The AtRAD51C gene is required for normal meiotic chromosome synapsis and double-stranded break repair in Arabidopsis

Meiotic prophase I is a complex process involving homologous chromosome (homolog) pairing, synapsis, and recombination. The budding yeast (Saccharomyces cerevisiae) RAD51 gene is known to be important for recombination and DNA repair in the mitotic cell cycle. In addition, RAD51 is required for meiosis and its Arabidopsis (Arabidopsis thaliana) ortholog is important for normal meiotic homolog pairing, synapsis, and repair of double-stranded breaks. In vertebrate cell cultures, the RAD51 paralog RAD51C is also important for mitotic homologous recombination and maintenance of genome integrity. However, the function of RAD51C in meiosis is not well understood. Here we describe the identification and analysis of a mutation in the Arabidopsis RAD51C ortholog, AtRAD51C. Although the atrad51c-1 mutant has normal vegetative and flower development and has no detectable abnormality in mitosis, it is completely male and female sterile. During early meiosis, homologous chromosomes in atrad51c-1 fail to undergo synapsis and become severely fragmented. In addition, analysis of the atrad51c-1 atspo11-1 double mutant showed that fragmentation was nearly completely suppressed by the atspo11-1 mutation, indicating that the fragmentation largely represents a defect in processing double-stranded breaks generated by AtSPO11-1. Fluorescence in situ hybridization experiments suggest that homolog juxtaposition might also be abnormal in atrad51c-1 meiocytes. These results demonstrate that AtRAD51C is essential for normal meiosis and is probably required for homologous synapsis.     

Multiple roles of Spo11 in meiotic chromosome behavior

Spo11, a type II topoisomerase, is likely to be required universally for initiation of meiotic recombination. However, a dichotomy exists between budding yeast and the animals Caenorhabditis elegans and Drosophila melanogaster with respect to additional roles of Spo11 in meiosis. In Saccharomyces cerevisiae, Spo11 is required for homolog pairing, as well as axial element (AE) and synaptonemal complex (SC) formation. All of these functions are Spo11 independent in C.elegans and D.melanogaster. We examined Spo11 function in a multicellular fungus, Coprinus cinereus. The C.cinereus spo11-1 mutant shows high levels of homolog pairing and occasionally forms full-length AEs, but no SC. In C.cinereus, Spo11 is also required for maintenance of meiotic chromosome condensation and proper spindle formation. Meiotic progression in spo11-1 is aberrant; late in meiosis basidia undergo programmed cell death (PCD). To our knowledge, this is the first example of meiotic PCD outside the animal kingdom. Ionizing radiation can partially rescue spo11-1 for both AE and SC formation and viable spore production, suggesting that the double-strand break function of Spo11 is conserved and is required for these functions.     

Chromosome synapsis defects and sexually dimorphic meiotic progression in mice lacking Spo11

Spo11, a protein first identified in yeast, is thought to generate the chromosome breaks that initiate meiotic recombination. We now report that disruption of mouse Spo11 leads to severe gonadal abnormalities from defective meiosis. Spermatocytes suffer apoptotic death during early prophase; oocytes reach the diplotene/dictyate stage in nearly normal numbers, but most die soon after birth. Consistent with a conserved function in initiating meiotic recombination, Dmc1/Rad51 focus formation is abolished. Spo11(-/-) meiocytes also display homologous chromosome synapsis defects, similar to fungi but distinct from flies and nematodes. We propose that recombination initiation precedes and is required for normal synapsis in mammals. Our results also support the view that mammalian checkpoint responses to meiotic recombination and/or synapsis defects are sexually dimorphic.     

Nuf2 is required for chromosome segregation during mouse oocyte meiotic maturation

Nuf2 plays an important role in kinetochore-microtubule attachment and thus is involved in regulation of the spindle assembly checkpoint in mitosis. In this study, we examined the localization and function of Nuf2 during mouse oocyte meiotic maturation. Myc₆-Nuf2 mRNA injection and immunofluorescent staining showed that Nuf2 localized to kinetochores from germinal vesicle breakdown to metaphase I stages, while it disappeared from the kinetochores at the anaphase I stage, but relocated to kinetochores at the MII stage. Overexpression of Nuf2 caused defective spindles, misaligned chromosomes, and activated spindle assembly checkpoint, and thus inhibited chromosome segregation and metaphase-anaphase transition in oocyte meiosis. Conversely, precocious polar body extrusion was observed in the presence of misaligned chromosomes and abnormal spindle formation in Nuf2 knock-down oocytes, causing aneuploidy. Our data suggest that Nuf2 is a critical regulator of meiotic cell cycle progression in mammalian oocytes.     

The Caenorhabditis elegans SCC-3 homologue is required for meiotic synapsis and for proper chromosome disjunction in mitosis and meiosis

The product of the Caenorhabditis elegans ORF F18E2.3 is homologous to the cohesin component Scc3p. By antibody staining the product of F18E2.3 is found in interphase and early meiotic nuclei. At pachytene it localizes to the axes of meiotic chromosomes but is no longer detectable on chromatin later in meiosis or in mitoses. Depletion of the gene product by RNAi results in aberrant mitoses and meioses. In meiosis, homologous pairing is defective during early meiotic prophase and at diakinesis there occur univalents consisting of loosely connected sister chromatids or completely separated sisters. The recombination protein RAD-51 accumulates in nuclear foci at higher numbers during meiotic prophase and disappears later than in wild-type worms, suggesting a defect in the repair of meiotic double-stranded DNA breaks. Embryos showing nuclei of variable size and anaphase bridges, indicative of mitotic segregation defects, are frequently observed. In the most severely affected gonads, nuclear morphology cannot be related to any specific stage. The cytological localization and the consequences of the lack of the protein indicate that C. elegans SCC-3 is essential for sister chromatid cohesion both in mitosis and in meiosis.     

A Role for SUMO in meiotic chromosome synapsis

During meiotic prophase, homologous chromosomes engage in a complex series of interactions that ensure their proper segregation at meiosis I. A central player in these interactions is the synaptonemal complex (SC), a proteinaceous structure elaborated along the lengths of paired homologs. In mutants that fail to make SC, crossing over is decreased, and chromosomes frequently fail to recombine; consequently, many meiotic products are inviable because of aneuploidy. Here, we have investigated the role of the small ubiquitin-like protein modifier (SUMO) in SC formation during meiosis in budding yeast. We show that SUMO localizes specifically to synapsed regions of meiotic chromosomes and that this localization depends on Zip1, a major building block of the SC. A non-null allele of the UBC9 gene, which encodes the SUMO-conjugating enzyme, impairs Zip1 polymerization along chromosomes. The Ubc9 protein localizes to meiotic chromosomes, coincident with SUMO staining. In the zip1 mutant, SUMO localizes to discrete foci on chromosomes. These foci coincide with axial associations, where proteins involved in synapsis initiation are located. Our data suggest a model in which SUMO modification of chromosomal proteins promotes polymerization of Zip1 along chromosomes. The ubc9 mutant phenotype provides the first evidence for a cause-and-effect relationship between sumoylation and synapsis.     

The maize desynaptic1 mutation disrupts meiotic chromosome synapsis

In most eukaryotes, homologous chromosomes undergo synapsis during the first meiotic prophase. A consequence of mutations that interfere with the fidelity or completeness of synapsis can be failure in the formation or maintenance of bivalents, resulting in univalent formation at diakinesis and production of unbalanced spores or gametes. Such mutations, termed desynaptic mutations, can result in complete or partial sterility. We have examined the effect of the maize desynaptic1-9101 mutation on synapsis, using the nuclear spread technique and electron microscopy to examine microsporocytes ranging from early pachytene until the diplotene stage of prophase I. Throughout the pachytene stage, there was an average of about 10 sites of lateral element divergence (indicating nonhomologous synapsis), and during middle and late pachytene, an average of two and three sites of foldback (intrachromosomal) synapsis, per mutant nucleus, respectively. By the diplotene stage, the number of sites of lateral element divergence had decreased to seven, and there was an average of one foldback synapsis site per nucleus. Lateral element divergence and foldback synapsis were not found in spread pachytene nuclei from normal plants. These results imply that the normal expression of the dsy1 gene is essential for the restriction of chromosome synapsis to homologues. The abundance of nonhomologous synapsis and the persistence of extended stretches of unsynapsed axial elements throughout the pachytene stage of dsy1–9101 meiocytes suggests that this mutation disrupts both the fidelity of homology search and the forward course of the synaptic process. This mutation may identify a maize mismatch repair gene. Dev. Genet. 21:146–159, 1997. © 1997 Wiley-Liss, Inc.     

The murine SCP3 gene is required for synaptonemal complex assembly, chromosome synapsis, and male fertility

During meiosis, the homologous chromosomes pair and recombine. An evolutionarily conserved protein structure, the synaptonemal complex (SC), is located along the paired meiotic chromosomes. We have studied the function of a structural component in the axial/lateral element of the SC, the synaptonemal complex protein 3 (SCP3). A null mutation in the SCP3 gene was generated, and we noted that homozygous mutant males were sterile due to massive apoptotic cell death during meiotic prophase. The SCP3-deficient male mice failed to form axial/lateral elements and SCs, and the chromosomes in the mutant spermatocytes did not synapse. While the absence of SCP3 affected the nuclear distribution of DNA repair and recombination proteins (Rad51 and RPA), as well as synaptonemal complex protein 1 (SCP1), a residual chromatin organization remained in the mutant meiotic cells.     

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.     

The Sgs1 helicase regulates chromosome synapsis and meiotic crossing over

BACKGROUND: In budding yeast, Sgs1 is the sole member of the RecQ family of DNA helicases. Like the human Bloom syndrome helicase (BLM), Sgs1 functions during both vegetative growth and meiosis. The sgs1 null mutant sporulates poorly and displays reduced spore viability. RESULTS: We have identified novel functions for Sgs1 in meiosis. Loss of Sgs1 increases the number of axial associations, which are connections between homologous chromosomes that serve as initiation sites for synaptonemal complex formation. In addition, mutation of SGS1 increases the number of synapsis initiation complexes and increases the rate of chromosome synapsis. Loss of Sgs1 also increases the number of meiotic crossovers without changing the frequency of gene conversion. The sgs1 defect in sporulation is due to checkpoint-induced arrest/delay at the pachytene stage of meiotic prophase. A non-null allele of SGS1 that specifically deletes the helicase domain is defective in the newly described meiotic functions of Sgs1, but wild-type for most vegetative functions and for spore formation. CONCLUSIONS: We have shown that the helicase domain of Sgs1 serves as a negative regulator of meiotic interchromosomal interactions. The activity of the wild-type Sgs1 protein reduces the numbers of axial associations, synapsis initiation complexes, and crossovers, and decreases the rate of chromosome synapsis. Our data argue strongly that axial associations marked by synapsis initiation complexes correspond to sites of reciprocal exchange. We propose that the Sgs1 helicase prevents a subset of recombination intermediates from becoming crossovers, and this distinction is made at an early stage in meiotic prophase.     

MRE11 is required for homologous synapsis and DSB processing in rice meiosis

Mre11, a conserved protein found in organisms ranging from yeast to multicellular organisms, is required for normal meiotic recombination. Mre11 interacts with Rad50 and Nbs1/Xrs2 to form a complex (MRN/X) that participates in double-strand break (DSB) ends processing. In this study, we silenced the MRE11 gene in rice and detailed its function using molecular and cytological methods. The OsMRE11-deficient plants exhibited normal vegetative growth but could not set seed. Cytological analysis indicated that in the OsMRE11-deficient plants, homologous pairing was totally inhibited, and the chromosomes were completely entangled as a formation of multivalents at metaphase I, leading to the consequence of serious chromosome fragmentation during anaphase I. Immunofluorescence studies further demonstrated that OsMRE11 is required for homologous synapsis and DSB processing but is dispensable for meiotic DSB formation. We found that OsMRE11 protein was located on meiotic chromosomes from interphase to late pachytene. This protein showed normal localization in zep1, Oscom1 and Osmer3, as well as in OsSPO11-1 ^RNAi plants, but not in pair2 and pair3 mutants. Taken together, our results provide evidence that OsMRE11 performs a function essential for maintaining the normal HR process and inhibiting non-homologous recombination during meiosis.     

The mouse polyubiquitin gene Ubb is essential for meiotic progression

Ubiquitin is encoded in mice by two polyubiquitin genes, Ubb and Ubc, that are considered to be stress inducible and two constitutively expressed monoubiquitin (Uba) genes. Here we report that targeted disruption of Ubb results in male and female infertility due to failure of germ cells to progress through meiosis I and hypogonadism. In the absence of Ubb, spermatocytes and oocytes arrest during meiotic prophase, before metaphase of the first meiotic division. Although cellular ubiquitin levels are believed to be maintained by a combination of functional redundancy among the four ubiquitin genes, stress inducibility of the two polyubiquitin genes, and ubiquitin recycling by proteasome-associated isopeptidases, our results indicate that ubiquitin is required for and consumed during meiotic progression. The striking similarity of the meiotic phenotype in Ubb^−/− germ cells to the sporulation defect in fission yeast (Schizosaccharomyces pombe) lacking a polyubiquitin gene suggests that a meiotic role of the polyubiquitin gene has been conserved throughout eukaryotic evolution.     

Zip4/Spo22 is required for class I CO formation but not for synapsis completion in Arabidopsis thaliana

In budding yeast meiosis, the formation of class I interference-sensitive crossovers requires the ZMM proteins. These ZMM proteins are essential in forming a mature synaptonemal complex, and a subset of these (Zip2, Zip3, and Zip4) has been proposed to compose the core of synapsis initiation complexes (SICs). Zip4/Spo22 functions with Zip2 to promote polymerization of Zip1 along chromosomes, making it a crucial SIC component. In higher eukaryotes, synapsis and recombination have often been correlated, but it is totally unknown how these two processes are linked. In this study, we present the characterization of a higher eukaryote SIC component homologue: Arabidopsis AtZIP4. We show that mutations in AtZIP4 belong to the same epistasis group as Atmsh4 and eliminate approximately 85% of crossovers (COs). Furthermore, genetic analyses on two adjacent intervals of Chromosome I established that the remaining COs in Atzip4 do not show interference. Lastly, immunolocalization studies showed that polymerization of the central element of the synaptonemal complex is not affected in Atzip4 background, even if it may proceed from fewer sites compared to wild type. These results reveal that Zip4 function in class I CO formation is conserved from budding yeast to Arabidopsis. On the other hand, and contrary to the situation in yeast, mutation in AtZIP4 does not prevent synapsis, showing that both aspects of the Zip4 function (i.e., class I CO maturation and synapsis) can be uncoupled.     

Spo11-accessory proteins link double-strand break sites to the chromosome axis in early meiotic recombination

Meiotic recombination between homologous chromosomes initiates via programmed DNA double-strand breaks (DSBs), generated by complexes comprising Spo11 transesterase plus accessory proteins. DSBs arise concomitantly with the development of axial chromosome structures, where the coalescence of axis sites produces linear arrays of chromatin loops. Recombining DNA sequences map to loops, but are ultimately tethered to the underlying axis. How and when such tethering occurs is currently unclear. Using ChIPchip in yeast, we show that Spo11-accessory proteins Rec114, Mer2, and Mei4 stably interact with chromosome axis sequences, upon phosphorylation of Mer2 by S phase Cdk. This axis tethering requires meiotic axis components (Red1/Hop1) and is modulated in?a domain-specific fashion by cohesin. Loss of Rec114, Mer2, and Mei4 binding correlates with loss of DSBs. Our results strongly suggest that hotspot sequences become tethered to axis sites by the DSB machinery prior to DSB formation.     

IGSF11 is required for pericentric heterochromatin dissociation during meiotic diplotene

Meiosis initiation and progression are regulated by both germ cells and gonadal somatic cells. However, little is known about what genes or proteins connecting somatic and germ cells are required for this regulation. Our results show that deficiency for adhesion molecule IGSF11, which is expressed in both Sertoli cells and germ cells, leads to male infertility in mice. Combining a new meiotic fluorescent reporter system with testicular cell transplantation, we demonstrated that IGSF11 is required in both somatic cells and spermatogenic cells for primary spermatocyte development. In the absence of IGSF11, spermatocytes proceed through pachytene, but the pericentric heterochromatin of nonhomologous chromosomes remains inappropriately clustered from late pachytene onward, resulting in undissolved interchromosomal interactions. Hi-C analysis reveals elevated levels of interchromosomal interactions occurring mostly at the chromosome ends. Collectively, our data elucidates that IGSF11 in somatic cells and germ cells is required for pericentric heterochromatin dissociation during diplotene in mouse primary spermatocytes.     

X-Autosome translocations,meiotic synapsis,chromosome evolution and speciation

Several theories have been proposed to explain the often-noted sterility of both reciprocal and Robertsonian X-autosome translocations in male mammals. However, there are a number of species in which all members of the species carry a Robertsonian X-autosome translocation. Meiosis in spermatocytes from these sterile vs. fertile animals is compared within the context of these theories. New technologies and insights into underlying mechanisms are summarized and suggestions presented for further studies.