Synapsis and chiasma formation in four meiotic mutants of tomato (Lycopersicon esculentum)

Meiosis in crops: from genes to genomes

Meiosis is a key feature of sexual reproduction. During meiosis homologous chromosomes replicate, recombine, and randomly segregate, followed by the segregation of sister chromatids to produce haploid cells. The unique genotypes of recombinant gametes are an essential substrate for the selection of superior genotypes in natural populations and in plant breeding. In this review we summarize current knowledge on meiosis in diverse monocot and dicot crop species and provide a comprehensive resource of cloned meiotic mutants in six crop species (rice, maize, wheat, barley, tomato, and Brassica species). Generally, the functional roles of meiotic proteins are conserved between plant species, but we highlight notable differences in mutant phenotypes. The physical lengths of plant chromosomes vary greatly; for instance, wheat chromosomes are roughly one order of magnitude longer than those of rice. We explore how chromosomal distribution for crossover recombination can vary between species. We conclude that research on meiosis in crops will continue to complement that in Arabidopsis, and alongside possible applications in plant breeding will facilitate a better understanding of how the different stages of meiosis are controlled in plant species.

Meiotic Crossover Patterning

Proper number and placement of meiotic crossovers is vital to chromosome segregation, with failures in normal crossover distribution often resulting in aneuploidy and infertility. Meiotic crossovers are formed via homologous repair of programmed double-strand breaks (DSBs). Although DSBs occur throughout the genome, crossover placement is intricately patterned, as observed first in early genetic studies by Muller and Sturtevant. Three types of patterning events have been identified. Interference, first described by Sturtevant in 1915, is a phenomenon in which crossovers on the same chromosome do not occur near one another. Assurance, initially identified by Owen in 1949, describes the phenomenon in which a minimum of one crossover is formed per chromosome pair. Suppression, first observed by Beadle in 1932, dictates that crossovers do not occur in regions surrounding the centromere and telomeres. The mechanisms behind crossover patterning remain largely unknown, and key players appear to act at all scales, from the DNA level to inter-chromosome interactions. There is also considerable overlap between the known players that drive each patterning phenomenon. In this review we discuss the history of studies of crossover patterning, developments in methods used in the field, and our current understanding of the interplay between patterning phenomena.

Meiotic recombination profiling of interspecific hybrid F1 tomato pollen by linked read sequencing

Genome wide screening of pooled pollen samples from a single interspecific F1 hybrid obtained from a cross between tomato, Solanum lycopersicum and its wild relative, Solanum pimpinellifolium using linked read sequencing of the haploid nuclei, allowed profiling of the crossover (CO) and gene conversion (GC) landscape. We observed a striking overlap between cold regions of CO in the male gametes and our previously established F6 recombinant inbred lines (RILs) population. COs were overrepresented in non-coding regions in the gene promoter and 5'UTR regions of genes. Poly-A/T and AT rich motifs were found enriched in 1 kb promoter regions flanking the CO sites. Non-crossover associated allelic and ectopic GCs were detected in most chromosomes, confirming that besides CO, GC represents also a source for genetic diversity and genome plasticity in tomato. Furthermore, we identified processed break junctions pointing at the involvement of both homology directed and non-homology directed repair pathways, suggesting a recombination machinery in tomato that is more complex than currently anticipated.

The telomere bouquet is a hub where meiotic double-strand breaks, synapsis, and stable homolog juxtaposition are coordinated in the zebrafish, Danio rerio

Meiosis is a cellular program that generates haploid gametes for sexual reproduction. While chromosome events that contribute to reducing ploidy (homologous chromosome pairing, synapsis, and recombination) are well conserved, their execution varies across species and even between sexes of the same species. The telomere bouquet is a conserved feature of meiosis that was first described nearly a century ago, yet its role is still debated. Here we took advantage of the prominent telomere bouquet in zebrafish, Danio rerio, and super-resolution microscopy to show that axis morphogenesis, synapsis, and the formation of double-strand breaks (DSBs) all take place within the immediate vicinity of telomeres. We established a coherent timeline of events and tested the dependence of each event on the formation of Spo11-induced DSBs. First, we found that the axis protein Sycp3 loads adjacent to telomeres and extends inward, suggesting a specific feature common to all telomeres seeds the development of the axis. Second, we found that newly formed axes near telomeres engage in presynaptic co-alignment by a mechanism that depends on DSBs, even when stable juxtaposition of homologous chromosomes at interstitial regions is not yet evident. Third, we were surprised to discover that ~30% of telomeres in early prophase I engage in associations between two or more chromosome ends and these interactions decrease in later stages. Finally, while pairing and synapsis were disrupted in both spo11 males and females, their reproductive phenotypes were starkly different; spo11 mutant males failed to produce sperm while females produced offspring with severe developmental defects. Our results support zebrafish as an important vertebrate model for meiosis with implications for differences in fertility and genetically derived birth defects in males and females.

Inherent to reproduction is the transmission of genetic information from one generation to the next. In sexually reproducing organisms, each parent contributes an equal amount of genetic information, packaged in chromosomes, to the offspring. Diploid organisms, like humans, have two copies of every chromosome, while their haploid gametes (e.g. eggs and sperm) have only one. This reduction in ploidy depends on the segregation of chromosomes during meiosis, resulting in gametes with one copy of each chromosome. Missegregation of the chromosomes in the parents leads to abnormal chromosome numbers in the offspring, which is usually lethal or has detrimental developmental effects. While it has been known for over a century that homologous chromosomes pair and recombine to facilitate proper segregation, how homologs find their partners has remained elusive. A structure that has been central to the discussion of homolog pairing is the bouquet, or the dynamic clustering of telomeres during early stages of meiosis. Here we use zebrafish to show that the telomere bouquet is the site where key events leading to homologous chromosome pairing are coordinated. Furthermore, we show that deletion of spo11, a gene required for proper recombination in most studied organisms, resulted in very different effects in males and females where males were sterile while females produced deformed progeny.

Meiosis, unreduced gametes, and parthenogenesis: implications for engineering clonal seed formation in crops

Meiosis and unreduced gametes. Sexual flowering plants produce meiotically derived cells that give rise to the male and female haploid gametophytic phase. In the ovule, usually a single precursor (the megaspore mother cell) undergoes meiosis to form four haploid megaspores; however, numerous mutants result in the formation of unreduced gametes, sometimes showing female specificity, a phenomenon reminiscent of the initiation of gametophytic apomixis. Here, we review the developmental events that occur during female meiosis and megasporogenesis at the light of current possibilities to engineer unreduced gamete formation. We also provide an overview of the current understanding of mechanisms leading to parthenogenesis and discuss some of the conceptual implications for attempting the induction of clonal seed production in cultivated plants.

Altered distribution of MLH1 foci is associated with changes in cohesins and chromosome axis compaction in an asynaptic mutant of tomato

In most multicellular eukaryotes, synapsis [synaptonemal complex (SC) formation] between pairs of homologous chromosomes during prophase I of meiosis is closely linked with crossing over. Asynaptic mutants in plants have reduced synapsis and increased univalent frequency, often resulting in genetically unbalanced gametes and reduced fertility. Surprisingly, some asynaptic mutants (like as1 in tomato) have wild-type or increased levels of crossing over. To investigate, we examined SC spreads from as1/as1 microsporocytes using both light and electron microscopic immunolocalization. We observed increased numbers of MLH1 foci (a crossover marker) per unit length of SC in as1 mutants compared to wild-type. These changes are associated with reduced levels of detectable cohesin proteins in the axial and lateral elements (AE/LEs) of SCs, and the AE/LEs of as1 mutants are also significantly longer than those of wild-type or another asynaptic mutant. These results indicate that chromosome axis structure, synapsis, and crossover control are all closely linked in plants.

Erratic male meiosis resulting in 2n pollen grain formation in a 4x cytotype (2n = 28) of Ranunculus laetus Wall. ex Royle

Two accessions were studied for male meiosis in Ranunculus laetus from the cold regions of Northwest Himalayas. One accession showed the presence of 14 bivalents at diakinesis and regular segregation of bivalents at anaphase I which lead to normal tetrad formation with four n microspores and consequently n pollen grains and 100% pollen fertility. Second accession from the same locality revealed the erratic meiosis characterized by the presence of all the 28 chromosomes as univalents in meiocytes at metaphase I. Univalent chromosomes failed to segregate during anaphases and produced restitution nuclei at meiosis I and II. These restitution nuclei resulted into dyads and triads which subsequently produced two types of apparently fertile pollen grains. On the basis of size, the two types of pollen grains were categorized as n (normal reduced) and 2n (unreduced, 1.5-times larger than the n pollen grains). The estimated frequency of 2n pollen grains from dyads and triads (61.59%) was almost the same as that of the observed one (59.90%), which indicated that 2n pollen grains in R. laetus were the result of dyads and triads. The present paper herein may provide an insight into the mechanisms of the formation of various intraspecific polyploids through sexual polyploidization in R. laetus.

Nuclear architecture and chromosome dynamics in the search of the pairing partner in meiosis in plants

The formation of haploid gametes in organisms with sexual reproduction requires regular bivalent chromosome pairing in meiosis. In many species, homologous chromosomes occupy separate territories at the onset of meiosis. To be paired at metaphase I, they need to be brought into a close proximity for interactions that include homology recognition and the establishment of some form of bonds. How homologues find each other is one of the least understood meiotic events. Plant species with large or medium sized genomes, such as wheat or maize, are excellent materials for the cytological analysis of chromosome dynamics at early meiosis, but genes that control meiosis have been identified mainly in small genome species such as Arabidopsis thaliana. This review is focused on the contribution studies on plants are providing to the knowledge of the initial steps of the meiotic process.

The mismatch repair protein MLH1 marks a subset of strongly interfering crossovers in tomato

In most eukaryotes, the prospective chromosomal positions of meiotic crossovers are marked during meiotic prophase by protein complexes called late recombination nodules (LNs). In tomato (Solanum lycopersicum), a cytological recombination map has been constructed based on LN positions. We demonstrate that the mismatch repair protein MLH1 occurs in LNs. We determined the positions of MLH1 foci along the 12 tomato chromosome pairs (bivalents) during meiotic prophase and compared the map of MLH1 focus positions with that of LN positions. On all 12 bivalents, the number of MLH1 foci was approximately 70% of the number of LNs. Bivalents with zero MLH1 foci were rare, which argues against random failure of detecting MLH1 in the LNs. We inferred that there are two types of LNs, MLH1-positive and MLH1-negative LNs, and that each bivalent gets an obligate MLH1-positive LN. The two LN types are differently distributed along the bivalents. Furthermore, cytological interference among MLH1 foci was much stronger than interference among LNs, implying that MLH1 marks the positions of a subset of strongly interfering crossovers. Based on the distances between MLH1 foci or LNs, we propose that MLH1-positive and MLH1-negative LNs stem from the same population of weakly interfering precursors.

Similarity of the domain structure of proteins as a basis for the conservation of meiosis

Meiosis is conserved in all eucaryotic kingdoms, and homologous rows of variability are revealed for the cytological traits of meiosis. To find the nature of these phenomenons, we reviewed the most-studied meiosis-specific proteins and studied them with the methods of bioinformatics. We found that synaptonemal complex proteins have no homology of amino-acid sequence, but are similar in the domain organization and three-dimensional (3D) structure of functionally important domains in budding yeast, nematode, Drosophila, Arabidopsis, and human. Recombination proteins of Rad51/Dmc1 family are conserved to the extent which permits them to make filamentous single-strand deoxyribonucleic acid (ssDNA)-protein intermediates of meiotic recombination. The same structural principles are valid for conservation of the ultrastructure of kinetochores, cell gap contacts, and nuclear pore complexes, such as in the cases when ultrastructure 3D parameters are important for the function. We suggest that self-assembly of protein molecules plays a significant role in building-up of all biological structures mentioned.

Chromosome synapsis in Arabidopsis: analysis of the transverse filament protein ZYP1 reveals novel functions for the synaptonemal complex

With respect to history, plants have provided an ideal system for cytogenetical analysis of the synaptonemal complex (SC). However, until recently, the identification of the genes that encode the SC in plants has proved elusive. In recent years, Arabidopsis thaliana was developed as a model system for plant meiosis research. As a result, there was substantial progress in the isolation of meiotic genes and this has recently led to the isolation of the first plant SC gene, ZYP1. The ZYP1 gene encodes a transverse filament (TF) protein that is predicted to have structural similarity to TF proteins found in other organisms. Analysis of plants deficient in ZYP1 expression has provided important insights into the function of the SC in plants. Loss of ZYP1 has only a limited effect on the overall level of recombination. However, it is associated with extensive nonhomologous recombination leading to multivalent formation at metaphase I. This phenomenon was not previously reported in other organisms. It is important to note that cytological analysis of the ZYP1 deficient lines indicates that SC formation is not required for the imposition of crossover interference.

Meiotic mutations in rye Secale cereale L

Spontaneous meiotic mutations of winter rye Secale cereale L. (2n = 14) were revealed in inbred F2 progenies, which were obtained by self-pollination of F1 hybrids resulting from crosses of individual plants of cultivar Vyatka or weedy rye with plants of self-fertile inbred lines. The mutations cause partial or complete sterility, and are maintained in heterozygote condition. Six types of mutations were distinguished as the result of cytological analysis of meiosis and genetic analysis. (1) Plants with nonallelic asynaptic mutations sy1 and sy9 lacked bivalents in 96.8 and 67.0% metaphase I cells, respectively, formed only axial elements but not the mature synaptonemal complex (SC), and had defects in telomere clustering in early prophase I. (2) Weak asynaptic mutant sy3 showed incomplete synapsis at the start of SC degradation at diplotene and lower chiasma number; yet only 2% meiocytes lacked bivalents in MI. (3) Mutations sy2, sy6, sy7, sy8, sy10, and sy19 caused nonhomologous synapsis; i.e., a varying number of univalents and occasional multivalents were observed in MI, which was preceded by switches of pairing partners and fold-back synapsis at mid-prophase I. (4) Mutation mei6 led to the formation of protrusions and minor branched structures of the SC lateral elements. (5) Allelic mutations mei8 and mei8-10 caused irregular chromatin condensation along the chromosome length in prophase I, which was accompanied by chromosome sticking and fragmentation in MI. (6) Allelic mutations mei5 and mei10 determined chromosome supercondensation, caused the disturbance of meiotic spindle assembly, arrested meiosis at various stages but did not affect formation of the pollen wall, thus arrested meiocytes got covered with the pollen wall. Analysis of double mutants revealed recessive epistatic interactions for some mutations; the epistatic group was sy9 > sy1 > sy3 > sy19. This reflects the sequence of meiotic events controlled by the corresponding genes. The expression of sy2 and sy19 proved to be modified by additional genes. Most meiotic mutations found in rye have analogs in other plants.

Meiotic recombination intermediates and mismatch repair proteins

Mismatch repair proteins are a highly diverse group of proteins that interact with numerous DNA structures during DNA repair and replication. Here we review data for the role of Msh4, Msh5, Mlh1, Mlh3 and Exo1 in crossing over. Based on the paradigm of interactions developed from studies of mismatch repair, we propose models for the mechanism of crossover implementation by Msh4/Msh5 and Mlh1/Mlh3.

Recent development of image analysis methods in plant chromosome research

Image analysis methods have provided effective tools in chromosome research along with the development both in computer software and hardware. A chromosome image analyzing system, CHIAS, for plant chromosomes was developed in 1985 and was subsequently revised so that with CHIAS3 one can take advantage of Internet use for downloading the program. In this review, the recent developments of imaging methods in plant chromosome research for automating chromosome identification, constructing a map of a pachytene chromosome, and patterning of interphase nuclei are described.

Recombination nodules in plants

The molecular events of recombination are thought to be catalyzed by proteins present in recombination nodules (RNs). Therefore, studying RN structure and function should give insights into the processes by which meiotic recombination is regulated in eukaryotes. Two types of RNs have been identified so far, early (ENs) and late (LNs). ENs appear at leptotene and persist into early pachytene while LNs appear in pachytene and remain into early diplotene. ENs and LNs can be distinguished not only on their time of appearance, but also by such characteristics as shape and size, relative numbers, and association with unsynapsed and/or synapsed chromosomal segments. The function(s) of ENs is not clear, but they may have a role in searching for DNA homology, synapsis, gene conversion and/or crossing over. LNs are well correlated with crossing over. Here, the patterns of ENs and LNs during prophase I in plants are reviewed.

Imposition of crossover interference through the nonrandom distribution of synapsis initiation complexes

Meiotic crossovers (COs) are nonrandomly distributed along chromosomes such that two COs seldom occur close together, a phenomenon known as CO interference. We have used genetic and cytological methods to investigate interference mechanisms in budding yeast. Assembly of the synaptonemal complex (SC) initiates at a few sites along each chromosome, triggered by a complex of proteins (including Zip2 and Zip3) called the synapsis initiation complex (SIC). We found that SICs, like COs, display interference, supporting the hypothesis that COs occur at synapsis initiation sites. Unexpectedly, we found that SICs show interference in mutants in which CO interference is abolished; one explanation is that these same mutations eliminate the subset of COs that normally occur at SICs. Since SICs are assembled in advance of SC and they are properly positioned even in the absence of SC formation, these data clearly demonstrate an aspect of interference that is independent of synapsis.

Development of a quantitative pachytene chromosome map in Oryza sativa by imaging methods

A higher GC content region of an Oryza sativa chromosome can be specifically visualized by double staining with propidium iodide (PI) and 4, 6-diamidino-2-phenylindole (DAPI). This procedure allows identification of chromosome 9 from the other rice chromosomes at the pachytene stage. Using rice chromosome 9 as a model, an imaging method to construct a pachytene chromosomal map was developed by quantifying the fluorescence profile (FP) of each chromomere. The pachytene map of chromosome 9 consists of twenty-two chromomeres including four chromomeres within the nucleolar organizing region (NOR) and satellite region. The pachytene map was compared with the corresponding somatic prometaphase map and the linkage map. The differences among the three maps indicate that each map depicts specific biological information, which is difficult to be substituted by the other maps.

Meiotic telomere clustering is inhibited by colchicine but does not require cytoplasmic microtubules

Telomere clustering, the defining feature of the bouquet, is an almost universal feature of meiotic prophase, yet its mechanism remains unknown. The microtubule-depolymerizing agent colchicine was found to inhibit bouquet formation. Telomeres in colchicine-treated cells remained scattered in the nuclear periphery, whereas untreated cells exhibited a prominent telomere cluster. Colchicine administered after the bouquet had formed did not affect telomere dispersal. The effect of colchicine on bouquet formation appeared to be separable from its effect on cytoplasmic microtubules; amiprophos methyl, a highly effective plant microtubule-depolymerizing drug, did not affect telomere clustering. Inhibition of bouquet formation was limited to colchicine and the related drug podophyllotoxin out of the variety of microtubule-depolymerizing drugs tested, suggesting that the target involved in bouquet formation has a structural specificity.

Chiasma formation in Arabidopsis thaliana accession Wassileskija and in two meiotic mutants

Meiotic chiasmata were analysed in metaphase I pollen mother cells (PMCs) of wild-type Arabidopsis thaliana and in two meiotic mutants. Fluorescence in situ hybridisation (FISH) with 45S rDNA and 5S rDNA as probes was used to identify the five chromosome pairs. A wild-type chiasma frequency of 9.24 per cell was found, consistent with estimated genetic recombination values. Individual bivalent chiasma frequencies varied according to chromosome size; chromosome 1 had the highest mean chiasma frequency (2.14) while the short acrocentric chromosomes had the lowest frequencies (1.54 and 1.56). FISH analysis was extended to two meiotic mutants (asy1 and dsy1) having low residual bivalent and chiasma frequencies. Mutant dsy1 gave no indication of chromosome preference for residual bivalent formation; instead it showed a general reduction in bivalent and chiasma frequencies. In asy1, the longest chromosome (1) had the lowest bivalent frequency and chiasma frequency while the short acrocentric chromosome 2 had the highest frequencies. This chromosome pair may be preferentially involved in synapsis and chiasma formation because of their association with the nucleolus. However, other factors may be operating since the other acrocentric chromosome (4), with similar size and structure to chromosome 2, did not share these chiasma properties.

The single-end invasion: an asymmetric intermediate at the double-strand break to double-holliday junction transition of meiotic recombination

We identify a novel meiotic recombination intermediate, the single-end invasion (SEI), which occurs during the transition from double-strand breaks (DSBs) to double-Holliday junction (dHJs). SEIs are products of strand exchange between one DSB end and its homolog. The structural asymmetry of SEIs indicates that the two ends of a DSB interact with the homolog in temporal succession, via structurally (and thus biochemically) distinct processes. SEIs arise surprisingly late in prophase, concomitant with synaptonemal complex (SC) formation. These and other data imply that SEIs are preceded by nascent DSB-partner intermediates, which then undergo selective differentiation into crossover and noncrossover types, with SC formation and strand exchange as downstream consequences. Late occurrence of strand exchange provides opportunity to reverse recombinational fate even after homologs are coaligned and/or synapsed. This feature can explain crossover suppression between homeologous and structurally heterozygous chromosomes.

The budding yeast Msh4 protein functions in chromosome synapsis and the regulation of crossover distribution

The budding yeast MSH4 gene encodes a MutS homolog produced specifically in meiotic cells. Msh4 is not required for meiotic mismatch repair or gene conversion, but it is required for wild-type levels of crossing over. Here, we show that a msh4 null mutation substantially decreases crossover interference. With respect to the defect in interference and the level of crossing over, msh4 is similar to the zip1 mutant, which lacks a structural component of the synaptonemal complex (SC). Furthermore, epistasis tests indicate that msh4 and zip1 affect the same subset of meiotic crossovers. In the msh4 mutant, SC formation is delayed compared to wild type, and full synapsis is achieved in only about half of all nuclei. The simultaneous defects in synapsis and interference observed in msh4 (and also zip1 and ndj1/tam1) suggest a role for the SC in mediating interference. The Msh4 protein localizes to discrete foci on meiotic chromosomes and colocalizes with Zip2, a protein involved in the initiation of chromosome synapsis. Both Zip2 and Zip1 are required for the normal localization of Msh4 to chromosomes, raising the possibility that the zip1 and zip2 defects in crossing over are indirect, resulting from the failure to localize Msh4 properly.

Meiotic recombination in sexual diploid and apomictic triploid dandelions (Taraxacum officinale L.)

Taraxacum officinale L. (dandelion) is a vigorous weed in Europe with diploid sexual populations in the southern regions and partially overlapping populations of diploid sexuals and triploid or tetraploid apomicts in the central and northern regions. Previous studies have demonstrated unexpectedly high levels of genetic variation in the apomictic populations, suggesting the occurrence of genetic segregation in the apomicts and (or) hybridization between sexual and apomictic individuals. In this study we analysed meiosis in both sexual diploid and apomictic triploid plants to find mechanisms that could account for the high levels of genetic variation in the apomicts. Microscopic study of microsporocytes in the triploid apomicts revealed that the levels of chromosome pairing and chiasma formation at meiotic prophase I were lower than in that of the sexual diploids, but still sufficient to assume recombination between the homologues. Nomarski DIC (differential interference contrast) microscopy of optically cleared megasporocytes in the apomicts demonstrated incidental formation of tetrads, which suggests that hybridization can occur in triploid apomicts.

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).

The leptotene-zygotene transition of meiosis

The leptotene/zygotene transition of meiosis, as defined by classical cytological studies, is the period when homologous chromosomes, already being discernible individualized entities, begin to be close together or touching over portions of their lengths. This period also includes the bouquet stage: Chromosome ends, which have already become integral components of the inner nuclear membrane, move into a polarized configuration, along with other nuclear envelope components. Chromosome movements, active or passive, also occur. The detailed nature of interhomologue interactions during this period, with special emphasis on the involvement of chromosome ends, and the overall role for meiosis and recombination of chromosome movement and, especially, the bouquet stage are discussed.

Cloning and characterization of MS5 from Arabidopsis: a gene critical in male meiosis

In this paper, we describe the cloning of the MS5 gene, a gene essential for male fertility in Arabidopsis. We previously defined the MS5 locus by characterizing an EMS-induced allele, ms5-1. We identified a new allele of MS5 (ms5-2) that was T-DNA-generated and used the T-DNA tag to clone the gene. Sequencing of mutant and wild-type alleles together with complementation of the ms5-1 mutant phenotype with a wild-type genomic clone confirmed the identity of the gene. Differences between the phenotypes of the two mutant alleles could be attributed to differences in mutant gene structure. The semi-dominant and dominant negative phenotypes of the ms5-2 mutant probably result from production of a truncated polypeptide. An unknown locus in Landsberg erecta can counteract the dominant negative phenotype of ms5-2. Mutations in MS5 cause the formation 'polyads'--tetrads with more than four pools of chromosomes after male meiosis. Similarities between the MS5 sequence and that of a number of proteins were found; two that may be significant were with a synaptonemal complex protein and with a regulatory subunit of a cyclin-dependent kinase. The MS5 gene is a member of a small gene family highly conserved amongst plant species.

Cytological characterization of four meiotic mutants of Arabidopsis isolated from T-DNA-transformed lines

A secondary screen of the Feldmann collection of T-DNA transformed Arabidopsis lines identified several meiotic mutants. We used a spreading technique combined with DAPI staining in a detailed cytogenetic analysis of meiotic chromosome behaviour in four of these mutants, all of which are putatively T-DNA tagged and therefore candidates for molecular and functional analysis of the mutated genes. Two of them are defined as 'synaptic' mutants, showing greatly reduced association of homologous chromosomes at metaphase I: one is asynaptic, showing failure of synapsis during prophase I, whereas the other is desynaptic and is characterized by normal but non-maintained synapsis. Another mutant is defective in meiotic cell cycle control and undergoes a third meiotic division, resembling a second division but without an additional round of chromosome duplication. A further mutant shows meiosis-limited chromosome disruption, resulting in extensive chromosome fragmentation combined with other defects. All four mutants experience very irregular chromosome distribution during the meiotic divisions, resulting in abnormal numbers and/or sizes of microspores, with resulting reduced fertility.

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

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

Tam1, a telomere-associated meiotic protein, functions in chromosome synapsis and crossover interference

The TAM1 gene of Saccharomyces cerevisiae is expressed specifically during meiosis and encodes a protein that localizes to the ends of meiotic chromosomes. In a tam1 null mutant, there is an increase in the frequency of chromosomes that fail to recombine and an associated increase in homolog nondisjunction at meiosis I. The tam1 mutant also displays an increased frequency of precocious separation of sister chromatids and a reduced efficiency of distributive disjunction. The defect in distributive disjunction may be attributable to overloading of the distributive system by the increased number of nonrecombinant chromosomes. Recombination is not impaired in the tam1 mutant, but crossover interference is reduced substantially. In addition, chromosome synapsis is delayed in tam1 strains. The combination of a defect in synapsis and a reduction in interference is consistent with previous studies suggesting a role for the synaptonemal complex in regulating crossover distribution. tam1 is the only known yeast mutant in which the control of crossover distribution is impaired, but the frequency of crossing over is unaffected. We discuss here possibilities for how a telomere-associated protein might function in chromosome synapsis and crossover interference.

Meiosis: how could it work?

A physical connection between homologs is required for reductional segregation at the first division of meiosis. This connection is usually provided by one or a few well-spaced crossovers. A speculative overview of processes leading to formation of these crossovers is presented.