The leptotene-zygotene transition of meiosis

How do meiotic chromosomes meet their homologous partners?: lessons from fission yeast

Homologous chromosome pairing is required for proper chromosome segregation and recombination during meiosis. The mechanism by which a pair of homologous chromosomes contact each other to establish pairing is not fully understood. When pairing occurs during meiotic prophase in the fission yeast, Schizosaccharomyces pombe, the nucleus oscillates between the cell poles and telomeres remain clustered at the leading edge of the moving nucleus. These meiosis-specific activities produce movements of telomere-bundled chromosomes. Several lines of evidence suggest that these movements facilitate homologous chromosome pairing by aligning homologous chromosomes and promoting contact between homologous regions. Since telomere clustering and nuclear or chromosome movements in meiotic prophase have been observed in a wide range of eukaryotic organisms, it is suggested that telomere-mediated chromosome movements are general activities that facilitate homologous chromosome pairing.

Checkpoint and DNA-repair proteins are associated with the cores of mammalian meiotic chromosomes

Meiotic checkpoints are manifested through protein complexes capable of detecting an abnormality in chromosome metabolism and signaling it to effector molecules that subsequently delay or arrest the progression of meiosis. Some checkpoints act during the first meiotic prophase to monitor the repair of chromosomal DSBs, predominantly by meiotic recombination, or to ensure the correct establishment of synapsis and its well-timed dissolution. In mammals, a number of checkpoint and repair proteins localize to the meiotic chromosomal cores, sometimes in the context of the synaptonemal complex (SC). Here we discuss possible functions of these proteins in the accomplishment of meiotic recombination and normal progression of the meiotic pathway. Also, we present arguments for a structural role of cores and SCs in the assembly of the repair and checkpoint protein complexes on the chromosomes.

Nuclear dispositions of subtelomeric and pericentromeric chromosomal domains during meiosis in asynaptic mutants of rye (Secale cereale L.)

The nuclear dispositions of subtelomeric and pericentromeric domains in pollen mother cells (PMCs) were tracked during meiosis in wildtype and two asynaptic mutants of rye (Secale cereale L.) by means of fluorescence in situ hybridization (FISH). Homozygotes for sy1 and sy9 non-allelic mutations form axial elements during leptotene of male meiosis, but fail to form synaptonemal complexes. Consequently, recombination is severely impaired, and high univalency is observed at metaphase I. Simultaneous FISH with pSc200 subtelomeric tandem repeat and CCS1 centromeric sequence revealed that at pre-meiotic interphase the two domains are in a bipolar Rabl orientation in both the PMCs and tapetal cells. At the onset of meiotic prophase, the subtelomeric regions in PMCs of wildtype and sy9 cluster into a typical bouquet conformation. The timing of this event in rye is comparable with that in wheat, and is earlier than that observed in other organisms, such as maize, yeast and mammals. This arrangement is retained until later in leptotene and zygotene when the pericentromeric domains disperse and the subtelomeric clusters fragment. The mutant phenotype of sy9 manifests itself during leptotene to zygotene, when the pericentromeric regions become distinctly more distended than in wildtype, and largely fail to pair during zygotene. This indicates that difference in the nature or timing of chromosome condensation in this region is the cause or consequence of asynapsis. By contrast, sy1 fails to form comparable aggregates of subtelomeric regions at leptotene in only half of the nuclei studied. Instead, two to five aggregates are formed that fail to disperse at later stages of meiotic prophase. In addition, the pericentromeric regions disperse prematurely at leptotene and do not associate in pairs at any subsequent stage. It is supposed that the sy1 mutation could disrupt the nuclear disposition of centromeres and telomeres at the end of pre-meiotic interphase, which could cause, or contribute to, its asynaptic phenotype.

Dynamic localization of bacterial and plasmid chromosomes

Plasmid-encoded partition genes determine the dynamic localization of plasmid molecules from the mid-cell position to the 1/4 and 3/4 positions. Similarly, bacterial homologs of the plasmid genes participate in controlling the bidirectional migration of the replication origin (oriC) regions during sporulation and vegetative growth in Bacillus subtilis, but not in Escherichia coli. In E. coli, but not B. subtilis, the chromosomal DNA is fully methylated by DNA adenine methyltransferase. The E. coli SeqA protein, which binds preferentially to hemimethylated nascent DNA strands, exists as discrete foci in vivo. A single SeqA focus, which is a SeqA-hemimethylated DNA cluster, splits into two foci that then abruptly migrate bidirectionally to the 1/4 and 3/4 positions during replication. Replicated oriC copies are linked to each other for a substantial period of generation time, before separating from each other and migrating in opposite directions. The MukFEB complex of E. coli and Smc of B. subtilis appear to participate in the reorganization of bacterial sister chromosomes.

B-type cyclins CLB5 and CLB6 control the initiation of recombination and synaptonemal complex formation in yeast meiosis

BACKGROUND

The life cycle of most eukaryotic organisms includes a meiotic phase, in which diploid parental cells produce haploid gametes. During meiosis a single round of DNA replication is followed by two rounds of chromosome segregation. In the first, or reductional, division (meiosis I), which is unique to meiotic cells, homologous chromosomes segregate from one another, whereas in the second, or equational, division (Meiosis II) sister centromeres disjoin. Meiotic DNA replication precedes the initiation of recombination by programmed Spo11-dependent DNA double-strand breaks. Recent reports that meiosis-specific cohesion is established during meiotic S phase and that the length of S phase is modified by recombination factors (Spo11 and Rec8) raise the possibility that replication plays a fundamental role in the recombination process.

RESULTS

To address how replication influences the initiation of recombination, we have used mutations in the B-type cyclin genes CLB5 and CLB6, which specifically prevent premeiotic replication in the yeast Saccharomyces cerevisiae. We find that clb5 and clb5 clb6 but not clb6 mutants are defective in DSB induction and prior associated changes in chromatin accessibility, heteroallelic recombination, and SC formation. The severity of these phenotypes in each mutant reflects the extent of replication impairment.

CONCLUSIONS

This assemblage of phenotypes reveals roles for CLB5 and CLB6 not only in DNA replication but also in other key events of meiotic prophase. Links between the function of CLB5 and CLB6 in activating meiotic DNA replication and their effects on subsequent events are discussed.

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

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

HO endonuclease-induced recombination in yeast meiosis resembles Spo11-induced events

In meiosis, gene conversions are accompanied by higher levels of crossing over than in mitotic cells. To determine whether the special properties of meiotic recombination can be attributed to the way in which Spo11p creates double-strand breaks (DSBs) at special hot spots in Saccharomyces cerevisiae, we expressed the site-specific HO endonuclease in meiotic cells. We could therefore compare HO-induced recombination in a well-defined region both in mitosis and meiosis, as well as compare Spo11p- and HO-induced meiotic events. HO-induced gene conversions in meiosis were accompanied by crossovers at the same high level (52%) as Spo11p-induced events. Moreover, HO-induced crossovers were reduced 3-fold by a msh4Delta mutation that similarly affects Spo11p-promoted events. In a spo11Delta diploid, where the only DSB is made by HO, crossing over was significantly higher (27%) than in mitotic cells (</=7%). This single meiotic DSB failed to induce the formation of a synaptonemal complex. We also show that HO-induced gene conversion tract lengths are shorter in meiotic than in mitotic cells. We conclude that a hallmark of meiotic recombination, the production of crossovers, is independent of the nature of Spo11p-generated DSBs at special hotspots, but some functions of Spo11p are required in trans to achieve maximum crossing over.

Mammalian meiotic telomeres: protein composition and redistribution in relation to nuclear pores

Mammalian telomeres consist of TTAGGG repeats, telomeric repeat binding factor (TRF), and other proteins, resulting in a protective structure at chromosome ends. Although structure and function of the somatic telomeric complex has been elucidated in some detail, the protein composition of mammalian meiotic telomeres is undetermined. Here we show, by indirect immunofluorescence (IF), that the meiotic telomere complex is similar to its somatic counterpart and contains significant amounts of TRF1, TRF2, and hRap1, while tankyrase, a poly-(ADP-ribose)polymerase at somatic telomeres and nuclear pores, forms small signals at ends of human meiotic chromosome cores. Analysis of rodent spermatocytes reveals Trf1 at mouse, TRF2 at rat, and mammalian Rap1 at meiotic telomeres of both rodents. Moreover, we demonstrate that telomere repositioning during meiotic prophase occurs in sectors of the nuclear envelope that are distinct from nuclear pore-dense areas. The latter form during preleptotene/leptotene and are present during entire prophase I.

Meiotic lamin C2: the unique amino-terminal hexapeptide GNAEGR is essential for nuclear envelope association

Meiotic lamin C2 is the only A-type lamin expressed during mammalian spermatogenesis. Typical for this short lamin is the unique hexapeptide GNAEGR, which substitutes the nonhelical amino terminus and part of the alpha-helical rod domain present in somatic lamins. Meiotic lamin C2 also lacks a carboxyl-terminal CaaX box, which is modified by isoprenylation and involved in nuclear envelope (NE) association of somatic isoforms. The mechanism by which lamin C2 becomes localized in the NE is totally unknown. Here we demonstrate that the hexapeptide GNAEGR is essential for this process: (i) Its deletion resulted in a diffuse distribution of lamin C2 within nuclei of transfected COS-7 cells; (ii) Mutated somatic lamin C, containing the sequence GNAEGR at its amino terminus, was located at the NE. The mass spectrometric analysis of the amino terminus of lamin C2 revealed that it is modified by myristoylation. Correspondingly, the substitution of the first glycine residue abolishes the NE association of lamin C2. We conclude that NE association of lamin C2 is achieved by a mechanism different from that of somatic lamins.

The mouse Spo11 gene is required for meiotic chromosome synapsis

The Spo11 protein initiates meiotic recombination by generating DNA double-strand breaks (DSBs) and is required for meiotic synapsis in S. cerevisiae. Surprisingly, Spo11 homologs are dispensable for synapsis in C. elegans and Drosophila yet required for meiotic recombination. Disruption of mouse Spo11 results in infertility. Spermatocytes arrest prior to pachytene with little or no synapsis and undergo apoptosis. We did not detect Rad51/Dmc1 foci in meiotic chromosome spreads, indicating DSBs are not formed. Cisplatin-induced DSBs restored Rad51/Dmc1 foci and promoted synapsis. Spo11 localizes to discrete foci during leptotene and to homologously synapsed chromosomes. Other mouse mutants that arrest during meiotic prophase (Atm -/-, Dmc1 -/-, mei1, and Morc(-/-)) showed altered Spo11 protein localization and expression. We speculate that there is an additional role for Spo11, after it generates DSBs, in synapsis.

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.

Disjunction of homologous chromosomes in meiosis I depends on proteolytic cleavage of the meiotic cohesin Rec8 by separin

It has been proposed but never proven that cohesion between sister chromatids distal to chiasmata is responsible for holding homologous chromosomes together while spindles attempt to pull them toward opposite poles during metaphase of meiosis I. Meanwhile, the mechanism by which disjunction of homologs is triggered at the onset of anaphase I has remained a complete mystery. In yeast, cohesion between sister chromatid arms during meiosis depends on a meiosis-specific cohesin subunit called Rec8, whose mitotic equivalent, Sccl, is cleaved at the metaphase to anaphase transition by an endopeptidase called separin. We show here that cleavage of Rec8 by separin at one of two different sites is necessary for the resolution of chiasmata and the disjunction of homologous chromosomes during meiosis.

Meiotic telomere protein Ndj1p is required for meiosis-specific telomere distribution, bouquet formation and efficient homologue pairing

We have investigated the requirements for NDJ1 in meiotic telomere redistribution and clustering in synchronized cultures of Saccharomyces cerevisiae. On induction of wild-type meiosis, telomeres disperse from premeiotic aggregates over the nuclear periphery, and then cluster near the spindle pole body (bouquet arrangement) before dispersing again. In ndj1Delta meiocytes, telomeres are scattered throughout the nucleus and fail to form perinuclear meiosis-specific distribution patterns, suggesting that Ndj1p may function to tether meiotic telomeres to the nuclear periphery. Since ndj1Delta meiocytes fail to cluster their telomeres at any prophase stage, Ndj1p is the first protein shown to be required for bouquet formation in a synaptic organism. Analysis of homologue pairing by two-color fluorescence in situ hybridization with cosmid probes to regions on III, IX, and XI revealed that disruption of bouquet formation is associated with a significant delay (>2 h) of homologue pairing. An increased and persistent fraction of ndj1Delta meiocytes with Zip1p polycomplexes suggests that chromosome polarization is important for synapsis progression. Thus, our observations support the hypothesis that meiotic telomere clustering contributes to efficient homologue alignment and synaptic pairing. Under naturally occurring conditions, bouquet formation may allow for rapid sporulation and confer a selective advantage.

Meiotic telomere distribution and Sertoli cell nuclear architecture are altered in Atm- and Atm-p53-deficient mice

The ataxia telangiectasia mutant (ATM) protein is an intrinsic part of the cell cycle machinery that surveys genomic integrity and responses to genotoxic insult. Individuals with ataxia telangiectasia as well as Atm(-/-) mice are predisposed to cancer and are infertile due to spermatogenesis disruption during first meiotic prophase. Atm(-/-) spermatocytes frequently display aberrant synapsis and clustered telomeres (bouquet topology). Here, we used telomere fluorescent in situ hybridization and immunofluorescence (IF) staining of SCP3 and testes-specific histone H1 (H1t) to spermatocytes of Atm- and Atm-p53-deficient mice and investigated whether gonadal atrophy in Atm-null mice is associated with stalling of telomere motility in meiotic prophase. SCP3-H1t IF revealed that most Atm(-/-) p53(-/-) spermatocytes degenerated during late zygotene, while a few progressed to pachytene and diplotene and some even beyond metaphase II, as indicated by the presence of a few round spermatids. In Atm(-/-) p53(-/-) meiosis, the frequency of spermatocytes I with bouquet topology was elevated 72-fold. Bouquet spermatocytes with clustered telomeres were generally void of H1t signals, while mid-late pachytene and diplotene Atm(-/-) p53(-/-) spermatocytes displayed expression of H1t and showed telomeres dispersed over the nuclear periphery. Thus, it appears that meiotic telomere movements occur independently of ATM signaling. Atm inactivation more likely leads to accumulation of spermatocytes I with bouquet topology by slowing progression through initial stages of first meiotic prophase and an ensuing arrest and demise of spermatocytes I. Sertoli cells (SECs), which contribute to faithful spermatogenesis, in the Atm mutants were found to frequently display numerous heterochromatin and telomere clusters-a nuclear topology which resembles that of immature SECs. However, Atm(-/-) SECs exhibited a mature vimentin and cytokeratin 8 intermediate filament expression signature. Upon IF with ATM antibodies, we observed ATM signals throughout the nuclei of human and mouse SECs, spermatocytes I, and haploid round spermatids. ATM but not H1t was absent from elongating spermatid nuclei. Thus, ATM appears to be removed from spermatid nuclei prior to the occurrence of DNA nicks which emanate as a consequence of nucleoprotamine formation.

Nuclear envelope associated protein that binds telomeric DNAs

Rana temporaria oocytes at the 6th diplotene stage of maturation contain a special structure, the karyosphere capsule, with chromosomes covered and detached from the nuclear envelope (NE), though at the previous stage the telomeres were attached to the membrane, as characteristic of germ cells. The DNA-protein complexes from band shift assays with proteins extracted from oocyte NEs and telomeric DNA fragment (T(2)G(4))(130) were isolated and injected into a guinea pig. In the present paper the only protein of 70 kDa recognized by antibody (AB) in the NE is named the Membrane Telomere Binding Protein (MTBP). Western blots with guinea pig AB and AB against telobox peptide from TRF2 show that protein of 60 kDa (probably TRF1) belongs to the chromatin, but MTBP (TRF2 according to immunoprecipitation) belongs to the NE. In the somatic cell nuclei both proteins are present and recognized by AB against telobox peptide, but AB raised recognize only MTBP/TRF2 due to the epitope different from telobox. Combined in situ hybridization with the vertebrate telomeric DNA sequences (T(2)AG(3))(135) and immunocytochemistry with the MTBP AB showed them to be colocalized within the mouse nucleus. As it was shown by immunofluorescense of NE spread, MTBP is organized in a distinct pattern that looks like a network made of double-dots. Electron microscope immunogold staining with both ABs showed that the protein is localized on the outer surface of the oocyte NE within cup-like structures attached to the membrane. This is the first clear evidence of a protein, which could be responsible for the attachment of telomeres to the nuclear membrane.

Extra-helical guanine interactions in DNA

We review the extra-helical guanine interactions present in many oligonucleotide crystals. Very often terminal guanines interact with other guanines in the minor groove of neighboring oligonucleotides through N2 x N3 hydrogen bonds. In other cases the interaction occurs with the help of Ni2+ ions. Guanine/netropsin stacking in the minor groove has also been found. From these studies we conclude that guanine may have multiple extra-helical interactions. In particular it may be considered a very effective minor groove binder, which could be used in the design of sequence selective binding drugs. Interactions through the major groove are seldom encountered, but might be present when DNA is stretched. Such interactions are also analyzed, since they might be important for homologous chromosome pairing during meiosis.

Restriction of ectopic recombination by interhomolog interactions during Saccharomyces cerevisiae meiosis

In Saccharomyces cerevisiae meiosis, recombination occurs frequently between sequences at the same location on homologs (allelic recombination) and can take place between dispersed homologous sequences (ectopic recombination). Ectopic recombination occurs less often than does allelic, especially when homologous sequences are on heterologous chromosomes. To account for this, it has been suggested that homolog pairing (homolog colocalization and alignment) either promotes allelic recombination or restricts ectopic recombination. The latter suggestion was tested by examining ectopic recombination in two cases where normal interhomolog relationships are disrupted. In the first case, one member of a homolog pair was replaced by a homologous (related but not identical) chromosome that has diverged sufficiently to prevent allelic recombination. In the second case, ndj1 mutants were used to delay homolog pairing and synapsis. Both circumstances resulted in a substantial increase in the frequency of ectopic recombination between arg4-containing plasmid inserts located on heterologous chromosomes. These findings suggest that, during normal yeast meiosis, progressive homolog colocalization, alignment, synapsis, and allelic recombination restrict the ability of ectopically located sequences to find each other and recombine. In the absence of such restrictions, the meiotic homology search may encompass the entire genome.

Centromere clustering is a major determinant of yeast interphase nuclear organization

During interphase in the budding yeast, Saccharomyces cerevisiae, centromeres are clustered near one pole of the nucleus as a rosette with the spindle pole body at its hub. Opposite to the centromeric pole is the nucleolus. Chromosome arms extend outwards from the centromeric pole and are preferentially directed towards the opposite pole. Centromere clustering is reduced by the ndc10 mutation, which affects a kinetochore protein, and by the microtubule poison nocodazole. This suggests that clustering is actively maintained or enforced by the association of centromeres with microtubules throughout interphase. Unlike the Rabl-orientation known from many higher eukaryotes, centromere clustering in yeast is not only a relic of anaphase chromosome polarization, because it can be reconstituted without the passage of cells through anaphase. Within the rosette, homologous centromeres are not arranged in a particular order that would suggest somatic pairing or genome separation.

Life history and developmental processes in the basidiomycete Coprinus cinereus

Coprinus cinereus has two main types of mycelia, the asexual monokaryon and the sexual dikaryon, formed by fusion of compatible monokaryons. Syngamy (plasmogamy) and karyogamy are spatially and temporally separated, which is typical for basidiomycetous fungi. This property of the dikaryon enables an easy exchange of nuclear partners in further dikaryotic-monokaryotic and dikaryotic-dikaryotic mycelial fusions. Fruiting bodies normally develop on the dikaryon, and the cytological process of fruiting-body development has been described in its principles. Within the specialized basidia, present within the gills of the fruiting bodies, karyogamy occurs in a synchronized manner. It is directly followed by meiosis and by the production of the meiotic basidiospores. The synchrony of karyogamy and meiosis has made the fungus a classical object to study meiotic cytology and recombination. Several genes involved in these processes have been identified. Both monokaryons and dikaryons can form multicellular resting bodies (sclerotia) and different types of mitotic spores, the small uninucleate aerial oidia, and, within submerged mycelium, the large thick-walled chlamydospores. The decision about whether a structure will be formed is made on the basis of environmental signals (light, temperature, humidity, and nutrients). Of the intrinsic factors that control development, the products of the two mating type loci are most important. Mutant complementation and PCR approaches identified further genes which possibly link the two mating-type pathways with each other and with nutritional regulation, for example with the cAMP signaling pathway. Among genes specifically expressed within the fruiting body are those for two galectins, beta-galactoside binding lectins that probably act in hyphal aggregation. These genes serve as molecular markers to study development in wild-type and mutant strains. The isolation of genes for potential non-DNA methyltransferases, needed for tissue formation within the fruiting body, promises the discovery of new signaling pathways, possibly involving secondary fungal metabolites.

Molecular characterisation of two paralogous SPO11 homologues in Arabidopsis thaliana

The Spo11 protein of yeast has been found to be covalently bound to double-strand breaks in meiosis, demonstrating a unique role of the protein in the formation of these breaks. Homologues of the SPO11 gene have been found in various eukaryotes, indicating that the machinery involved in meiotic recombination is conserved in eukaryotes. Here we report on SPO11 homologues in plants. In contrast to what is known from other eukaryotes, Arabidopsis thaliana carries in its genome at least two SPO11 homologues, AtSPO11-1 and AtSPO11-2. Both genes are not more closely related to each other than to other eukaryotic SPO11 homologues, indicating that they did not arise via a recent duplication event during higher plant evolution. For both genes three different poly-adenylation sites were found. AtSPO11-1 is expressed not only in generative but also to a lesser extent in somatic tissues. We were able to detect in different organs various AtSPO11-1 cDNAs in which introns were differently spliced-a surprising phenomenon also reported for SPO11 homologues in mammals. In the case of AtSPO11-2 we found that the 3' end of the mRNA is overlapping with a mRNA produced by a gene located in inverse orientation next to it. This points to a possible antisense regulation mechanism. Our findings hint to the intriguing possibility that, at least for plants, Spo11-like proteins might have more and possibly other biological functions than originally anticipated for yeast.

Meiotic chromosomes: integrating structure and function

Meiotic chromosomes have been studied for many years, in part because of the fundamental life processes they represent, but also because meiosis involves the formation of homolog pairs, a feature which greatly facilitates the study of chromosome behavior. The complex events involved in homolog juxtaposition necessitate prolongation of prophase, thus permitting resolution of events that are temporally compressed in the mitotic cycle. Furthermore, once homologs are paired, the chromosomes are connected by a specific structure: the synaptonemal complex. Finally, interaction of homologs includes recombination at the DNA level, which is intimately linked to structural features of the chromosomes. In consequence, recombination-related events report on diverse aspects of chromosome morphogenesis, notably relationships between sisters, development of axial structure, and variations in chromatin status. The current article reviews recent information on these topics in an historical context. This juxtaposition has suggested new relationships between structure and function. Additional issues were addressed in a previous chapter (551).

Evidence for the coincident initiation of homolog pairing and synapsis during the telomere-clustering (bouquet) stage of meiotic prophase

To improve knowledge of the prerequisites for meiotic chromosome segregation in higher eukaryotes, we analyzed the spatial distribution of a pair of homologs before and during early meiotic prophase. Three-dimensional images of fluorescence in situ hybridization (FISH) were used to localize a single pair of homologs in diploid nuclei of a chromosome-addition line of oat, oat-maize9b. The system provided a robust assay for pairing based on cytological colocalization of FISH signals. Using a triple labeling scheme for simultaneous imaging of chromatin, telomeres and the homolog pair, we determined the timing of pairing in relation to the onset of three sequential hallmarks of early meiotic prophase: chromatin condensation (the leptotene stage), meiotic telomere clustering (the bouquet stage) and the initiation of synapsis (the zygotene stage). We found that the two homologs were mostly unpaired up through middle leptotene, at which point their spherical cloud-like domains began to transform into elongated and stretched-out domains. At late leptotene, the homologs had completely reorganized into long extended fibers, and the beginning of the bouquet stage was conspicuously marked by the de novo clustering of telomeres at the nuclear periphery. The homologs paired and synapsed during the bouquet stage, consistent with the timing of pairing observed for several oat 5S rDNA loci. In summary, results from analysis of more than 100 intact nuclei lead us to conclude that pairing and synapsis of homologous chromosomes are largely coincident processes, ruling out a role for premeiotic pairing in this system. These findings suggest that the genome-wide remodeling of chromatin and telomere-mediated nuclear reorganization are prerequisite steps to the DNA sequence-based homology-search process in higher eukaryotes.

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.

A mouse homolog of the Saccharomyces cerevisiae meiotic recombination DNA transesterase Spo11p

The Saccharomyces cerevisiae Spo11 protein is thought to catalyze formation of the DNA double-strand breaks that initiate meiotic recombination. We have cloned cDNA and genomic DNA for a mouse gene encoding a protein with significant sequence similarity to conserved domains found in proteins of the Spo11p family. This putative mouse Spo11 gene maps to the distal region of chromosome 2 (homologous to human chromosome 20q13.2-q13.3) and comprises at least 12 exons, spanning approximately 15-18 kb. Strong expression of the Spo11 message is seen in juvenile and adult testis by RNA in situ hybridization, RT-PCR, and Northern blot, with much weaker expression in thymus and brain. In situ hybridization detects expression in oocytes of embryonic ovary, but not of adult ovary. RT-PCR and in situ hybridization analyses of a time course of juvenile testis development indicate that Spo11 expression begins in early meiotic Prophase I, prior to the pachytene stage, with increasing accumulation of mRNA through the pachytene stage. Taken together, these results strongly suggest that this gene encodes the functional homolog of yeast Spo11p, which in turn suggests that the mechanism of meiotic recombination initiation is conserved between yeast and mammals.

Active-site mutations in the Xrn1p exoribonuclease of Saccharomyces cerevisiae reveal a specific role in meiosis

Xrn1p of Saccharomyces cerevisiae is a major cytoplasmic RNA turnover exonuclease which is evolutionarily conserved from yeasts to mammals. Deletion of the XRN1 gene causes pleiotropic phenotypes, which have been interpreted as indirect consequences of the RNA turnover defect. By sequence comparisons, we have identified three loosely defined, common 5'-3' exonuclease motifs. The significance of motif II has been confirmed by mutant analysis with Xrn1p. The amino acid changes D206A and D208A abolish singly or in combination the exonuclease activity in vivo. These mutations show separation of function. They cause identical phenotypes to that of xrn1Delta in vegetative cells but do not exhibit the severe meiotic arrest and the spore lethality phenotype typical for the deletion. In addition, xrn1-D208A does not cause the severe reduction in meiotic popout recombination in a double mutant with dmc1 as does xrn1Delta. Biochemical analysis of the DNA binding, exonuclease, and homologous pairing activity of purified mutant enzyme demonstrated the specific loss of exonuclease activity. However, the mutant enzyme is competent to promote in vitro assembly of tubulin into microtubules. These results define a separable and specific function of Xrn1p in meiosis which appears unrelated to its RNA turnover function in vegetative cells.

Spo76p is a conserved chromosome morphogenesis protein that links the mitotic and meiotic programs

Spo76p is conserved and related to the fungal proteins Pds5p and BIMD and the human AS3 prostate proliferative shutoff-associated protein. Spo76p localizes to mitotic and meiotic chromosomes, except at metaphase(s) and anaphase(s). During meiotic prophase, Spo76p assembles into strong lines in correlation with axial element formation. As inferred from spo76-1 mutant phenotypes, Spo76p is required for sister chromatid cohesiveness, chromosome axis morphogenesis, and chromatin condensation during critical transitions at mitotic prometaphase and meiotic midprophase. Spo76p is also required for meiotic interhomolog recombination, likely at postinitiation stage(s). We propose that a disruptive force coordinately promotes chromosomal axial compaction and destabilization of sister connections and that Spo76p restrains and channels the effects of this force into appropriate morphogenetic mitotic and meiotic outcomes.

DNA recombination: the replication connection

Chromosomal double-strand breaks (DSBs) arise after exposure to ionizing radiation or enzymatic cleavage, but especially during the process of DNA replication itself. Homologous recombination plays a critical role in repair of such DSBs. There has been significant progress in our understanding of two processes that occur in DSB repair: gene conversion and recombination-dependent DNA replication. Recent evidence suggests that gene conversion and break-induced replication are related processes that both begin with the establishment of a replication fork in which both leading- and lagging-strand synthesis occur. There has also been much progress in characterization of the biochemical roles of recombination proteins that are highly conserved from yeast to humans.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.