The dsy10 Mutation of Arabidopsis results in desynapsis and a general breakdown in meiosis

The proper pairing and recombination of chromosomes during prophase is essential for the formation of gametes during meiosis. As part of studies to identify genes required for homologous chromosome pairing and recombination during meiosis in plants, we characterized a number of T-DNA-tagged, male-sterile mutants of Arabidopsis. Preliminary cytological studies on one line, 7219 which is male and female sterile, suggested that the mutation may disrupt meiosis and result in the formation of aberrant microsporocytes and microspores. In this report we present the results of a detailed analysis of meiosis in microsporocytes of sterile plants to elucidate the nature of the 7219 mutation. Analysis indicates that the mutation usually results in a desynaptic phenotype, with ten sister chromatids observed prior to metaphase I in most cells. Based on this, we named the mutation dsy10. The presence of several other meiotic defects suggests that dsy10 may not be a typical desynaptic mutant. Received: 15 December 2000 / Accepted: 19 April 2001     

An analysis of univalent segregation in meiotic mutants of Arabidopsis thaliana: a possible role for synaptonemal complex

During first meiotic prophase, homologous chromosomes are normally kept together by both crossovers and synaptonemal complexes (SC). In most eukaryotes, the SC disassembles at diplotene, leaving chromosomes joined by chiasmata. The correct co-orientation of bivalents at metaphase I and the reductional segregation at anaphase I are facilitated by chiasmata and sister-chromatid cohesion. In the absence of meiotic reciprocal recombination, homologs are expected to segregate randomly at anaphase I. Here, we have analyzed the segregation of homologous chromosomes at anaphase I in four meiotic mutants of Arabidopsis thaliana, spo11-1-3, dsy1, mpa1, and asy1, which show a high frequency of univalents at diplotene. The segregation pattern of chromosomes 2, 4, and 5 was different in each mutant. Homologous univalents segregated randomly in spo11-1-3, whereas they did not in dsy1 and mpa1. An intermediate situation was observed in asy1. Also, we have found a parallelism between this behavior and the synaptic pattern displayed by each mutant. Thus, whereas spo11-1-3 and asy1 showed low amounts of SC stretches, dsy1 and mpa1 showed full synapsis. These findings suggest that in Arabidopsis there is a system, depending on the SC formation, that would facilitate regular disjunction of homologous univalents to opposite poles at anaphase I.     

Double-stranded DNA breaks and gene functions in recombination and meiosis

Meiotic prophase I is a long and complex phase. Homologous recombination is an important process that occurs between homologous chromosomes during meiotic prophase I. Formation of chiasmata, which hold homologous chromosomes together until the metaphase I to anaphase I transition, is critical for proper chromosome segregation. Recent studies have suggested that the SPO 11 proteins have conserved functions in a number of organisms in generating sites of double-stranded DNA breaks （DSBs） that are thought to be the starting points of homologous recombination. Processing of these sites of DSBs requires the function of RecA homologs, such as RAD5 1, DMC 1, and others, as suggested by mutant studies; thus the failure to repair these meiotic DSBs results in abnormal chromosomal alternations, leading to disrupted meiosis. Recent discoveries on the functions of these RecA homologs have improved the understanding of the mechanisms underlying meiotic homologous recombination.     

Comparative analysis of female and male meiosis in three meiotic mutants of tomato.

Female meiosis was analysed in squash preparations of ovules from three meiotic mutants and wild-type plants of tomato. In the completely asynaptic mutant as6, chromosome pairing and chiasma formation were virtually absent in both sexes. In the partially asynaptic mutant asb, with intermediate levels of chromosome pairing at pachytene, there were a higher number of chiasmate chromosome arms in female meiosis than in male meiosis, whereas in the desynaptic mutant as5 there were normal levels of chromosome pairing at pachytene and a similar reduction in chiasma frequency in the two sexes. In wild-type tomato, we found slightly higher numbers of chiasmate chromosome arms in female meiosis than in male meiosis. We propose that the higher female chiasma frequencies in mutant asb and wild-type tomato result from a longer duration of female meiotic prophase. This would allow chromosomes more time to pair and recombine. It is possible that a longer duration of prophase I does not affect mutants as5 and as6, either because the meiotic defect acts before the pairing process begins (in as6) or because it acts at a later stage and involves chiasma maintenance (in as5).     

The desynaptic (dy) and desynaptic1 (dsy1) mutations in maize (Zea mays L) cause distinct telomere-misplacement phenotypes during meiotic prophase

During meiotic prophase, telomeres actively attach themselves to the nuclear envelope and cluster in an arrangement called the bouquet. The bouquet is unique to meiosis, highly conserved, and thought to facilitate homologous chromosome synapsis. Analy sis of three-dimensional fluorescence in situ hybridization (3-D FISH) image data has been employed to characterize the bouquet in fixed pollen mother cells of maize (Zea mays L.). In order to examine the function of the bouquet further, several meiotic mutants were screened for telomeric defects using 3-D FISH as an assay. Two mutants, desynaptic (dy) and desynaptic1 (dsy1), were found to exhibit novel telomere-misplacement phenotypes. In both cases, the telomere-associated mutant phenotypes occurred prior to what was previously reported as the earliest affected stage. Three alleles of the desynaptic1 mutation (dsy1-1, dsy1-9101, and dsy1-9307) resulted in a partial bouquet phenotype at the zygotene stage of meiotic prophase. By contrast, dy nuclei contained apparently normal bouquets, but then resulted in a premature intranuclear localization of telomeres at the pachytene stage, when telomeres normally disperse but remain attached to the nuclear envelope. The dsy1 mutation is known to impair the fidelity and progression of homologous synapsis, whereas the dy mutation is known to reduce recombination rates. If the telomere misplacements are primary defects of these mutants, then these data would be consistent with the hypothesis that meiotic telomeres have at least two separable functions, one involving proper homologous chromosome synapsis at the bouquet stage and another involving post-bouquet cross-over control.     

The<Emphasis Type="Italic">atspo11-1</Emphasis> mutation rescues <Emphasis Type="Italic">atxrcc3</Emphasis> meiotic chromosome fragmentation

Homologous recombination events occurring during meiotic prophase I ensure the proper segregation of homologous chromosomes at the first meiotic division. These events are initiated by programmed double-strand breaks produced by the Spo11 protein and repair of such breaks by homologous recombination requires a strand exchange activity provided by the Rad51 protein. We have recently reported that the absence of AtXrcc3, an ArabidopsisRad51 paralogue, leads to extensive chromosome fragmentation during meiosis, first visible in diplotene of meiotic prophase I. The present study clearly shows that this fragmentation results from un- or mis-repaired AtSpo11-1 induced double-strand breaks and is thus due to a specific defect in the meiotic recombination process.     

Extreme heterogeneity in the molecular events leading to the establishment of chiasmata during meiosis i in human oocytes

In humans, ~50% of conceptuses are chromosomally aneuploid as a consequence of errors in meiosis, and most of these aneuploid conceptuses result in spontaneous miscarriage. Of these aneuploidy events, 70% originate during maternal meiosis, with the majority proposed to arise as a direct result of defective crossing over during meiotic recombination in prophase I. By contrast, <1%-2% of mouse germ cells exhibit prophase I-related nondisjunction events. This disparity among mammalian species is surprising, given the conservation of genes and events that regulate meiotic progression. To understand the mechanisms that might be responsible for the high error rates seen in human females, we sought to further elucidate the regulation of meiotic prophase I at the molecular cytogenetic level. Given that these events occur during embryonic development in females, samples were obtained during a defined period of gestation (17-24 weeks). Here, we demonstrate that human oocytes enter meiotic prophase I and progress through early recombination events in a similar temporal framework to mice. However, at pachynema, when chromosomes are fully paired, we find significant heterogeneity in the localization of the MutL homologs, MLH1 and MLH3, among human oocyte populations. MLH1 and MLH3 have been shown to mark late-meiotic nodules that correlate well with--and are thought to give rise to--the sites of reciprocal recombination between homologous chromosomes, which suggests a possible 10-fold variation in the processing of nascent recombination events. If such variability persists through development and into adulthood, these data would suggest that as many as 30% of human oocytes are predisposed to aneuploidy as a result of prophase I defects in MutL homolog-related events.     

Dynamics of chromosome organization and pairing during meiotic prophase in fission yeast

Interactions between homologous chromosomes (pairing, recombination) are of central importance for meiosis. We studied entire chromosomes and defined chromosomal subregions in synchronous meiotic cultures of Schizosaccharomyces pombe by fluorescence in situ hybridization. Probes of different complexity were applied to spread nuclei, to delineate whole chromosomes, to visualize repeated sequences of centromeres, telomeres, and ribosomal DNA, and to study unique sequences of different chromosomal regions. In diploid nuclei, homologous chromosomes share a joint territory even before entry into meiosis. The centromeres of all chromosomes are clustered in vegetative and meiotic prophase cells, whereas the telomeres cluster near the nucleolus early in meiosis and maintain this configuration throughout meiotic prophase. Telomeres and centromeres appear to play crucial roles for chromosome organization and pairing, both in vegetative cells and during meiosis. Homologous pairing of unique sequences shows regional differences and is most frequent near centromeres and telomeres. Multiple homologous interactions are formed independently of each other. Pairing increases during meiosis, but not all chromosomal regions become closely paired in every meiosis. There is no detectable axial compaction of chromosomes in meiotic prophase. S. pombe does not form mature synaptonemal complexes, but axial element-like structures (linear elements), which were analyzed in parallel. Their appearance coincides with pairing of interstitial chromosomal regions. Axial elements may define minimal structures required for efficient pairing and recombination of meiotic chromosomes.     

Analysis of close stable homolog juxtaposition during meiosis in mutants of Saccharomyces cerevisiae

A unique aspect of meiosis is the segregation of homologous chromosomes at the meiosis I division. The pairing of homologous chromosomes is a critical aspect of meiotic prophase I that aids proper disjunction at anaphase I. We have used a site-specific recombination assay in Saccharomyces cerevisiae to examine allelic interaction levels during meiosis in a series of mutants defective in recombination, chromatin structure, or intracellular movement. Red1, a component of the chromosome axis, and Mnd1, a chromosome-binding protein that facilitates interhomolog interaction, are critical for achieving high levels of allelic interaction. Homologous recombination factors (Sae2, Rdh54, Rad54, Rad55, Rad51, Sgs1) aid in varying degrees in promoting allelic interactions, while the Srs2 helicase appears to play no appreciable role. Ris1 (a SWI2/SNF2 related protein) and Dot1 (a histone methyltransferase) appear to play minor roles. Surprisingly, factors involved in microtubule-mediated intracellular movement (Tub3, Dhc1, and Mlp2) appear to play no appreciable role in homolog juxtaposition, unlike their counterparts in fission yeast. Taken together, these results support the notion that meiotic recombination plays a major role in the high levels of homolog interaction observed during budding yeast meiosis.     

A cytoplasmic dynein heavy chain is required for oscillatory nuclear movement of meiotic prophase and efficient meiotic recombination in fission yeast.

Meiotic recombination requires pairing of homologous chromosomes, the mechanisms of which remain largely unknown. When pairing occurs during meiotic prophase in fission yeast, the nucleus oscillates between the cell poles driven by astral microtubules. During these oscillations, the telomeres are clustered at the spindle pole body (SPB), located at the leading edge of the moving nucleus and the rest of each chromosome dangles behind. Here, we show that the oscillatory nuclear movement of meiotic prophase is dependent on cytoplasmic dynein. We have cloned the gene encoding a cytoplasmic dynein heavy chain of fission yeast. Most of the cells disrupted for the gene show no gross defect during mitosis and complete meiosis to form four viable spores, but they lack the nuclear movements of meiotic prophase. Thus, the dynein heavy chain is required for these oscillatory movements. Consistent with its essential role in such nuclear movement, dynein heavy chain tagged with green fluorescent protein (GFP) is localized at astral microtubules and the SPB during the movements. In dynein-disrupted cells, meiotic recombination is significantly reduced, indicating that the dynein function is also required for efficient meiotic recombination. In accordance with the reduced recombination, which leads to reduced crossing over, chromosome missegregation is increased in the mutant. Moreover, both the formation of a single cluster of centromeres and the colocalization of homologous regions on a pair of homologous chromosomes are significantly inhibited in the mutant. These results strongly suggest that the dynein-driven nuclear movements of meiotic prophase are necessary for efficient pairing of homologous chromosomes in fission yeast, which in turn promotes efficient meiotic recombination.     

Cis-Acting Determinants Affecting Centromere Function, Sister-Chromatid Cohesion and Reciprocal Recombination during Meiosis in Saccharomyces Cerevisiae

We have employed a system that utilizes homologous pairs of human DNA-derived yeast artificial chromosomes (YACs) as marker chromosomes to assess the specific role (s) of conserved centromere DNA elements (CDEI, CDEII and CDEIII) in meiotic chromosome disjunction fidelity. Thirteen different centromere (CEN) mutations were tested for their effects on meiotic centromere function. YACs containing a wild-type CEN DNA sequence segregate with high fidelity in meiosis I (99% normal segregation) and in meiosis II (96% normal segregation). YACs containing a 31-bp deletion mutation in centromere DNA element II (CDEIIδ31) in either a heterocentric (mutant/wild type), homocentric (mutant/mutant) or monosomic (mutant/--) YAC pair configuration exhibited high levels (16-28%) of precocious sister-chromatid segregation (PSS) and increased levels (1-6%) of nondisjunction meiosis I (NDI). YACs containing this mutation also exhibit high levels (21%) of meiosis II nondisjunction. Interestingly, significant alterations in homolog recombination frequency were observed in the exceptional PSS class of tetrads, suggesting unusual interactions between prematurely separated sister chromatids and their homologous nonsister chromatids. We also have assessed the meiotic segregation effects of rare gene conversion events occurring at sites located immediately adjacent to or distantly from the centromere region. Proximal gene conversion events were associated with extremely high levels (60%) of meiosis I segregation errors (including both PSS and NDI), whereas distal events had no apparent effect. Taken together, our results indicate a critical role for CDEII in meiosis and underscore the importance of maintaining sister-chromatid cohesion for proper recombination in meiotic prophase and for proper disjunction in meiosis I.     

The pam1 gene is required for meiotic bouquet formation and efficient homologous synapsis in maize (Zea mays L.)

The clustering of telomeres on the nuclear envelope (NE) during meiotic prophase to form the bouquet arrangement of chromosomes may facilitate homologous chromosome synapsis. The pam1 (plural abnormalities of meiosis 1) gene is the first maize gene that appears to be required for telomere clustering, and homologous synapsis is impaired in pam1. Telomere clustering on the NE is arrested or delayed at an intermediate stage in pam1. Telomeres associate with the NE during the leptotene-zygotene transition but cluster slowly if at all as meiosis proceeds. Intermediate stages in telomere clustering including miniclusters are observed in pam1 but not in wild-type meiocytes. The tight bouquet normally seen at zygotene is a rare event. In contrast, the polarization of centromeres vs. telomeres in the nucleus at the leptotene-zygotene transition is the same in mutant and wild-type cells. Defects in homologous chromosome synapsis include incomplete synapsis, nonhomologous synapsis, and unresolved interlocks. However, the number of RAD51 foci on chromosomes in pam1 is similar to that of wild type. We suggest that the defects in homologous synapsis and the retardation of prophase I arise from the irregularity of telomere clustering and propose that pam1 is involved in the control of bouquet formation and downstream meiotic prophase I events.     

Coordinating the events of the meiotic prophase

Meiosis is a specialized type of cell division leading to the production of gametes. During meiotic prophase I, homologous chromosomes interact with each other and form bivalents (pairs of homologous chromosomes). Three major meiotic processes--chromosome pairing, synapsis and recombination--are involved in the formation of bivalents. Many recent reports have uncovered complex networks of interactions between these processes. Chromosome pairing is largely dependent on the initiation and progression of recombination in fungi, mammals and plants, but not in Caenorhabditis elegans or Drosophila. Synapsis and recombination are also tightly linked. Understanding the coordination between chromosome pairing, synapsis and recombination lends insight into many poorly explained aspects of meiosis, such as the nature of chromosome homology recognition.     

SUN1 is required for telomere attachment to nuclear envelope and gametogenesis in mice

Prior to the pairing and recombination between homologous chromosomes during meiosis, telomeres attach to the nuclear envelope and form a transient cluster. However, the protein factors mediating meiotic telomere attachment to the nuclear envelope and the requirement of this attachment for homolog pairing and synapsis have not been determined in animals. Here we show that the inner nuclear membrane protein SUN1 specifically associates with telomeres between the leptotene and diplotene stages during meiotic prophase I. Disruption of Sun1 in mice prevents telomere attachment to the nuclear envelope, efficient homolog pairing, and synapsis formation in meiosis. Massive apoptotic events are induced in the mutant gonads, leading to the abolishment of both spermatogenesis and oogenesis. This study provides genetic evidence that SUN1-telomere interaction is essential for telomere dynamic movement and is required for efficient homologous chromosome pairing/synapsis during mammalian gametogenesis.     

DNA polymerase mu interacts with a meiosis-specific RecA homolog Lim15 during meiosis in Coprinus cinereus

Meiosis is a fundamental process in eukaryotes. Homologous chromosomes are paired and recombined during meiotic prophase I, which results in variation among the gametes. However, the mechanism of recombination between the maternal and paternal chromosome is unknown. In this study, we report on the identification of interaction between Coprinus cinereus DNA polymerase mu (CcPol mu) and CcLim15/Dmc1, a meiosis-specific RecA-like protein, during meiosis. Interaction between these two proteins was confirmed using a GST-pull down assay. A two-hybrid assay revealed that the N-terminus of CcPol mu, which includes the BRCT domain, is responsible for binding the C-terminus of CcLim15. Furthermore, co-immunoprecipitation experiments indicate that these two proteins also interact in the crude extract of the meiotic cell. A significant proportion of CcPol mu and CcLim15 is shown to co-localize in nuclei from the leptotene/zygotene stage to the early pachytene stage during meiotic prophase I. Moreover, CcLim15 enhances polymerase activity of CcPol mu early in the reaction. These results suggest that CcPol mu might be recruited by CcLim15 and elongate the D-loop structure during homologous recombination in meiosis.     

Isolation of Com1, a New Gene Required to Complete Meiotic Double-Strand Break-Induced Recombination in Saccharomyces Cerevisiae

We have designed a screen to isolate mutants defective during a specific part of meiotic prophase I of the yeast Saccharomyces cerevisiae. Genes required for the repair of meiotic double-strand breaks or for the separation of recombined chromosomes are targets of this mutant hunt. The specificity is achieved by selecting for mutants that produce viable spores when recombination and reductional segregation are prevented by mutations in SPO11 and SPO13 genes, but fail to yield viable spores during a normal Rec(+) meiosis. We have identified and characterized a mutation com1-1, which blocks processing of meiotic double-strand breaks and which interferes with synaptonemal complex formation, homologous pairing and, as a consequence, spore viability after induction of meiotic recombination. The COM1/SAE2 gene was cloned by complementation, and the deletion mutant has a phenotype similar to com1-1. com1/sae2 mutants closely resemble the phenotype of rad50S, as assayed by phase-contrast microscopy for spore formation, physical and genetic analysis of recombination, fluorescence in situ hybridization to quantify homologous pairing and immunofluorescence and electron microscopy to determine the capability to synapse axial elements.     

RAD51 and DMC1 form mixed complexes associated with mouse meiotic chromosome cores and synaptonemal complexes.

The eukaryotic RecA homologues RAD51 and DMC1 function in homology recognition and formation of joint-molecule recombination intermediates during yeast meiosis. The precise immunolocalization of these two proteins on the meiotic chromosomes of plants and animals has been complicated by their high degree of identity at the amino acid level. With antibodies that have been immunodepleted of cross-reactive epitopes, we demonstrate that RAD51 and DMC1 have identical distribution patterns in extracts of mouse spermatocytes in successive prophase I stages, suggesting coordinate functionality. Immunofluorescence and immunoelectron microscopy with these antibodies demonstrate colocalization of the two proteins on the meiotic chromosome cores at early prophase I. We also show that mouse RAD51 and DMC1 establish protein-protein interactions with each other and with the chromosome core component COR1(SCP3) in a two-hybrid system and in vitro binding analyses. These results suggest that the formation of a multiprotein recombination complex associated with the meiotic chromosome cores is essential for the development and fulfillment of the meiotic recombination process.     

AtSPO11-1 is necessary for efficient meiotic recombination in plants

The Saccharomyces cerevisiae Spo11 protein catalyses DNA double-strand breaks (DSBs) that initiate meiotic recombination. The model plant Arabidopsis thaliana possesses at least three SPO11 homologues. T-DNA and ethyl-methane sulfonate mutagenesis allowed us to show that meiotic progression is altered in plants in which the AtSPO11-1 gene is disrupted. Both male and female meiocytes formed very few bivalents. Furthermore, no fully synapsed chromosomes were observed during prophase I. Later, in meiosis I, we observed that chromosomes segregated randomly, leading to the production of a large proportion of non-functional gametes. These meiotic aberrations were associated with a drastic reduction in meiotic recombination. Thus, our data show that initiation of meiotic recombination by SPO11- induced DSBs is a mechanism conserved in plants. Furthermore, unlike Drosophila and Caenorhabditis elegans, but like fungi, SPO11 is necessary for normal synapsis in plants.