Chiasma Formation Research Articles

The role of DNA topoisomerase 1α (AtTOP1α) in regulating arabidopsis meiotic recombination and chromosome segregation.

Meiosis is a critical process in sexual reproduction, and errors during this cell division can significantly impact fertility. Successful meiosis relies on the coordinated action of numerous genes involved in DNA replication, strand breaks, and subsequent rejoining. DNA topoisomerase enzymes play a vital role by regulating DNA topology, alleviating tension during replication and transcription. To elucidate the specific function of DNA topoisomerase 1α ( ) in male reproductive development of Arabidopsis thaliana, we investigated meiotic cell division in Arabidopsis flower buds. Combining cytological and biochemical techniques, we aimed to reveal the novel contribution of to meiosis. Our results demonstrate that the absence of leads to aberrant chromatin behavior during meiotic division. Specifically, the top1α1 mutant displayed altered heterochromatin distribution and clustered centromere signals at early meiotic stages. Additionally, this mutant exhibited disruptions in the distribution of 45s rDNA signals and a reduced frequency of chiasma formation during metaphase I, a crucial stage for genetic exchange. Furthermore, the atm-2×top1α1 double mutant displayed even more severe meiotic defects, including incomplete synapsis, DNA fragmentation, and the presence of polyads. These observations collectively suggest that plays a critical role in ensuring accurate meiotic progression, promoting homologous chromosome crossover formation, and potentially functioning in a shared DNA repair pathway with ATAXIA TELANGIECTASIA MUTATED (ATM) in Arabidopsis microspore mother cells.

Asynapsis and meiotic restitution in tomato male meiosis induced by heat stress.

Susceptibility of the reproductive system to temperature fluctuations is a recurrent problem for crop production under a changing climate. The damage is complex as multiple processes in male and female gamete formation are affected, but in general, particularly pollen production is impaired. Here, the impact of short periods of elevated temperature on male meiosis of tomato (Solanum lycopersicon L.) is reported. Meiocytes in early stage flower buds exposed to heat stress (>35°C) exhibit impaired homolog synapsis resulting in partial to complete omission of chiasmata formation. In the absence of chiasmata, univalents segregate randomly developing unbalanced tetrads and polyads resulting in aneuploid spores. However, most heat-stressed meiotic buds primarily contain balanced dyads, indicating a propensity to execute meiotic restitution. With most meiocytes exhibiting a complete loss of chiasma formation and concomitantly showing a mitotic-like division, heat stress triggers first division restitution resulting in clonal spores. These findings corroborate with the plasticity of male meiosis under heat and establish a natural route for the induction of sexual polyploidization in plants and the engineering of clonal seed.

ASYNAPSIS 1 ensures crossover fidelity in polyploid wheat by promoting homologous recombination and suppressing non-homologous recombination.

During meiosis, the chromosome axes and synaptonemal complex mediate chromosome pairing and homologous recombination to maintain genomic stability and accurate chromosome segregation. In plants, ASYNAPSIS 1 (ASY1) is a key component of the chromosome axis that promotes inter-homolog recombination, synapsis and crossover formation. Here, the function of ASY1 has been cytologically characterized in a series of hypomorphic wheat mutants. In tetraploid wheat, asy1 hypomorphic mutants experience a reduction in chiasmata (crossovers) in a dosage-specific manner, resulting in failure to maintain crossover (CO) assurance. In mutants with only one functional copy of ASY1, distal chiasmata are maintained at the expense of proximal and interstitial chiasmata, indicating that ASY1 is required to promote chiasma formation away from the chromosome ends. Meiotic prophase I progression is delayed in asy1 hypomorphic mutants and is arrested in asy1 null mutants. In both tetraploid and hexaploid wheat, single asy1 mutants exhibit a high degree of ectopic recombination between multiple chromosomes at metaphase I. To explore the nature of the ectopic recombination, Triticum turgidum asy1b-2 was crossed with wheat-wild relative Aegilops variabilis. Homoeologous chiasmata increased 3.75-fold in Ttasy1b-2/Ae. variabilis compared to wild type/Ae. variabilis, indicating that ASY1 suppresses chiasma formation between divergent, but related chromosomes. These data suggest that ASY1 promotes recombination along the chromosome arms of homologous chromosomes whilst suppressing recombination between non-homologous chromosomes. Therefore, asy1 mutants could be utilized to increase recombination between wheat wild relatives and elite varieties for expediting introgression of important agronomic traits.

Continuous double-strand break induction and their differential processing sustain chiasma formation during Caenorhabditis elegans meiosis.

Faithful chromosome segregation into gametes depends on Spo11-induced DNA double-strand breaks (DSBs). These yield single-stranded 3' tails upon resection to promote crossovers (COs). While early Mre11-dependent end resection is the predominant pathway in most organisms, Exo1 or Dna2/BLM can also contribute to the efficient processing of meiotic DSBs. Although its enzymatic activity has been thoroughly dissected, the temporal dynamics underlying Spo11 activity have remained mostly elusive. We show that, in Caenorhabditis elegans, SPO-11-mediated DSB induction takes place throughout early meiotic prophase I until mid-late pachynema. We find that late DSBs are essential for CO formation and are preferentially processed by EXO-1 and DNA-2 in a redundant fashion. Further, EXO-1-DNA-2-mediated resection ensures completion of conservative DSB repair and discourages activation of KU-dependent end joining. Taken together, our data unveil important temporal aspects of DSB induction and identify previously unknown functional implications for EXO-1-DNA-2-mediated resection activity in C.elegans.

Location and Identification on Chromosome 3B of Bread Wheat of Genes Affecting Chiasma Number.

Understanding meiotic crossover (CO) variation in crops like bread wheat (Triticum aestivum L.) is necessary as COs are essential to create new, original and powerful combinations of genes for traits of agronomical interest. We cytogenetically characterized a set of wheat aneuploid lines missing part or all of chromosome 3B to identify the most influential regions for chiasma formation located on this chromosome. We showed that deletion of the short arm did not change the total number of chiasmata genome-wide, whereas this latter was reduced by ~35% while deleting the long arm. Contrary to what was hypothesized in a previous study, deletion of the long arm does not disturb the initiation of the synaptonemal complex (SC) in early meiotic stages. However, progression of the SC is abnormal, and we never observed its completion when the long arm is deleted. By studying six different deletion lines (missing different parts of the long arm), we revealed that at least two genes located in both the proximal (C-3BL2-0.22) and distal (3BL7-0.63-1.00) deletion bins are involved in the control of chiasmata, each deletion reducing the number of chiasmata by ~15%. We combined sequence analyses of deletion bins with RNA-Seq data derived from meiotic tissues and identified a set of genes for which at least the homoeologous copy on chromosome 3B is expressed and which are involved in DNA processing. Among these genes, eight (CAP-E1/E2, DUO1, MLH1, MPK4, MUS81, RTEL1, SYN4, ZIP4) are known to be involved in the recombination pathway.

Assessing the Heat Tolerance of Meiosis in Spanish Landraces of Tetraploid Wheat Triticum turgidum.

Heat stress alters the number and distribution of meiotic crossovers in wild and cultivated plant species. Hence, global warming may have a negative impact on meiosis, fertility, and crop productions. Assessment of germplasm collections to identify heat-tolerant genotypes is a priority for future crop improvement. Durum wheat, Triticum turgidum, is an important cultivated cereal worldwide and given the genetic diversity of the durum wheat Spanish landraces core collection, we decided to analyse the heat stress effect on chiasma formation in a sample of 16 landraces of T. turgidum ssp. turgidum and T. turgidum ssp. durum, from localities with variable climate conditions. Plants of each landrace were grown at 18–22 °C and at 30 °C during the premeiotic temperature-sensitive stage. The number of chiasmata was not affected by heat stress in three genotypes, but decreased by 0.3–2 chiasmata in ten genotypes and more than two chiasmata in the remaining three ones. Both thermotolerant and temperature-sensitive genotypes were found in the two subspecies, and in some of the agroecological zones studied, which supports that genotypes conferring a heat tolerant meiotic phenotype are not dependent on subspecies or geographical origin. Implications of heat adaptive genotypes in future research and breeding are discussed.

Is partial desynapsis in cauliflower (Brassica oleracea L. var. botrytis) pollen mother cells linked to aneuploidy in the crop?

Trisomic cauliflower plants (Brassica oleracea L. var. botrytis) display abnormal curd phenotypes that seriously decrease commercial value of the crop. Despite extensive breeding efforts, selection of genotypes producing euploid gametes remains unsuccessful due to unknown genetic and environmental factors. To reveal an eventual role of an-euploid gametes, we analyzed chromosome pairing, chiasma formation and chromosome segregation in pollen mother cells of selected cauliflower genotypes. To this end we compared three genotypes exhibiting Low with < 5%, Moderate with 5–10% and High with > 10% aberrant offspring, respectively. Although chromosome pairing at pachytene was regular, cells at diakinesis and metaphase I showed variable numbers of univalents, suggesting partial desynapsis. Cells at anaphase I–telophase II exhibit various degrees of unbalanced chromosome numbers, that may explain the aneuploid offspring. Immunofluorescence probed with an MLH1 antibody demonstrated fluorescent foci in all genotypes, but their lower numbers do not correspond to the number of putative chiasmata. Interchromosomal connections between chromosomes and bivalents are common at diakinesis and metaphase I, and they contain centromeric and 45S rDNA tandem repeats, but such chromatin connections seem not to affect proper disjoin of the half bivalents at anaphase I. Moreover, male meiosis in the Arabidopsis APETALA1/CAULIFLOWER double mutant with the typical cauliflower phenotype does show interchromosomal connections, but there are no indications for partial desynapsis. The causality of the curd development on the desynapsis in cauliflower is still a matter of debate.

Meiosis Remodeled: Inclusion of New Parts to Poppit Bead Models Enhances Understanding of Meiosis

A long-standing tradition uses strings of poppit beads of different colors to model meiosis, especially to show how segments of paired homologous chromosomes are recombined. Our use of orthodontic latex bands to model cohesion of sister chromatids, and plastic coffee stirrers as microtubules, extends what can normally be achieved with &lsquo;standard&rsquo; commercial kits of beads, so emphasizing the importance of four key elements of meiosis: (a) the role of chromosome replication before meiosis itself begins; (b) pairing and exchange (chiasma formation) of homologous chromosomes during meiosis I; (c) centromere (kinetochore) attachment and orientation within/on the spindle during meiosis I and meiosis II; and (d) the differential loss of arm and centromere cohesion at onset of anaphase I and anaphase II. These are essential elements of meiosis that students best need to visualize, not just read and think about. Bead modeling leads them in that direction, as our gallery of figures and accompanying text show. <em>Primary image:&nbsp;</em>Unassembled components of &lsquo;PoppitMeiosis&rsquo; &ndash; a poppit bead exercise aimed at student learning of meiosis. Beads are snapped together to model bivalent chromosomes (on the right side), with double-stick tape (top) representing the synaptonemal complex, orthodontic latex bands representing cohesion rings, and coffee stirrers representing microtubule bundles that connect centromeres to the spindle poles.

Replication protein A large subunit (RPA1a) limits chiasma formation during rice meiosis.

Replication protein A (RPA), a single-stranded DNA-binding protein, plays essential role in homologous recombination. However, because deletion of RPA causes embryonic lethality in mammals, the exact function of RPA in meiosis remains unclear. In this study, we generated an rpa1a mutant using CRISPR/Cas9 technology and explored its function in rice (Oryza sativa) meiosis. In rpa1a, 12 bivalents were formed at metaphase I, just like in wild-type, but chromosome fragmentations were consistently observed at anaphase I. Fluorescence in situ hybridization assays indicated that these fragmentations were due to the failure of the recombination intermediates to resolve. Importantly, the mutant had a highly elevated chiasma number, and loss of RPA1a could completely restore the 12 bivalent formations in the zmm (for ZIP1-4, MSH4/5, and MER3) mutant background. Protein-protein interaction assays showed that RPA1a formed a complex with the methyl methansulfonate and UV sensitive 81 (and the Fanconi anemia complementation group M-Bloom syndrome protein homologs (RECQ4A)-Topoisomerase3α-RecQ-mediated genome instability 1 complex to regulate chiasma formation and processing of the recombination intermediates. Thus, our data establish a pivotal role for RPA1a in promoting the accurate resolution of recombination intermediates and in limiting redundant chiasma formation during rice meiosis.

Meiotic analyses of Passiflora L. parents and interspecific F1 hybrids (HD15 progeny)

Passiflora F1 hybrids, with ornamental potential, were confirmed via GISH and analyzed for meiotic compatibility. This study aimed to analyze the meiotic behavior of P. gardneri Mast. and P. gibertii N.E. Brown and four interspecific hybrids resulting from this cross, with the aim of inferring genetic compatibility between parents and the implications of hybridization on the hybrids’ genomes. Flower buds were collected for classical cytogenetic studies and anthesis flowers for pollen viability (PV) using (1) Alexander's solution and (2) fluorescein diacetate. Pollen grains were counted, measured and identified as viable or unviable, with the PV considered to be low. Genomic in situ hybridization (GISH) technique was used in prophase I. Analysis of meiotic behavior showed bivalent formation among the homologous in the parents and homeologous in the hybrids; some irregularities were found, such as lagging chromosomes, asynchrony, spindle fiber irregularities and chromosomal bridges. Post meiosis resulted in anomalous cells such as monads, dyads, triads and polyads. All results were subjected to analysis of variance, with significant differences for all analyses between hybrids, with the exception of monad formation. GISH confirmed the hybrid character and pairing between homeologous chromosomes. The results obtained in this study established the degree of homology between the parent species through analysis of chromosome pairing and chiasma formation, confirmed by GISH. Both parent species showed potential for use as paternal parents.

Male and female meiosis evince differential patterns in chiasma formation: a case study of ornamental plant, Delphinium ajacis L.

Chromosomal behaviour during megasporogenesis and microsporogenesis has been studied in ornamental Delphinium ajacis L. Meiosis in female sex cell initiates later than male. The floral buds which carry egg mother cell (EMC) at diplotene stage has pollen mother cells (PMCs) at tetrad stage of meiosis suggesting protandry. Although the 16 chromosomes formed regular eight bivalents in both the sex cells, they differed in overall chiasma frequency which was 32.95% higher in EMCs and found to be 18.52 ± 2.12 per cell. In PMCs, the average chiasma frequency recorded was 13.93 ± 1.40 per cell. Interestingly, this variation in chiasma frequency was largely confined to the two large bivalents which shared 42.61% chiasma per EMC. The use of Q-Q plot, Box plot and Whisker plot showed departure in the chiasma frequency distributions in EMCs and PMCs from the normal distribution pattern. The difference in chiasma frequency in the two sex cells was significant at all levels as indicated by the low P values of 3.094 × 10-11 obtained from nonparametric test, i.e. Wilcoxon rank-sum test. It is suggested that the two different mechanisms of recombination are operational in the two sex cells, and the sex differences of chiasma frequency could have arisen due to differential epigenetic modifications of the chromatin which pattern the double-strand breaks, and the position and frequency of crossing over visible as chiasmata.

Usp26 mutation in mice leads to defective spermatogenesis depending on genetic background

Spermatogenesis is a reproductive system process that produces sperm. Ubiquitin specific peptidase 26 (USP26) is an X chromosome-linked deubiquitinase that is specifically expressed in the testes. It has long been controversial whether USP26 variants are associated with human male infertility. Thus, in the present study, we introduced a mutation into the Usp26 gene in mice and found that Usp26 mutant males backcrossed to a DBA/2 background, but not a C57BL/6 background, were sterile or subfertile and had atrophic testes. These findings indicate that the effects of the Usp26 mutation on male reproductive capacity were influenced by genetic background. Sperm in the cauda epididymis of Usp26 mutant mice backcrossed to a DBA/2 background were decreased in number and showed a malformed head morphology compared to those of wild-type mice. Additionally, histological examinations of the testes revealed that the number of round and elongated spermatids were dramatically reduced in Usp26 mutant mice. The mutant mice exhibited unsynapsed chromosomes in pachynema and defective chiasma formation in diplonema, which presumably resulted in apoptosis of metaphase spermatocytes and subsequent decrease of spermatids. Taken together, these results indicate that the deficiencies in fertility and spermatogenesis caused by mutation of Usp26 were dependent on genetic background.

Achiasmy and sex chromosome evolution

Abstract Achiasmy is a phenomenon whereby chiasma (“chromosomal cross–over”) formation is absent during meiosis in one or both sexes. It is one of the strategies that have evolved to cease recombination between sex chromosomes in various taxa. Absence of recombination makes Y (W) chromosome diverge from X (Z) chromosome via natural selection on linked genes. In the present review, we first review the research history of achiasmy in Drosophila species and summarize the prevalence of achiasmy in a wide range of animal taxa and the rapid turnover of achiasmy. We then briefly describe the mechanism of achiasmatic meiosis in Drosophila melanogaster and the evolution of meiotic genes in Drosophila species. Second, we discuss sex chromosome evolution in achiasmatic species. If meiotic recombination had been suppressed between sex chromosomes, heterogametic sex chromosomes have no chance of meiotic recombination. Therefore, the neo–Y chromosome which originated from Y–autosome fusion in achiasmatic species has no chance of recombination and no pre-existing genetic diversity since its origination. On the contrary, if the neo-Y chromosome had chance of recombination, it can maintain ancestral polymorphisms. We compare three different evolutionary trajectories of neo–Y chromosome in Drosophila americana, Drosophila miranda, and Drosophila albomicans. Lastly, we discuss the origin of Y chromosome in achiasmatic species. In particular, we introduce hypotheses proposed in three insect species that achiasmatic meiosis may facilitate the recruitment of an accessory B chromosome to a Y (W) chromosome.

EMS Induced Desynaptic Male Sterile Lines in Buckwheat (Fagopyrum esculentum Moench)

Cytological analysis of EMS (Ethyl methane sulphonate) treated population of Fagopyrum esculentum revealed an abnormal behaviour of microsporogenesis that affected the meiotic events resulting in the formation of abnormal meiotic products that prevent the gamete formation and impair pollen fertility. The two desynaptic mutants were recorded, showing distinctive variations in the morphology as compared to the control plants. The seeds of Fagopyrum esculentum were treated with EMS solution in different concentrations such as 0.1, 0.3 and 0.5% using potassium phosphate buffer (pH 7) for 5 h. During cytological investigation, 0.5% concentration of EMS enhanced the univalent frequency per cell at diakinesis/Metaphase I, respectively along with unequal segregation at anaphase I which is pronounced to be higher in contrast to bivalents. Because of higher frequency of univalents desynapsis has been categorized as medium-strong type. EMS induced desynaptic plants showed abnormal meiotic behaviour leading to pollen sterility and no seed setting was recorded. The chemical mutagen has acted on recombination genes system which is accountable for synapsis and chiasma formation and further disrupted the complete chiasma assembly. The desynaptic mutant is a potential tool that provides genetic information on the maintenance of chiasma and the study offers the possibility for formation of aneuploids production which may be exploited successfully through chemical mutagenesis in breeding programmes.

Competition for Chiasma Formation Between Identical and Homologous (But Not Identical) Chromosomes in Synthetic Autotetraploids of Arabidopsis thaliana.

Polyploid organisms provide additional opportunities to study meiosis in a more complex context since more than two potential homologous chromosomes are available. When the chromosome complement of a diploid individual is duplicated, each chromosome is accompanied by one identical and two homologous chromosomes within the same nucleus. In this situation, a competition in pairing/synapsis/chiasma formation between identical and homologous (but not necessarily identical) chromosomes can occur. Several studies have been conducted in different species to address whether there are preferences in crossover formation between identical rather than homologous chromosomes. In this study, multivalent and chiasma frequencies were cytologically analyzed in synthetic autotetraploids of Arabidopsis thaliana including the accessions Col, Ler, and the Col/Ler hybrid. Fluorescence in situ hybridization was conducted to identify each chromosome at metaphase I. The new Col and Ler tetraploids showed high multivalent frequencies, exceeding the theoretical 66.66% expected on a simple random end-pairing model, thus indicating that there are more than two autonomous synaptic sites per chromosome despite their small size. However, a significant excess of bivalent pairs was found in the Col/Ler hybrid, mainly due to the contribution of chromosomes 2 and 3. The mean chiasma frequencies of the three artificial autotetraploids were about twofold the corresponding mean cell chiasma frequencies of their diploid counterparts. The relative contribution of each chromosome to the total chiasma frequency was similar in the three genotypes, with the exception of a lower contribution of chromosome 3 in the hybrid. Preferences for chiasma formation between identical and homologous chromosomes were analyzed in Col/Ler 4x, taking advantage of the cytological differences between the accessions: variations in the size of the 45S rDNA region on the short arm of chromosome 2 and changes in the size and localization of the 5S rDNA region in chromosome 3. We observed a different behavior of chromosomes 2 and 3, i.e., random chiasma formation between identical and homologous chromosomes 2, and preferences for chiasma formation between homologous chromosomes 3. Hence, our results reveal the existence of chromosome-specific mechanisms responsible for these preferences.

Enigmatic Phenomenon of Secondary Association among Bivalents in Guar (Cyamopsis tetragonoloba (L.) Taub.)

Cytogenetical assessment of pollen mother cells of Guar (Cyamopsis tetragonoloba (L.) Taub.) in response to treatment of chemical mutagen was done. Ethyl Methane Sulphonate (EMS) revealed possession of an enigmatic behaviour among chromosomal bivalents i.e. formation of secondary association. Chromosomes exhibited a unique tendency of getting associated in close proximity and form discrete pairs. The phenomenon of secondary chromosomal association has been ascribed to the existence of similar cohesive homology or malfunctioning in chiasma formation among the associating bivalents. The association of bivalents was witnessed to be executed in either end to end fashion or sideways association. Various other chromosomal abnormalities were found, but secondary association was the preponderant abnormality which exhibited a continuous increase in its magnitude with increase in EMS concentration. Along with secondary associations, stickiness of chromosomes at anaphase was observed in considerable frequency. A significant influence of chromosomal association on the post meiotic product was also experienced, which in turn, led to influence the meiotic index. Abnormal sporads in the form of triads, dyads, monads and polyads, alongwith normal tetrad were seen. Pollen fertility percent was seen to decline from control to the highest concentration of EMS due to anomalies of preceding stages.

Variable Patterning of Chromatin Remodeling, Telomere Positioning, Synapsis, and Chiasma Formation of Individual Rye Chromosomes in Meiosis of Wheat-Rye Additions

Meiosis, the type of cell division that halves the chromosome number, shows a considerable degree of diversity among species. Unraveling molecular mechanisms of the meiotic machinery has been mainly based on meiotic mutants, where the effects of a change were assessed on chromosomes of the particular species. An alternative approach is to study the meiotic behavior of the chromosomes introgressed into different genetic backgrounds. As an allohexaploid, common wheat tolerates introgression of chromosomes from related species, such as rye. The behavior of individual pairs of rye homologues added to wheat has been monitored in meiotic prophase I and metaphase I. Chromosome 4R increased its length in early prophase I much more than other chromosomes studied, implying chromosome specific patterns of chromatin organization. Chromosome conformation affected clustering of telomeres but not their dispersion. Telomeres of the short arm of submetacentric chromosomes 4R, 5R, and 6R failed more often to be included in the telomere cluster either than the telomeres of the long arms or telomeres of metacentrics such as 2R, 3R, and 7R. The disturbed migration of the telomeres of 5RS and 6RS was associated with failure of synapsis and chiasma formation. However, despite the failed convergence of its telomere, the 4RS arm developed normal synapsis, perhaps because the strong increase of its length in early prophase I facilitated homologous encounters in intercalary regions. Surprisingly, chiasma frequencies in both arms of 4R were reduced. Similarly, the short arm of metacentric chromosome 2R often failed to form chiasmata despite normal synapsis. Chromosomes 1R, 3R, and 7R showed a regular meiotic behavior. These observations are discussed in the context of the behavior that these chromosomes show in rye itself.

SHOC1 is a ERCC4-(HhH)2-like protein, integral to the formation of crossover recombination intermediates during mammalian meiosis.

Chromosome segregation errors during meiosis result in the formation of aneuploid gametes and are the leading cause of pregnancy loss and birth defects in humans. Proper chromosome segregation requires pairwise associations of maternal and paternal homologous chromosomes. Chiasmata, which are the cytological manifestations of crossovers (COs), provide a physical link that holds the homologs together as a pair, facilitating their orientation on the spindle at meiosis I. Although CO-promoting activities ensure a balanced number and position of COs, their identity and mechanism of action in mammals remain understudied. Previous work in yeast and Arabidopsis has shown that Zip2 and Shoc1 are ortholog proteins with an important role in promoting the formation of COs. Our work is the first study in mammals showing the in vivo and in vitro function of mouse and human SHOC1. We show that purified recombinant human SHOC1, an XPF/MUS81 family member, preferentially binds branched DNA molecules but apparently lacks in vitro endonuclease activity, despite its conserved ERCC4-(HhH)2 core structure. Cytological observations suggest that initial steps of recombination are normal in a majority of spermatocytes from SHOC1 hypomorphic mice. However, late stages of recombination appear abnormal, as chromosomal localization of MLH1 is reduced. In agreement, chiasma formation is reduced, and cells arrest at metaphase I with a few lagging chromosomes and subsequent apoptosis. This analysis of SHOC1-deficient mice and the selective localization of SHOC1 to a subset of recombination sites show that SHOC1 acts at key mid-stage steps of the CO formation process. The formation of chromosome axial elements and homologous pairing are apparently normal, but synapsis is altered with SYCP1 frequently failing to extend the full length of the chromosome axes. Finally, we describe that SHOC1 interacts with TEX11, another protein important for the formation of COs, connecting SHOC1 to chromosome axis and structure.

Cytogenetical Study of Induced Desynaptic Variants in &lt;i&gt;Phaseolus vulgaris&lt;/i&gt; L.

The cytological screening of gamma ray treated plants revealed few desynaptic plants where an enhanced frequency of univalents were found at metaphase I along with few bivalents. The desynaptic plants were obtained at 200 Gy dose of gamma rays and were characterised as weak and medium strong type. The univalents remained unpaired till later meiotic stages and resulted in the unequal separation of chromosomes at anaphase I. The unequal separation of chromosomes as a consequence of desynapsis lead to the formation of micronuclei and gametes with imbalanced chromosomes, thus affecting the post-meiotic stages also. The desynaptic plants possess very few pods and acquire high pollen inviability. The study suggests the ability of gamma rays in the creation of male sterile lines in Phaseolus vulgaris L. The gamma rays either act on the genes responsible for synapsis and chiasma formation or disrupt the synaptonemal complex affecting chiasmate dissociation. The desynaptic mutants could be used as a potential source for gathering information on chiasma maintenance mechanism as well as to study the genetic mechanism and consequences of male sterility in plants. Further, the male sterile lines could also be profitably utilized in hybridization breeding programmes to produce hybrid seeds.

Microsatellite-centromere mapping in Japanese scallop (Patinopecten yessoensis) through half-tetrad analysis in gynogenetic diploid families

Gene-centromere mapping is an essential prerequisite for understanding the composition and structure of genomes. Half-tetrad analysis is a powerful tool for mapping genes and understanding chromosomal behavior during meiosis. The Japanese scallop (Patinopecten yessoensis), a cold-tolerant species inhabiting the northwestern Pacific coast, is a commercially important marine bivalve in Asian countries. In this study, inheritance of 32 informative microsatellite loci was examined in 70-h D-shaped larvae of three induced meiogynogenetic diploid families of P. yessoensis for centromere mapping using half-tetrad analysis. The ratio of gynogenetic diploids was proven to be 100%, 100% and 96% in the three families, respectively. Inheritance analysis in the control crosses showed that 51 of the 53 genotypic ratios observed were in accordance with Mendelian expectations at the 5% level after Bonferroni correction. Seven of the 32 microsatellite loci showed the existence of null alleles in control crosses. The second division segregation frequency (y) of the microsatellite loci ranged from 0.07 to 0.85 with a mean of 0.38, suggesting the existence of positive interference after a single chiasma formation in some chromosomes in the scallop. Microsatellite-centromere distances ranged from 4 cM to 42 cM under the assumption of complete interference. Information on the positions of centromeres in relation to the microsatellite loci will represent a contribution towards the assembly of genetic maps in the commercially important scallop species.