunoprecipitated, and an N-terminal portion of Mei5 has been shown to interact with Dmc1 in a two-hybrid assay (22).The fission yeast Schizosaccharomyces pombe encodes two proteins, Swi5 and Sfr1, which share sequence homology with Sae3 and Mei5, respectively (22). Swi5 and Sfr1 have been shown to stimulate the strand exchange activity of Rhp51 (the S. pombe Rad51 homologue) and Dmc1 (25). Although some results indicate functional similarity of Swi5-Sfr1 and Mei5-Sae3, there are also clear differences. The Mei5-Sae3 complex of budding yeast is expressed solely during meiosis, and no mitotic phenotypes have been reported for mei5 or sae3 mutants (22, 24, 26). In contrast, the Swi5-Sfr1 complex of fission yeast is expressed in mitotic and meiotic cells, and mutations in SWI5 have been shown to cause defects in mitotic recombination (27). Furthermore, although mei5 and sae3 mutants are phenotypically similar to dmc1 mutants, swi5 and sfr1 mutants display more severe meiotic defects during fission yeast meiosis than do dmc1 mutants (27–29). These data suggest that although Swi5-Sfr1 clearly contributes to Rad51 activity in fission yeast, it is possible that the activity of Mei5-Sae3 is restricted to stimulating Dmc1 in budding yeast.In this study, a biochemical approach is used to test the budding yeast Mei5-Sae3 complex for properties expected of a recombinase assembly mediator. We show that Mei5-Sae3 binds both ssDNA and double-stranded DNA (dsDNA) but binds ssDNA preferentially. We also show that Mei5-Sae3 can overcome the inhibitory effects of RPA on the ssDNA binding and strand assimilation activities of Dmc1. Finally, we show that Mei5-Sae3 and RPA bind one another directly. These results indicate that Mei5-Sae3 acts directly as a mediator protein for assembly of Dmc1.     

Mnd1/Hop2 facilitates Dmc1-dependent interhomolog crossover formation in meiosis of budding yeast

During meiosis, each chromosome must pair with its homolog and undergo meiotic crossover recombination in order to segregate properly at the first meiotic division. Recombination in meiosis in Saccharomyces cerevisiae relies on two Escherichia coli recA homologs, Rad51 and Dmc1, as well as the more recently discovered heterodimer Mnd1/Hop2. Meiotic recombination in S. cerevisiae mnd1 and hop2 single mutants is initiated via double-strand breaks (DSBs) but does not progress beyond this stage; heteroduplex DNA, joint molecules, and crossovers are not detected. Whereas hop2 and mnd1 single mutants are profoundly recombination defective, we show that mnd1 rad51, hop2 rad51, and mnd1 rad17 double mutants are able to carry out crossover recombination. Interestingly, noncrossover recombination is absent, indicating a role for Mnd1/Hop2 in the designation of DSBs for noncrossover recombination. We demonstrate that in the rad51 mnd1 double mutant, recombination is more likely to occur between repetitive sequences on nonhomologous chromosomes. Our results support a model in which Mnd1/Hop2 is required for DNA-DNA interactions that help ensure Dmc1-mediated stable strand invasion between homologous chromosomes, thereby preserving genomic integrity.     

AtMND1 is required for homologous pairing during meiosis in Arabidopsis

Background  

Pairing of homologous chromosomes at meiosis is an important requirement for recombination and balanced chromosome segregation among the products of meiotic division. Recombination is initiated by double strand breaks (DSBs) made by Spo11 followed by interaction of DSB sites with a homologous chromosome. This interaction requires the strand exchange proteins Rad51 and Dmc1 that bind to single stranded regions created by resection of ends at the site of DSBs and promote interactions with uncut DNA on the homologous partner. Recombination is also considered to be dependent on factors that stabilize interactions between homologous chromosomes. In budding yeast Hop2 and Mnd1 act as a complex to promote homologous pairing and recombination in conjunction with Rad51 and Dmc1.     

Changes in histone acetylation during mouse oocyte meiosis

We examined global changes in the acetylation of histones in mouse oocytes during meiosis. Immunocytochemistry with specific antibodies against various acetylated lysine residues on histones H3 and H4 showed that acetylation of all the lysines decreased to undetectable or negligible levels in the oocytes during meiosis, whereas most of these lysines were acetylated during mitosis in preimplantation embryos and somatic cells. When the somatic cell nuclei were transferred into enucleated oocytes, the acetylation of lysines decreased markedly. This type of deacetylation was inhibited by trichostatin A, a specific inhibitor of histone deacetylase (HDAC), thereby indicating that HDAC is able to deacetylate histones during meiosis but not during mitosis. Meiosis-specific deacetylation may be a consequence of the accessibility of HDAC1 to the chromosome, because HDAC1 colocalized with the chromosome during meiosis but not during mitosis. As histone acetylation is thought to play a role in propagating the gene expression pattern to the descendent generation during mitosis, and the gene expression pattern of differentiated oocytes is reprogrammed during meiosis to allow the initiation of a new program by totipotent zygotes of the next generation, our results suggest that the oocyte cytoplasm initializes a program of gene expression by deacetylating histones.     

2P1, a novel male mouse cDNA specifically expressed during meiosis

We screened a mouse germinal cell expression library with a probe derived from Sob1, a human testis-specific cDNA, and identified 2P1, a new mouse cDNA. A database search revealed that 2P1 was 91% identical to ORF1 of E3-3, a rat gene probably involved in the regulation of alternative splicing. Sequencing showed that 2P1 has a destabilization motif in its 3'-untranslated region. Northern blotting showed strong gene expression in the testis and weak expression in the epididymis, with no signal detected in other tissues. RT-PCR analysis confirmed testis and epididymis expression. In situ hybridization revealed that 2P1 mRNA was absent in spermatogonia but expressed in spermatocytes. This last result was confirmed by RT-PCR of FACS isolated primary spermatocytes (pachytene stage). Using RT-PCR, purified spermatids were also shown to express 2P1.     

Survivin regulates Plk1 localization to kinetochore in mouse oocyte meiosis

Survivin is a member of inhibitors of apoptosis proteins (IAPs), and also belongs to be a member of the chromosomal passenger complex (CPC) which has multiple functions including inhibition of apoptosis and regulation of cell division and SAC activity. Plk1 (polo-like kinase 1) associates with the spindle poles and also distributes to the kinetochores and is shown to involve in spindle organization, APC/C activation and cytokinesis in many models. Our recent work has shown that Survivin is a critical regulator of chromosome segregation and spindle assembly checkpoint (SAC) in meiosis. In the present study, we found that Plk1 co-localized with Survivin at metaphase I (MI) and telophase I (TI) stage after GVBD. Plk1 dispersed into the oocyte cytoplasm or accumulated near the chromosomes after the depletion of Survivin by morpholino (MO) injection. Our results showed that the localization of Plk1 to kinetochores required the involvement of Survivin.     

Partner choice during meiosis is regulated by Hop1-promoted dimerization of Mek1

Meiotic recombination differs from mitotic recombination in that DSBs are repaired using homologous chromosomes, rather than sister chromatids. This change in partner choice is due in part to a barrier to sister chromatid repair (BSCR) created by the meiosis-specific kinase, Mek1, in a complex with two other meiosis-specific proteins, Hop1 and Red1. HOP1 contains two functional domains, called the N and C domains. Analysis of a point mutation that specifically inactivates the C domain (hop1-K593A) reveals that the N domain is sufficient for Hop1 localization to chromosomes and for Red1 and Hop1 interactions. The C domain is needed for spore viability, for chromosome synapsis, and for preventing DMC1-independent DSB repair, indicating it plays a role in the BSCR. All of the hop1-K593A phenotypes can be bypassed by fusion of ectopic dimerization domains to Mek1, suggesting that the function of the C domain is to promote Mek1 dimerization. Hop1 is a DSB-dependent phosphoprotein, whose phosphorylation requires the presence of the C domain, but is independent of MEK1. These results suggest a model in which Hop1 phosphorylation in response to DSBs triggers dimerization of Mek1 via the Hop1 C domain, thereby enabling Mek1 to phosphorylate target proteins that prevent repair of DSBs by sister chromatids.     

Mouse Emi2 is required to enter meiosis II by reestablishing cyclin B1 during interkinesis

During interkinesis, a metaphase II (MetII) spindle is built immediately after the completion of meiosis I. Oocytes then remain MetII arrested until fertilization. In mouse, we find that early mitotic inhibitor 2 (Emi2), which is an anaphase-promoting complex inhibitor, is involved in both the establishment and the maintenance of MetII arrest. In MetII oocytes, Emi2 needs to be degraded for oocytes to exit meiosis, and such degradation, as visualized by fluorescent protein tagging, occurred tens of minutes ahead of cyclin B1. Emi2 antisense morpholino knockdown during oocyte maturation did not affect polar body (PB) extrusion. However, in interkinesis the central spindle microtubules from meiosis I persisted for a short time, and a MetII spindle failed to assemble. The chromatin in the oocyte quickly decondensed and a nucleus formed. All of these effects were caused by the essential role of Emi2 in stabilizing cyclin B1 after the first PB extrusion because in Emi2 knockdown oocytes a MetII spindle was recovered by Emi2 rescue or by expression of nondegradable cyclin B1 after meiosis I.     

A protein complex containing Mei5 and Sae3 promotes the assembly of the meiosis-specific RecA homolog Dmc1

Meiotic recombination requires the meiosis-specific RecA homolog Dmc1 as well as the mitotic RecA homolog Rad51. Here, we show that the two meiosis-specific proteins Mei5 and Sae3 are necessary for the assembly of Dmc1, but not for Rad51, on chromosomes including the association of Dmc1 with a recombination hot spot. Mei5, Sae3, and Dmc1 form a ternary and evolutionary conserved complex that requires Rad51 for recruitment to chromosomes. Mei5, Sae3, and Dmc1 are mutually dependent for their chromosome association, and their absence prevents the disassembly of Rad51 filaments. Our results suggest that Mei5 and Sae3 are loading factors for the Dmc1 recombinase and that the Dmc1-Mei5-Sae3 complex is integrated onto Rad51 ensembles and, together with Rad51, plays both catalytic and structural roles in interhomolog recombination during meiosis.     

Replication protein A is sequentially phosphorylated during meiosis

Phosphorylation of the cellular single-stranded DNA-binding protein, replication protein A (RPA), occurs during normal mitotic cell cycle progression and also in response to genotoxic stress. In budding yeast, these reactions require the ATM homolog Mec1, a central regulator of the DNA replication and DNA damage checkpoint responses. We now demonstrate that the middle subunit of yeast RPA (Rfa2) becomes phosphorylated in two discrete steps during meiosis. Primary Rfa2 phosphorylation occurs early in meiotic progression and is independent of DNA replication, recombination and Mec1. In contrast, secondary Rfa2 phosphorylation is activated upon initiation of recombination and requires Mec1. While the primary Rfa2 phosphoisomer is detectable throughout most of meiosis, the secondary Rfa2 phosphoisomer is only transiently generated and begins to disappear soon after recombination is complete. Extensive secondary Rfa2 phosphorylation is observed in a recombination mutant defective for the pachytene checkpoint, indicating that Mec1-dependent Rfa2 phosphorylation does not function to maintain meiotic delay in response to DNA double-strand breaks. Our results suggest that Mec1-dependent RPA phosphorylation could be involved in regulating recombination rather than cell cycle or meiotic progression.     

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.     

Nuclear compartmentalization is abolished during fission yeast meiosis

In eukaryotic cells, the nuclear envelope partitions the nucleus from the cytoplasm. The fission yeast Schizosaccharomyces pombe undergoes closed mitosis in which the nuclear envelope persists rather than being broken down, as in higher eukaryotic cells. It is therefore assumed that nucleocytoplasmic transport continues during the cell cycle. Here we show that nuclear transport is, in fact, abolished specifically during anaphase of the second meiotic nuclear division. During that time, both nucleoplasmic and cytoplasmic proteins disperse throughout the cell, reminiscent of the open mitosis of higher eukaryotes, but the architecture of the S. pombe nuclear envelope itself persists. This functional alteration of the nucleocytoplasmic barrier is likely induced by spore wall formation, because ectopic induction of sporulation signaling leads to premature dispersion of nucleoplasmic proteins. A photobleaching assay demonstrated that nuclear envelope permeability increases abruptly at the onset of anaphase of the second meiotic division. The permeability was not altered when sporulation was inhibited by blocking the trafficking of forespore-membrane vesicles from the endoplasmic reticulum to the Golgi. The evidence indicates that yeast gametogenesis produces vesicle transport-mediated forespore membranes by inducing nuclear envelope permeabilization.     

Septin 7 is required for orderly meiosis in mouse oocytes

Septin 7 is a conserved GTP-binding protein. In this study, we examined the localization and functions of Septin 7 during mouse oocyte meiotic maturation. Immunofluorescent analysis showed that intrinsic Septin 7 localized to the spindles from the pro-MI stage to the MII stage. Knockdown of Septin 7 by siRNA microinjection caused abnormal spindles and affected extrusion of the first polar body. Septin 7 mRNA tagged with myc was injected into GV stage oocytes to overexpress Septin 7. Overexpressed Myc-Septin 7 localized to the spindle and beneath the plasma membrane displaying long filaments. Fluorescence intensity of spindle α-tubulin in myc-Septin 7-injected oocytes was weaker than that of the control group, demonstrating that Septin 7 may influence recruitment of α-tubulin to spindles. MII oocytes injected with myc-Septin 7 exhibited abnormal chromosome alignment, and parthenogenetic activation failed to allow extrusion of the second polar body, suggesting that overexpression of Septin 7 may affect extrusion of the polar body by disturbing the alignment of chromosomes and regulating α-tubulin recruitment to spindles. In summary, Septin 7 may regulate meiotic cell cycle progression by affecting microtubule cytoskeletal dynamics in mouse oocytes.     

Septin 7 is required for orderly meiosis in mouse oocytes

Septin 7 is a conserved GTP-binding protein. In this study, we examined the localization and functions of Septin 7 during mouse oocyte meiotic maturation. Immunofluorescent analysis showed that intrinsic Septin 7 localized to the spindles from the pro-MI stage to the MII stage. Knockdown of Septin 7 by siRNA microinjection caused abnormal spindles and affected extrusion of the first polar body. Septin 7 mRNA tagged with myc was injected into GV stage oocytes to overexpress Septin 7. Overexpressed Myc-Septin 7 localized to the spindle and beneath the plasma membrane displaying long filaments. Fluorescence intensity of spindle α-tubulin in myc-Septin 7-injected oocytes was weaker than that of the control group, demonstrating that Septin 7 may influence recruitment of α-tubulin to spindles. MII oocytes injected with myc-Septin 7 exhibited abnormal chromosome alignment, and parthenogenetic activation failed to allow extrusion of the second polar body, suggesting that overexpression of Septin 7 may affect extrusion of the polar body by disturbing the alignment of chromosomes and regulating α-tubulin recruitment to spindles. In summary, Septin 7 may regulate meiotic cell cycle progression by affecting microtubule cytoskeletal dynamics in mouse oocytes.     

Homeostatic control of recombination is implemented progressively in mouse meiosis

Humans suffer from high rates of fetal aneuploidy, often arising from the absence of meiotic crossover recombination between homologous chromosomes. Meiotic recombination is initiated by double-strand breaks (DSBs) generated by the SPO11 transesterase. In yeast and worms, at least one buffering mechanism, crossover homeostasis, maintains crossover numbers despite variation in DSB numbers. We show here that mammals exhibit progressive homeostatic control of recombination. In wild-type mouse spermatocytes, focus numbers for early recombination proteins (RAD51, DMC1) were highly variable from cell to cell, whereas foci of the crossover marker MLH1 showed little variability. Furthermore, mice with greater or fewer copies of the Spo11 gene--with correspondingly greater or fewer numbers of early recombination foci--exhibited relatively invariant crossover numbers. Homeostatic control is enforced during at least two stages, after the formation of early recombination intermediates and later while these intermediates mature towards crossovers. Thus, variability within the mammalian meiotic program is robustly managed by homeostatic mechanisms to control crossover formation, probably to suppress aneuploidy. Meiotic recombination exemplifies how order can be progressively implemented in a self-organizing system despite natural cell-to-cell disparities in the underlying biochemical processes.