Yeast genetics and the fall of the classical view of meiosis

REGIONS OF RELATIVE GC% UNIFORMITY ARE RECOMBINATIONAL ISOLATORS

Among species within a phylogenetic group, genomic GC% values can cover a wide range that is particularly evident at third codon positions. However, among genes within a genome, genic GC% values can also cover a wide range that is, again, particularly evident at third codon positions. Individual genes and genomes each have a "homostabilizing propensity" to adopt a relatively uniform GC%. Each gene (a "microisochore") occupies a discrete GC% niche of relatively uniform base composition amongst its fellow genes, which can collectively span a wide GC% range. Homostabilization serves to recombinationally isolate both genome sectors (facilitating gene duplication and differentiation) and genomes (facilitating genome duplication and differentiation; e.g., speciation). Although they may sometimes be in conflict, the individualities of genomes, and of genes within those genomes, are separately sustained by a common mechanism, uniformity of GC%. The protection against inadvertent recombination afforded by GC% differentiation is, in the general case, a prerequisite for phenotypic differentiation.

Molecular sex: The importance of base composition rather than homology when nucleic acids hybridize

On learning that nucleic acid hybridization had been achieved in a test tube, Huxley hailed the discovery of "molecular sex." The description was apt, since sex involves recombination, which requires hybridization that, in turn, depends on a successful homology search. Conversely, when the homology search fails, recombination fails. In yeast, this failure has been attributed to "simple sequence divergence." But sequence divergence does not impair nucleic acid hybridization simply. Most natural single-stranded nucleic acids are predisposed to adopt higher-order structures containing stem-loops. Tomizawa showed that the rate-limiting step in the hybridization of single-stranded sequences is an initial "kissing" exploration between complementary loops, which must first be appropriately extruded and aligned. Successful duplex formation requires successful synchronization of matching higher-ordered structures, which depends, not so much on the degree of similarity between their base sequences as on the closeness of their base compositions (GC%). In these terms, we can understand how the anti-recombinational effect of GC% differences supports the duplication both of genes within a genome and of genomes within a genus (speciation).

Activation of pericentromeric and telomeric heterochromatin in cultured lymphocytes from old individuals

The functional characteristics of chromosomes (level of total heterochromatin, chromosome instability, and sister chromatid exchanges [SCEs]) were studied in cultured lymphocytes derived from 80- to 91-year-old and 18- to 30-year-old (control group) individuals under the single and combined effect of CoCl(2) and bioregulator Livagen. The results obtained showed that chromosome heterochromatinization (condensation of eu- and heterochromatin regions) had progressively increased with aging and led to inactivation of a number of once functioning "active genes." The peptide bioregulator Livagen could induce reactivation (deheterochromatinization) of chromatin to modify heterochromatinized chromosomal regions in cultured lymphocytes of aged individuals. Our results indicated that metal ions (CoCl(2)) caused a significant increase in the level of chromosomal aberrations in old donors in comparison with the control group (P < 0.05). The peptide bioregulator Livagen was effective in decreasing the number of changes induced by the CoCl(2) 3.4 +/- 0.6% (control group 4.2 +/- 0.7%). Co(2+) ions single and Co(2+) ions in combination with the Livagen changed the distribution of SCE over chromosomes: pericentromeric heterochromatin was more sensitive to the effect of CoCl(2) (15.4 +/- 1.8% SCE), while SCE were mostly registered in telomeric heterochromatin under the combined effect of CoCl(2) and Livagen 12.0 +/- 1.2% SCE (control group 4.5 +/- 0.6% and 2.8 +/- 0.5% SCE, respectively). Thus, we have first demonstrated that Co(2+) ions separately and in combination with the bioregulator Livagen have different chromosomal target regions as demonstrated by SCE induction, deheterochromatinization of precentromeric and telomeric heterochromatin in lymphocytes from old individuals.

Meiosis: inducing variation by reduction

A brief introduction is presented with some thought on the origin of meiosis. Subsequently, a sequential overview of the diverse processes that take place during meiosis is provided, with an eye to similarities and differences between the different eukaryotic systems. In the final part, we try to summarize the available core meiotic mutants and make a comprehensive comparison for orthologous genes between fungal, plant, and animal systems.

Chromosomes of the budding yeast Saccharomyces cerevisiae

The mitotic chromosomes of the baker's yeast, Saccharomyces cerevisiae, cannot be visualized by standard cytological methods. Only the study of meiotic bivalents and the synaptonemal complex and the visualization of chromosome-sized DNA molecules on pulsed-field gels have provided some insight into chromosome structure and behavior. More recently, advanced techniques such as in situ hybridization, the illumination of chromosomal loci by GFP-tagged DNA-binding proteins, and immunostaining of chromosomal proteins have promoted our knowledge about yeast chromosomes. These novel cytological approaches in combination with the yeast's advanced biochemistry and genetics have produced a great wealth of information on the interplay between molecular and cytological processes and have strengthened the role of yeast as a leading cell biological model organism. Recent cytological studies have revealed much about the chromosomal organization in interphase nuclei and have contributed significantly to our current understanding of chromosome condensation, sister chromatid cohesion, and centromere orientation in mitosis. Moreover, important details about the biochemistry and ultrastructure of meiotic pairing and recombination have been revealed by combined cytological and molecular approaches. This article covers several aspects of yeast chromosome structure, including their organization within interphase nuclei and their behavior during mitosis and meiosis.

Meiotic segregation of a homeologous chromosome pair

During meiosis, the alignment of homologous chromosomes facilitates their subsequent migration away from one another to opposite spindle poles at anaphase I. Recombination is part of the mechanism by which chromosomes identify their homologous partners, and serves to link the homologs in a way that, in some organisms, has been shown to promote proper attachment to the meiotic spindle. We have built a diploid strain that contains a pair of homeologous chromosomes V': one is derived from Saccharomyces cerevisiae and one originates from S. carlsbergensis. Sequence analysis reveals that these chromosomes share 71% sequence identity. The homeologs experience high levels of meiotic double-stranded breaks. Despite their relatedness and their competence to initiate recombination, the meiotic segregation behavior of the homeologous chromosomes suggests that, in most meioses, they are partitioned by a meiotic segregation system that has been shown previously to partition non-exchange chromosomes and pairs with no homology. Though the homeologous chromosomes show a degree of meiotic segregation fidelity similar to that of other non-exchange pairs, our data provide evidence that their limited sequence homology may provide some bias in meiotic partner choice.

The Synaptonemal complex component C(2)M regulates meiotic crossing over in Drosophila

BACKGROUND

The synaptonemal complex (SC) is a proteinaceous structure that forms between homologously paired meiotic chromosomes. Previous studies have suggested that the SC is required for meiotic crossing over in Drosophila. However, only one component of this structure, C(3)G, has been identified in Drosophila.

RESULTS

Mutations in c(2)M cause a reduced frequency of meiotic crossing over due, in part, to how recombination events are resolved. Cytological evidence suggests that C(2)M is a component of the SC and is required for the assembly of C(3)G (a putative transverse filament of the SC) along the chromosomes. Additionally, C(2)M localizes along the chromosomes in the absence of C(3)G. Despite having a defect in C(3)G localization, c(2)M mutants unexpectedly affect crossing over less severely than a c(3)G mutant. There is virtually no crossing over in a c(3)G mutant, but c(2)M or c(2)M; c(3)G double mutants produce a substantial number of crossovers. The appearance of C(3)G-independent crossovers in c(2)M mutants suggests that C(2)M prevents recombination in the absence of complete SC formation.

CONCLUSIONS

We have identified a new Drosophila SC component, C(2)M, that promotes the formation of crossovers. Furthermore, the appearance of C(3)G-independent crossovers in c(2)M mutants suggests a novel role in preventing recombination in the absence of complete SC.

The yeast kinetochore protein Slk19 is required to prevent aberrant chromosome segregation in meiosis and mitosis

BACKGROUND

Slk19 is a coiled-coil protein, which locates to the kinetochores of S. cerevisiae. Most cells lacking Slk19 undergo incomplete meiosis and form dyads during sporulation. Endogenous chromosomes appeared to be predominantly divided in an equational manner during single-division meiosis of slk19 null mutants.

RESULTS

We have monitored the segregation of artificial chromosomes (YACs) in slk19 null mutants during both single-division meiosis and complete meiosis. In contrast to the results obtained with endogenous chromosomes, YACs only rarely undergo equational segregation during single division meiosis, although high rates of aberrant segregation were detected. This accounts for the high frequency of lethal spores among dyads of slk19 delta null mutants. The fraction of slk19 delta cells that were able to form tetrads solely exhibited YAC segregation defects in meiosis II, whereas the segregation of YACs in meiosis I was normal in these cells. This result might indicate that correct chromosome division in meiosis I is a prerequisite for tetrad formation. slk19 null mutants also showed YAC instability in mitosis and reduced survival after the induction of mitotic spindle damage.

CONCLUSION

Slk19 is required to avoid aberrant segregation of chromosomes in meiosis I and II and in mitosis. We suggest that the absence of Slk19 leads to uncoupling of chromosome movement from completion of microtubule attachment and resolution of chromosome cohesion.

Homolog pairing and two kinds of bouquets in the meiotic prophase of rye, Secale cereale

Chromosome configurations and structures during meiotic prophase were investigated by staining large repeated DNA sequences localized in the subtelomeric regions of all the chromosomes in rye, Secale cereale, in order to clarify when and how homolog pairing and bouquet formation occur. The changes of the spatial locations of chromosomes in the nucleus were investigated by the use of laser confocal microscopy, together with the surface-spreading method of silver nitrate staining to detect the formation of the synaptonemal complex. Homolog pairing in which homologs of four chromatids of a pair of homologs were coaligned in parallel but remained distinctly separate was microscopically detected for the first time in the present study. Homolog pairing showed the following characteristics: (1) it occurred at the leptotene-zygotene transition stage, prior to the formation of nodules and the synaptonemal complex; (2) the chromatin structure of chromosomes was in a state of decondensation; (3) it required no telomere clustering. These data suggest that homolog pairing represents a structure that indicates incipient recombination. After the homolog pairing stage, two kinds of bouquet configuration were found in zygotene. The commonly observed type was a loose bouquet, in which the subtelomeric regions were loosely aggregated. The other type was a definite bouquet, in which almost all the subtelomeric regions were conjugated, but this type was observed only in a limited number of the meiotic prophase cells of some individuals. It was concluded that the former represents the configuration of homologous recombination and the latter that of ectopic recombination.

c(3)G encodes a Drosophila synaptonemal complex protein

The meiotic mutant c(3)G (crossover suppressor on 3 of Gowen) abolishes both synaptonemal complex (SC) formation and meiotic recombination, whereas mutations in the mei-W68 and mei-P22 genes prevent recombination but allow normal SC to form. These data, as well as a century of cytogenetic studies, support the argument that meiotic recombination between homologous chromosomes in Drosophila females requires synapsis and SC formation. We have cloned the c(3)G gene and shown that it encodes a protein that is structurally similar to SC proteins from yeast and mammals. Immunolocalization of the C(3)G protein, as well as the analysis of a C(3)G-eGFP expression construct, reveals that C(3)G is present in a thread-like pattern along the lengths of chromosomes in meiotic prophase, consistent with a role as an SC protein present on meiotic bivalents. The availability of a marker for SC in Drosophila allowed the investigation of the extent of synapsis in exchange-defective mutants. These studies indicate that SC formation is impaired in certain meiotic mutants and that the synaptic defect correlates with the exchange defects. Moreover, the observation of interference among the residual exchanges in these mutant oocytes implies that complete SC formation is not required for crossover interference in Drosophila.

Novel meiosis-specific isoform of mammalian SMC1

Structural maintenance of chromosomes (SMC) proteins fulfill pivotal roles in chromosome dynamics. In yeast, the SMC1-SMC3 heterodimer is required for meiotic sister chromatid cohesion and DNA recombination. Little is known, however, about mammalian SMC proteins in meiotic cells. We have identified a novel SMC protein (SMC1beta), which-except for a unique, basic, DNA binding C-terminal motif-is highly homologous to SMC1 (which may now be called SMC1alpha) and is not present in the yeast genome. SMC1beta is specifically expressed in testes and coimmunoprecipitates with SMC3 from testis nuclear extracts, but not from a variety of somatic cells. This establishes for mammalian cells the concept of cell-type- and tissue-specific SMC protein isoforms. Analysis of testis sections and chromosome spreads of various stages of meiosis revealed localization of SMC1beta along the axial elements of synaptonemal complexes in prophase I. Most SMC1beta dissociates from the chromosome arms in late-pachytene-diplotene cells. However, SMC1beta, but not SMC1alpha, remains chromatin associated at the centromeres up to metaphase II. Thus, SMC1beta and not SMC1alpha is likely involved in maintaining cohesion between sister centromeres until anaphase II.

Asymmetrically distributed oligonucleotide repeats in the Caenorhabditis elegans genome sequence that map to regions important for meiotic chromosome segregation

The roundworm Caenorhabditis elegans has a haploid karyotype containing six linear chromosomes. The termini of worm chromosomes have been proposed to play an important role in meiotic prophase, either when homologs are participating in a genome-wide search for their proper partners or in the initiation of synapsis. For each chromosome one end appears to stimulate crossing-over with the correct homolog; the other end lacks this property. We have used a bioinformatics approach to identify six repetitive sequence elements in the sequenced C.elegans genome whose distribution closely parallels these putative meiotic pairing centers (MPC) or homolog recognition regions (HRR). We propose that these six DNA sequence elements, which are largely chromosome specific, may correspond to the genetically defined HRR/MPC elements.

A Caenorhabditis elegans cohesion protein with functions in meiotic chromosome pairing and disjunction

We have studied four Caenorhabditis elegans homologs of the Rad21/Scc1/Rec8 sister-chromatid cohesion protein family. Based on the RNAi phenotype and protein localization, it is concluded that one of them, W02A2.6p, is the likely worm ortholog of yeast Rec8p. The depletion of C. elegans W02A2.6p (called REC-8) by RNAi, induced univalent formation and splitting of chromosomes into sister chromatids at diakinesis. Chromosome synapsis at pachytene was defective, but primary homology recognition seemed unaffected, as a closer-than-random association of homologous fluorescence in situ hybridization (FISH) signals at leptotene/zygotene was observed. Depletion of REC-8 also induced chromosome fragmentation at diakinesis. We interpret these fragments as products of unrepaired meiotic double-stranded DNA breaks (DSBs), because fragmentation was suppressed in a spo-11 background. Thus, REC-8 seems to be required for successful repair of DSBs. The occurrence of DSBs in REC-8-depleted meiocytes suggests that DSB formation does not depend on homologous synapsis. Anti-REC-8 immunostaining decorated synaptonemal complexes (SCs) at pachytene and chromosomal axes in bivalents and univalents at diakinesis. Between metaphase I and metaphase II, REC-8 is partially lost from the chromosomes. The partial loss of REC-8 from chromosomes between metaphase I and metaphase II suggests that worm REC-8 might function similarly to yeast Rec8p. The loss of yeast Rec8p from chromosome arms at meiosis I and centromeres at meiosis II coordinates the disjunction of homologs and sister chromatids at the two meiotic divisions.

Chargaff's legacy

Of Chargaff's four rules on DNA base composition, only his first parity rule was incorporated into mainstream biology as the DNA double helix. Now, the cluster rule, the second parity rule, and the GC rule, reveal the multiple levels of information in our genomes and potential conflicts between them. In these terms we can understand how double-stranded RNA became an intracellular alarm signal, how potentially recombining nucleic acids can distinguish between 'self' and 'not-self' so leading to the origin of species, how isochores evolved to facilitate gene duplication, and how unlikely it is that any mutation can ever remain truly neutral.

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.

Association of mammalian SMC1 and SMC3 proteins with meiotic chromosomes and synaptonemal complexes

In somatic cells, the heterodimeric Structural Maintenance of Chromosomes (SMC) proteins are involved in chromosome condensation and gene dosage compensation (SMC2 and 4), and sister chromatid cohesion and DNA recombination (SMC1 and 3). We report here evidence for an involvement of mammalian SMC1 and SMC3 proteins in meiosis. Immunofluorescence analysis of testis sections showed intense chromatin association in meiotic prophase cells, weaker staining in round spermatids and absence of the SMC proteins in elongated spermatids. In spermatocyte nuclei spreads, the SMC1 and SMC3 proteins localize in a beaded structure along the axial elements of synaptonemal complexes of pachytene and diplotene chromosomes. Both SMC proteins are present in rat spermatocytes and enriched in preparations of synaptonemal complexes. Several independent experimental approaches revealed interactions of the SMC proteins with synaptonemal complex-specific proteins SCP2 and SCP3. These results suggest a model for the arrangement of SMC proteins in mammalian meiotic chromatin.

A candidate recombination modifier gene for Zea mays L

Maize meiotic mutant desynaptic (dy) was tested as a candidate recombination modifier gene because its effect is manifested in prophase I. Recombination rates for desynaptic (dy) and its wild type were compared in two ways: (1) segregation analysis using six linked molecular markers on chromosome 1L and (2) cytogenetic analysis using fluorescence in situ hybridization (FISH)-aided meiotic configurations observed in metaphase I. Chromosome 1L map lengths among the six linked markers were 45-63 cM for five F2 dy/dy plants, significantly lower than the wild-type F2 map distance of 72 cM. Chromosomes 2 and 6 were marked with rDNA FISH probes, and their map lengths were estimated from FISH-adorned meiotic configurations using the expectation-maximization algorithm. Chiasma frequencies for dy/dy plants were significantly reduced for both arms of chromosome 2, for chromosome arm 6L, and for eight unidentified chromosomes. There was a notable exception for the nucleolus-organizing region-bearing arm chromosome arm 6S, where dy increased chiasma frequency. Maize meiotic mutant desynaptic is a recombination modifier gene based on cytogenetic and segregation analyses.

Effect of the pairing gene Ph1 and premeiotic colchicine treatment on intra- and interchromosome pairing of isochromosomes in common wheat

The analysis of the pattern of isochromosome pairing allows one to distinguish factors affecting presynaptic alignment of homologous chromosomes from those affecting synapsis and crossing-over. Because the two homologous arms in an isochromosome are invariably associated by a common centromere, the suppression of pairing between these arms (intrachromosome pairing) would indicate that synaptic or postsynaptic events were impaired. In contrast, the suppression of pairing between an isochromosome and its homologous chromosome (interchromosome pairing), without affecting intrachromosome pairing, would suggest that homologous presynaptic alignment was impaired. We used such an isochromosome system to determine which of the processes associated with chromosome pairing was affected by the Ph1 gene of common wheat-the main gene that restricts pairing to homologues. Ph1 reduced the frequency of interchromosome pairing without affecting intrachromosome pairing. In contrast, intrachromosome pairing was strongly reduced in the absence of the synaptic gene Syn-B1. Premeiotic colchicine treatment, which drastically decreased pairing of conventional chromosomes, reduced interchromosome but not intrachromosome pairing. The results support the hypothesis that premeiotic alignment is a necessary stage for the regularity of meiotic pairing and that Ph1 relaxes this alignment. We suggest that Ph1 acts on premeiotic alignment of homologues and homeologues as a means of ensuring diploid-like meiotic behavior in polyploid wheat.

Aspects of three-dimensional chromosome reorganization during the onset of human male meiotic prophase

The three-dimensional morphology and distribution of human chromosomes 3 were studied in nuclei of spermatogonia and spermatocytes I from formaldehyde-fixed human testis sections. Chromosome arms, pericentromeres and telomeric regions were painted by a three-color, five-probe fluorescence in situ hybridization protocol. Light optical serial sections of premeiotic and meiotic nuclei obtained by confocal laser scanning microscopy revealed that premeiotic chromosomes 3 are separate from each other and occupy variably shaped territories, which are sectored in distinct 3 p- and q-arm domains. Three-dimensional reconstructions of the painted chromosome domains by a Voronoi tessellation approach showed that mean chromosome volumes did not differ significantly among the premeiotic and meiotic stages investigated. A significant increase in surface area and reduction of dimensionless ‘roundness factor’ estimates of arm domains indicated that the restructuring of spatially separate chromosome territories initiates during preleptotene. Telomeric regions, which in meiotic stem cells located predominantly in arm-domain chromatin, showed a redistribution towards the domain surface during this stage. At leptotene homologues were generally misaligned and displayed intimate intermingling of non-homologous chromatin. Pairing initiated at the ends of bent zygotene chromosomes, which displayed a complex surface structure with discernible sister chromatids. The results indicate that, in mammals, homology search is executed during leptotene, after remodeling of chromosome territories.

Meiotic recombination in C. elegans initiates by a conserved mechanism and is dispensable for homologous chromosome synapsis

Chromosome segregation at meiosis I depends on pairing and crossing-over between homologs. In most eukaryotes, pairing culminates with formation of the proteinaceous synaptonemal complex (SC). In budding yeast, recombination initiates through double-strand DNA breaks (DSBs) and is thought to be essential for SC formation. Here, we examine whether this mechanism for initiating meiotic recombination is conserved, and we test the dependence of homologous chromosome synapsis on recombination in C. elegans. We find that a homolog of the yeast DSB-generating enzyme, Spo11p, is required for meiotic exchange in this metazoan, and that radiation-induced breaks partially alleviate this dependence. Thus, initiation of recombination by DSBs is apparently conserved. However, homologous synapsis is independent of recombination in the nematode, since it occurs normally in a C. elegans spo-11 null mutant.

MEIOTIC CHROMOSOME ORGANIZATION AND SEGREGATION IN PLANTS

During meiosis, homologous chromosomes are brought together to be recombined and segregated into separate haploid gametes. This requires two cell divisions, an elaborate prophase with five substages, and specialized mechanisms that regulate the association of sister chromatids. This review focuses on plant chromosomes and chromosome-associated structures, such as recombination nodules and kinetochores, that ensure accurate meiotic chromosome segregation.

Effect of the pairing gene Ph1 on centromere misdivision in common wheat

The cytologically diploid-like meiotic behavior of hexaploid wheat (i.e., exclusive bivalent pairing of homologues) is largely controlled by the pairing homoeologous gene Ph1. This gene suppresses pairing between homoeologous (partially homologous) chromosomes of the three closely related genomes that compose the hexaploid wheat complement. It has been previously proposed that Ph1 regulates meiotic pairing by determining the pattern of premeiotic arrangement of homologous and homoeologous chromosomes. We therefore assume that Ph1 action may be targeted at the interaction of centromeres with spindle microtubules--an interaction that is critical for movement of chromosomes to their specific interphase positions. Using monosomic lines of common wheat, we studied the effect of this gene on types and rates of centromere division of univalents at meiosis. In the presence of the normal two doses of Ph1, the frequency of transverse breakage (misdivision) of the centromere of univalent chromosomes was high in both first and second meiotic divisions; whereas with zero dose of the gene, this frequency was drastically reduced. The results suggest that Ph1 is a trans-acting gene affecting centromere-microtubules interaction. The findings are discussed in the context of the effect of Ph1 on interphase chromosome arrangement.

Meiotic synapsis in the absence of recombination

Although in Saccharomyces cerevisiae the initiation of meiotic recombination, as indicated by double-strand break formation, appears to be functionally linked to the initiation of synapsis, meiotic chromosome synapsis in Drosophila females occurs in the absence of meiotic exchange. Electron microscopy of oocytes from females homozygous for either of two meiotic mutants (mei-W68 and mei-P22), which eliminate both meiotic crossing over and gene conversion, revealed normal synaptonemal complex formation. Thus, synapsis in Drosophila is independent of meiotic recombination, consistent with a model in which synapsis is required for the initiation of meiotic recombination. Furthermore, the basic processes of early meiosis may have different functional or temporal relations, or both, in yeast and Drosophila.

The mechanisms involved in formation of deletions and duplications of 15q11-q13

Haplotype analysis was undertaken in 20 cases of 15q11-q13 deletion associated with Prader-Willi syndrome (PWS) or Angelman syndrome (AS) to determine if these deletions arose through unequal meiotic crossing over between homologous chromosomes. Of these, six cases of PWS and three of AS were informative for markers on both sides of the deletion. For four of six cases of paternal 15q11-q13 deletion (PWS), markers on both sides of the deletion breakpoints were inferred to be of the same grandparental origin, implying an intrachromosomal origin of the deletion. Although the remaining two PWS cases showed evidence of crossing over between markers flanking the deletion, this was not more frequent than expected by chance given the genetic distance between proximal and distal markers. It is therefore possible that all PWS deletions were intrachromosomal in origin with the deletion event occurring after normal meiosis I recombination. Alternatively, both sister chromatid and homologous chromosome unequal exchange during meiosis may contribute to these deletions. In contrast, all three cases of maternal 15q11-q13 deletion (AS) were associated with crossing over between flanking markers, which suggests significantly more recombination than expected by chance (p = 0.002). Therefore, there appears to be more than one mechanism which may lead to PWS/AS deletions or the resolution of recombination intermediates may differ depending on the parental origin of the deletion. Furthermore, 13 of 15 cases of 15q11-q13 duplication, triplication, or inversion duplication had a distal duplication breakpoint which differed from the common distal deletion breakpoint. The presence of at least four distal breakpoint sites in duplications indicates that the mechanisms of rearrangement may be complex and multiple repeat sequences may be involved.

Chiasmata, crossovers, and meiotic chromosome segregation

Meiotic recombination events are probably critical for the completion of several meiotic processes. In addition, recombination is likely to be involved in the events that lead up to synapsis of homologues in meiotic prophase. Recombination events that ultimately become resolved as exchanges are needed for the formation of chiasmata. Chiasmata maintain the association of paired homologues following loss of the synaptonemal complex and participate in the mechanism that signals that the bivalent has attached to the spindle in a bipolar orientation that will result in meiosis I disjunction.

Double-strand break-induced recombination in eukaryotes

Genetic recombination is of fundamental importance for a wide variety of biological processes in eukaryotic cells. One of the major questions in recombination relates to the mechanism by which the exchange of genetic information is initiated. In recent years, DNA double strand breaks (DSBs) have emerged as an important lesion that can initiate and stimulate meiotic and mitotic homologous recombination. In this review, we examine the models by which DSBs induce recombination, describe the types of recombination events that DSBs stimulate, and compare the genetic control of DSB-induced mitotic recombination in budding and fission yeasts.

The transcriptional basis of chromosome pairing

Pairing between homologous chromosomes is essential for successful meiosis; generally only paired homologs recombine and segregate correctly into haploid germ cells. Homologs also pair in some somatic cells (e.g. in diploid and polytene cells of Drosophila). How homologs find their partners is a mystery. First, I review some explanations of how they might do so; most involve base-pairing (i.e. DNA-DNA) interactions. Then I discuss the remarkable fact that chromosomes only pair when they are transcriptionally active. Finally, I present a general model for pairing based upon the DNA-protein interactions involved in transcription. Each chromosome in the haploid set has a unique array of transcription units strung along its length. Therefore, each chromatin fibre will be folded into a unique array of loops associated with clusters of polymerases and transcription factors; only homologs share similar arrays. As these loops and clusters, or transcription factories, move continually, they make and break contact with others. Correct pairing would be nucleated when a promoter in a loop tethered to one factory binds to a homologous polymerizing site in another factory, before transcription stabilizes the association. This increases the chances that adjacent promoters will bind to their homologs, so that chromosomes eventually become zipped together with their partners. Pairing is then the inevitable consequence of transcription of partially-condensed chromosomes.

Heterochromatin: a meiotic matchmaker?

During meiosis, the pairing of chromosomes is crucial for a successful partitioning of the genetic material and its transmission into the developing gamete. The classical view of meiotic pairing involves recombination between homologues as an integral part of the pairing mechanism. But, in cases where no recombination occurs, how do chromosome partners find one another? And how do they pair up and then segregate appropriately? Recently, a combination of molecular genetics and fluorescence in situ hybridization appears to have provided an answer to these questions by demonstrating a crucial role for heterochromatin in chromosome pairing.

Centromere and telomere movements during early meiotic prophase of mouse and man are associated with the onset of chromosome pairing

The preconditions and early steps of meiotic chromosome pairing were studied by fluorescence in situ hybridization (FISH) with chromosome-specific DNA probes to mouse and human testis tissue sections. Premeiotic pairing of homologous chromosomes was not detected in spermatogonia of the two species. FISH with centromere- and telomere-specific DNA probes in combination with immunostaining (IS) of synaptonemal complex (SC) proteins to testis sections of prepuberal mice at days 4-12 post partum was performed to study sequentially the meiotic pairing process. Movements of centromeres and then telomeres to the nuclear envelope, and of telomeres along the nuclear envelope leading to the formation of a chromosomal bouquet were detected during mouse prophase. At the bouquet stage, pairing of a mouse chromosome-8-specific probe was observed. SC-IS and simultaneous telomere FISH revealed that axial element proteins appear as large aggregates in mouse meiocytes when telomeres are attached to the nuclear envelope. Axial element formation initiates during tight telomere clustering and transverse filament-IS indicated the initiation of synapsis during this stage. Comparison of telomere and centromere distribution patterns of mouse and human meiocytes revealed movements of centromeres and then telomeres to the nuclear envelope and subsequent bouquet formation as conserved motifs of the pairing process. Chromosome painting in human spermatogonia revealed compacted, largely mutually exclusive chromosome territories. The territories developed into long, thin threads at the onset of meiotic prophase. Based on these results a unified model of the pairing process is proposed.

Synaptonemal complexes: structure and function

Synaptonemal complexes (SCs) are zipper-like structures which are assembled between homologous chromosomes during the prophase of the first meiotic division. Their assembly and disassembly correlate with the successive chromatin rearrangements of meiotic prophase, namely the condensation, pairing, recombination and disjunction of homologous chromosomes. It was originally thought that SCs created the preconditions for the homologous crossing over of chromosomes by bringing corresponding parts of homologous chromosomes in close apposition. However, this view has been gradually undermined during recent years, and ideas about the roles of SCs have radically changed. SCs are now considered to be structures that both control the number and distribution of reciprocal exchanges between homologous chromosomes (cross-overs) and convert cross-overs into functional chiasmata. How SCs fulfil these roles remains to be elucidated.

Targeted gene disruption of Hsp70-2 results in failed meiosis, germ cell apoptosis, and male infertility

In addition to the five 70-kDa heat shock proteins (HSP70) common to germ cells and somatic tissues of mammals, spermatogenic cells synthesize HSP70-2 during meiosis. To determine if this unique stress protein has a critical role in meiosis, we used gene-targeting techniques to disrupt Hsp70-2 in mice. Male mice homozygous for the mutant allele (Hsp70-2 -/-) did not synthesize HSP70-2, lacked postmeiotic spermatids and mature sperm, and were infertile. However, neither meiosis nor fertility was affected in female Hsp70-2 -/- mice. We previously found that HSP70-2 is associated with synaptonemal complexes in the nucleus of meiotic spermatocytes from mice and hamsters. While synaptonemal complexes assembled in Hsp70-2 -/- spermatocytes, structural abnormalities became apparent in these cells by late prophase, and development rarely progressed to the meiotic divisions. Furthermore, analysis of nuclei and genomic DNA indicated that the failure of meiosis in Hsp70-2 -/- mice was coincident with a dramatic increase in spermatocyte apoptosis. These results suggest that HSP70-2 participates in synaptonemal complex function during meiosis in male germ cells and is linked to mechanisms that inhibit apoptosis.

Targeted gene disruption of Hsp70-2 results in failed meiosis, germ cell apoptosis, and male infertility

In addition to the five 70-kDa heat shock proteins (HSP70) common to germ cells and somatic tissues of mammals, spermatogenic cells synthesize HSP70-2 during meiosis. To determine if this unique stress protein has a critical role in meiosis, we used gene-targeting techniques to disrupt Hsp70-2 in mice. Male mice homozygous for the mutant allele (Hsp70-2 -/-) did not synthesize HSP70-2, lacked postmeiotic spermatids and mature sperm, and were infertile. However, neither meiosis nor fertility was affected in female Hsp70-2 -/- mice. We previously found that HSP70-2 is associated with synaptonemal complexes in the nucleus of meiotic spermatocytes from mice and hamsters. While synaptonemal complexes assembled in Hsp70-2 -/- spermatocytes, structural abnormalities became apparent in these cells by late prophase, and development rarely progressed to the meiotic divisions. Furthermore, analysis of nuclei and genomic DNA indicated that the failure of meiosis in Hsp70-2 -/- mice was coincident with a dramatic increase in spermatocyte apoptosis. These results suggest that HSP70-2 participates in synaptonemal complex function during meiosis in male germ cells and is linked to mechanisms that inhibit apoptosis.

A recA-like gene in Drosophila melanogaster that is expressed at high levels in female but not male meiotic tissues

The RecA protein is the central enzyme in prokaryotic recombination. It catalyzes pairing and strand exchange between homologous DNA molecules, and functions in both DNA repair and genetic recombination. The RecA-like proteins Rad51 and Dmc1 of yeast are both required for meiotic recombination and the former is also necessary for repair of double-strand breaks in vegetative cells. Genes encoding Rad51 homologs have been isolated recently from several higher eukaryotes. This paper describes the isolation and molecular characterization of a genomic DNA fragment from Drosophila melanogaster containing the coding sequence for a RecA-like protein. This protein exhibits strong sequence homology with the Rad51 proteins of budding yeast, fission yeast, chickens, mouse and humans, and slightly less (but still strong) homology with yeast Dmc1. Both in situ hybridization and Southern analysis indicate that the Rad51 gene is present only once per genome in Drosophila (at 99D on chromosome arm 3R). However, there are at least three other fragments that cross-hybridize strongly at low stringency. RNA blotting analysis detects a single transcript of about 1.35 kb that is present throughout development at low levels. Transcript levels are induced at least tenfold in ovaries, as measured by RNase protection analysis, suggestive of a role in female meiosis. Transcript levels are significantly lower in testes than in bulk RNA of adult males, however, indicating that Rad51 may be repressed in meiosis of Drosophila males.

HSP70-2 is part of the synaptonemal complex in mouse and hamster spermatocytes

Mouse spermatogenic cells are known to express HSP70-2, a member of the HSP70 family of heat-shock proteins. The purpose of the present study was to characterize further the expression and localization of HSP70-2 in meiotic cells of mice and hamsters. After separating mouse spermatogenic cells into cytoplasmic and nuclear fractions, proteins were separated by two-dimensional gel electrophoresis and detected with HSP-specific antibodies. Of several HSP70 proteins identified in the cytoplasm, only HSC70 and HSP70-2 were also detected in the nucleus. Immunocytological analyses of spermatocyte prophase cells revealed that HSP70-2 was associated with the synaptonemal complex. Surface-spread synaptonemal complexes at pachytene and diplotene stages labeled distinctly with the antiserum to HSP70-2. Synaptonemal complexes from fetal mouse oocytes failed to show any evidence of HSP70-2. Reverse-transcriptase-polymerase chain reaction (RT-PCR) analyses of gene expression confirmed this sex specificity; Hsp70-2 mRNA was detected in mouse testes, but not ovaries. These findings are suggestive of a previously unsuspected sexual dimorphism in structure and/or function of the synaptonemal complex.

Meiotic recombination and germ cell aneuploidy

Data on human trisomic conceptuses suggest that the extra chromosome commonly has a maternal origin, and the amount and position of crossing-over on nondisjoined chromosomes is commonly altered. These observations may provide important clues to the etiology of human germ cell aneuploidy, especially in regard to evaluating whether environmental factors play a role. There is concordance of effects of environmental agents on fungi, plants, and animals, which suggests that the overall process of meiosis is well conserved and that chemical and physical agents can affect meiotic recombination, leading to aneuploidy. It seems likely that meiosis in humans will fit the general pattern of meiosis in terms of sensitivity to radiation and chemicals. Thus studies on other organisms provide some insight into the procedures necessary for obtaining useful human data. For example, frequencies of spontaneous meiotic recombination are not uniform per physical length in Drosophila, and different regions of a chromosome respond differently to treatment. Treatments that relieve constraints on the distribution of meiotic exchange, without changing greatly the overall frequency of exchange, may increase the number of univalents and give the impression that there are chromosome-specific responses. Recombination studies that monitor one or a few relatively short genetic regions may also give a false impression of the effects of a treatment on recombination. In addition, meiotic mutants in Saccharomyces and Drosophila highlight a number of processes that are important for production of an exchange event and the utility of that event in the proper segregation of both homologues and sisters. They also suggest that tests for pairing at pachytene, chiasmata at diplotene, and genetic crossing-over may give different results.

Meiotic metaphase arrest in animal oocytes: its mechanisms and biological significance

Metaphase arrest in meiosis I or II before fertilization is a common and unique feature of oogenesis in many animal species. How and why oocytes from many species are arrested at metaphase, rather than after the completion of meiosis, has long remained a mystery. This article reviews recent advances in our understanding of the mechanisms and biological significance of meiotic metaphase arrest in animal oocytes.

Reciprocal relationship between stem-loop potential and substitution density in retroviral quasispecies under positive Darwinian selection

Nucleic acids have the potential to form intrastrand stem-loops if complementary bases are suitably located. Computer analyses of poliovirus and retroviral RNAs have revealed a reciprocal relationship between "statistically significant" stem-loop potential and "sequence variability." The statistically significant stem-loop potential of a nucleic acid segment has been defined as a function of the difference between the folding energy of the natural segment (FONS) and the mean folding energy of a set of randomized (shuffled) versions of the natural segment (FORS-M). Since FONS is dependent on both base composition and base order, whereas FORS-M is solely dependent on base composition (a genomic characteristic), it follows that statistically significant stem-loop potential (FORS-D) is a function of base order (a local characteristic). In retroviral genomes, as in all DNA genomes studied, positive FORS-D values are widely distributed. Thus there have been pressures on base order both to encode specific functions and to encode stem-loops. As in the case of DNA genomes under positive Darwinian selection pressure, in HIV-1 specific function appears to dominate in rapidly evolving regions. Here high sequence variability, expressed as substitution density (not indel density), is associated with negative FORS-D values (impaired base-order-dependent stem-loop potential). This suggests that in these regions HIV-1 genomes are under positive selection pressure by host defenses. The general function of stem-loops is recombination. This is a vital process if, from among members of viral "quasispecies," functional genomes are to be salvaged. Thus, for rapidly evolving RNA genomes, it is as important to conserve base-order-dependent stem-loop potential as to conserve other functions.

Identification of functional domains in the Sep1 protein (= Kem1, Xrn1), which is required for transition through meiotic prophase in Saccharomyces cerevisiae

The Sep1 (also known as Kem1, Xrn1, Rar5, DST2/Stpbeta) protein of Saccharomyces cerevisiae is an Mr 175,000 multifunctional exonuclease with suspected roles in RNA turnover and in the microtubular cytoskeleton as well as in DNA recombination and DNA replication. The most striking phenotype of SEP1 null mutations is quantitative arrest during meiotic prophase at the pachytene stage. We have constructed a set of N- and C-terminal as well as internal deletions of the large SEP1 gene. Analysis of these deletion mutations on plasmids in a host carrying a null allele (sep1 ) revealed that at least 270 amino acids from the C-terminus of the wild-type protein were dispensable for complementing the slow growth and benomyl hypersensitivity of a null mutant. In contrast, any deletion at the N-terminus abrogated complementing activity for these phenotypes. The sequences essential for function correspond remarkably well with the regions of Sep1 that are homologous to its Schizosaccharomyces pombe counterpart Exo2. In addition, these experiments showed that, despite the high intracellular levels of Sep1, over-expression of this protein above these levels is detrimental to the cell. We discuss the potential cellular roles of the Sep1 protein as a microtubule-nucleic acid interface protein linking its suspected function in the microtubular cytoskeleton with its role as a nucleic acid binding protein.

The search for DNA homology does not limit stable homologous pairing promoted by RecA protein

BACKGROUND

The basic molecular mechanisms that govern the search for DNA homology and subsequent homologous pairing during genetic recombination are not understood. RecA is the central homologous recombination protein of Escherichia coli; because several RecA homologues have been identified in eukaryotic cells, it is likely that the mechanisms employed by RecA are conserved throughout evolution. Analysis of the kinetics of the homologous search and pairing reactions catalyzed by RecA should therefore provide insights of general relevance into the mechanisms by which macromolecules locate, and interact with, specific DNA targets.

RESULTS

RecA forms three-stranded synaptic complexes with a single-stranded oligonucleotide and a homologous region in duplex DNA. The kinetics of this initial pairing reaction were characterized using duplex DNA molecules of various concentrations and complexities containing a single target site, as well as various concentrations of homologous single-stranded oligonucleotides. The formation of the synaptic complex follows apparent second-order reaction kinetics with a rate proportional to the concentrations of both the homologous single-stranded oligonucleotide and the target sites within the duplex DNA. The reaction rate is independent of the complexity of duplex DNA in the reaction. We propose a kinetic scheme in which the RecA-single-stranded DNA filament interacts with duplex DNA and locates its target in a relatively fast reaction. We also suggest that complex conformational changes occur during the subsequent rate-limiting step.

CONCLUSIONS

We conclude that, during the formation of synaptic complexes by RecA, the search for homology is not rate-limiting, and that the iteration frequency of the search is around 10(2)-10(3) s-1. This value agrees well with what has been calculated as the minimum number for such a frequency in genome-wide searches, and limits the possible structures involved in the search for homology to those involving very soft (low energy) interactions. Furthermore, from the order of the reaction at the DNA concentrations found in eukaryotic nuclei, and the rate constant of the overall reaction, we predict that the search for homology is also not the rate-limiting step in the genome-wide searches implicated in meiosis and in gene targeting.

Germ line specific factors in chemical mutagenesis

Chemical mutagenesis test results have not revealed evidence of germ line specific mutagens. However, conventional assays have indicated that there are male-female differences in mutagenic response, as well as quantitative/qualitative differences in induced mutations which depend upon the particular cell stage exposed. Many factors inherent in the germ line can be speculated to influence chemical transport to, and interaction with, target cell populations to result in mutagenic outcomes. The level of uncertainty regarding the general operation of such factors, in combination with the limited availability of chemical test data designed to address comparative somatic and germ cell mutagenesis, leaves open the question of whether there are mutagens specifically affecting germ cells. This argues for a conservative approach to interpreting germ cell risk from somatic cell mutation analysis.

Male mice defective in the DNA mismatch repair gene PMS2 exhibit abnormal chromosome synapsis in meiosis

Using gene targeting in embryonic stem cells, we have derived mice with a null mutation in a DNA mismatch repair gene homolog, PMS2. We observed microsatellite instability in the male germline, in tail, and in tumor DNA of PMS2-deficient animals. We therefore conclude that PMS2 is involved in DNA mismatch repair in a variety of tissues. PMS2-deficient animals appear prone to sarcomas and lymphomas. PMS2-deficient males are infertile, producing only abnormal spermatozoa. Analysis of axial element and synaptonemal complex formation during prophase of meiosis I indicates abnormalities in chromosome synapsis. These observations suggest links among mismatch repair, genetic recombination, and chromosome synapsis in meiosis.