Homologous Chromosomes In Meiosis Research Articles

Polyploidy in the history of breeding at the Yaltushkiv Experimental Breeding Station: new methods of differentiation and stabilisation of sugar beet tetraploids

Purpose. To analyse the history of the polyploid breeding at the Yaltushkiv Experimental Breeding Station, in particular the improvement of genetic models of new hybrids, new methods of induction of meiotic tetraploids, differentiation and selection of breeding genotypes by genome ploidy.
 Methods. Cytological analysis of metaphase chromosomes of the apical meristems of the growth points of seed plants and beets of the first year for the selection of tetraploid plants 4x = 36 chromosomes; cytophotometric analysis using computer programs AP Partec to study ploidy based on the quantitative content of nuclear DNA; field methods (selection of tetraploid seed plants according to the types of pollen grains, formation of synthetic populations of tetraploid multigerm plants on a cytogenetic basis according to the structure of bivalent conjugation of homologous chromosomes in meiosis.
 Results. In the process of tetraploid induction, cytological methods of ploidy analysis at the station were improved using computer programs of AP Partec for the selection of sugar beets 4x by the quantitative content of nuclear DNA in the cell on plants of the first year of vegetation and for the formation of tetraploid populations after colchicine induction in seed plants. In the process of studying the structure of polyploid populations by ploidy, the percentage of tetraploids varied from 82.0±2.22% in Ya/Hen pollinator to 58.1±2.81% in Ya/Man pollinator. It was established that the selection for fertility and pollen uniformity depends on the genetic characteristics of the breeding genotypes and affects the pollen-forming ability in the first generations after induction with colchicine, reducing the number of 4x IIa and IIb biotypes according to cytological classification. Using the example of the multigerm pollinator Ya/Roi, it was established that selection for sugar content and productivity had, to a certain extent, a stabilising effect on the degree of ploidy. The biotechnological methods used to deposit 4x genotypes with a high percentage of bivalent conjugation of homologous chromosomes in metaphase I of meiosis (70–90%) contributed to the cytogenetic stability of polyploid gametes and their high yield – 92.9% of 4x in the next generation of seeds. It was investigated that the productivity of triploid hybrids obtained with the use of the tetraploid pollinator Ya/DA 4x based on the germplasm of the wild species Beta patula at the early stages of vegetative development varied in triploid plants from 325 to 465 g, and the sugar content in some numbers reached 21.5%.
 Conclusions. The development of polyploid selection at the Yaltushkiv Experimental Breeding Station evolved from the development polyhybrids to the development of triploid hybrids based on CMS using tetraploid multigerm pollinators. For the formation of populations of tetraploid multigerm pollinators, the methods of fluorescent cytophotometry and computer programs AP Partec were used, as well as biotechnological methods for creating synthetic populations of tetraploid multigerm pollinators on a cytogenetic basis, which ensure the yield of tetraploids in seeds up to 92.9%.

Phosphorylation of adducin-1 by TPX2 promotes interpolar microtubule homeostasis and precise chromosome segregation in mouse oocytes

BackgroundADD1 (adducin-1) and TPX2 (targeting protein for Xklp2) are centrosomal proteins and regulate mitotic spindle assembly. Mammalian oocytes that segregate homologous chromosomes in Meiosis I and sister chromatids in Meiosis II with a spindle lacking centrosomes are more prone to chromosome segregation errors than in mitosis. However, the regulatory mechanisms of oocyte spindle assembly and the functions of ADD1 and TPX2 in this process remain elusive.ResultWe found that the expression levels and localization of ADD1, S726 phosphorylated ADD1 (p-ADD1), and TPX2 proteins exhibited spindle assembly-dependent dynamic changes during mouse oocyte meiosis. Taxol treatment, which stabilizes the microtubule polymer and protects it from disassembly, made the signals of ADD1, p-ADD1, and TPX2 present in the microtubule organizing centers of small asters and spindles. Knockdown of approximately 60% of ADD1 protein levels destabilized interpolar microtubules in the meiotic spindle, resulting in aberrant chromosome alignment, reduced first polar body extrusion, and increased aneuploidy in metaphase II oocytes, but did not affect K-fiber homeostasis and the expression and localization of TPX2. Strikingly, TPX2 deficiency caused increased protein content of ADD1, but decreased expression and detachment of p-ADD1 from the spindle, thereby arresting mouse oocytes at the metaphase I stage with collapsed spindles.ConclusionPhosphorylation of ADD1 at S726 by TPX2 mediates acentriolar spindle assembly and precise chromosome segregation in mouse oocytes.

Rec8 Cohesin: A Structural Platform for Shaping the Meiotic Chromosomes.

Meiosis is critically different from mitosis in that during meiosis, pairing and segregation of homologous chromosomes occur. During meiosis, the morphology of sister chromatids changes drastically, forming a prominent axial structure in the synaptonemal complex. The meiosis-specific cohesin complex plays a central role in the regulation of the processes required for recombination. In particular, the Rec8 subunit of the meiotic cohesin complex, which is conserved in a wide range of eukaryotes, has been analyzed for its function in modulating chromosomal architecture during the pairing and recombination of homologous chromosomes in meiosis. Here, we review the current understanding of Rec8 cohesin as a structural platform for meiotic chromosomes.

The Zebrafish Meiotic Cohesin Complex Protein Smc1b Is Required for Key Events in Meiotic Prophase I.

The eukaryotic structural maintenance of chromosomes (SMC) proteins are involved in key processes of chromosome structure and dynamics. SMC1β was identified as a component of the meiotic cohesin complex in vertebrates, which aids in keeping sister chromatids together prior to segregation in meiosis II and is involved in association of homologous chromosomes in meiosis I. The role of SMC1β in meiosis has primarily been studied in mice, where mutant male and female mice are infertile due to germ cell arrest at pachytene and metaphase II stages, respectively. Here, we investigate the function of zebrafish Smc1b to understand the role of this protein more broadly in vertebrates. We found that zebrafish smc1b is necessary for fertility and has important roles in meiosis, yet has no other apparent roles in development. Therefore, smc1b functions primarily in meiosis in both fish and mammals. In zebrafish, we showed that smc1b mutant spermatocytes initiated telomere clustering in leptotene, but failed to complete this process and progress into zygotene. Furthermore, mutant spermatocytes displayed a complete failure of synapsis between homologous chromosomes and homolog pairing only occurred at chromosome ends. Interestingly, meiotic DNA double strand breaks occurred in the absence of Smc1b despite failed pairing and synapsis. Overall, our findings point to an essential role of Smc1b in the leptotene to zygotene transition during zebrafish spermatogenesis. In addition, ovarian follicles failed to form in smc1b mutants, suggesting an essential role in female meiosis as well. Our results indicate that there are some key differences in Smc1b requirement in meiosis among vertebrates: while Smc1b is not required for homolog pairing and synapsis in mice, it is essential for these processes in zebrafish.

Meiosis I Kinase Regulators: Conserved Orchestrators of Reductional Chromosome Segregation.

Research over the last two decades has identified a group of meiosis-specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring that sister kinetochores face towards the same pole (mono-orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk.

Prdm9 Intersubspecific Interactions in Hybrid Male Sterility of House Mouse.

The classical definition posits hybrid sterility as a phenomenon when two parental taxa each of which is fertile produce a hybrid that is sterile. The first hybrid sterility gene in vertebrates, Prdm9, coding for a histone methyltransferase, was identified in crosses between two laboratory mouse strains derived from Mus mus musculus and M. m. domesticus subspecies. The unique function of PRDM9 protein in the initiation of meiotic recombination led to the discovery of the basic molecular mechanism of hybrid sterility in laboratory crosses. However, the role of this protein as a component of reproductive barrier outside the laboratory model remained unclear. Here, we show that the Prdm9 allelic incompatibilities represent the primary cause of reduced fertility in intersubspecific hybrids between M. m. musculus and M. m. domesticus including 16 musculus and domesticus wild-derived strains. Disruption of fertility phenotypes correlated with the rate of failure of synapsis between homologous chromosomes in meiosis I and with early meiotic arrest. All phenotypes were restored to normal when the domesticus Prdm9dom2 allele was substituted with the Prdm9dom2H humanized variant. To conclude, our data show for the first time the male infertility of wild-derived musculus and domesticus subspecies F1 hybrids controlled by Prdm9 as the major hybrid sterility gene. The impairment of fertility surrogates, testes weight and sperm count, correlated with increasing difficulties of meiotic synapsis of homologous chromosomes and with meiotic arrest, which we suppose reflect the increasing asymmetry of PRDM9-dependent DNA double-strand breaks.

The Association of SYCP3 T657C Polymorphism with Early Pregnancy Loss in Russian Women

Background. Aneuploidy is a consequence of the chromosome nondisjunction. Majority of aneuploid embryos die in utero between the 6th and 10th week, resulting in early pregnancy loss of approximately 15% of all clinically recognized pregnancies. SYCP3 plays an important role in pairing and recombination of homologous chromosomes in meiosis I, and it has been shown that the lack of this gene leads to sterility in male and subfertility in female mice due to chromosome nondisjunction. So SYCP3 is a candidate gene to evaluate the hypothesis of fetal aneuploidy arisen from meiosis gene mutations as a cause for human early pregnancy loss. Methods. In our study, the SYCP3 T657C polymorphism in 100 Russian women with early pregnancy loss (EPL) that were classified into 2 subgroups: with sporadic pregnancy loss (SPL, n=50) and recurrent pregnancy loss (RPL, n=50), as well as 56 normal fertile women (control), was examined to determine whether there is an association between the polymorphism and early pregnancy loss in Russian population. Genomic DNA was extracted from peripheral blood samples using the standard procedure of a commercially available kit. Genotyping of SYCP3 gene was determined by allele-specific polymerase chain reaction method. Data were analyzed using SPSS statistical software version 22. The chi-square test and Fisher’s exact test were used to compare genotype and allele frequencies between analyzed groups. The odds ratio (OR) and 95% confidence intervals (CI) were used for risk estimation. Results. Frequency of the heterozygous genotype (CT) was significantly higher in the group of women with early pregnancy loss as well as in women with sporadic pregnancy loss (p<0.05). In women with RPL, we found that the frequency of this genotype tended to increase but could not make a significant difference when compared with the control group. Conclusion. Our findings postulate that the T657C polymorphism of the SYCP3 gene is possibly associated with a sporadic early pregnancy loss. Актуальность. Анеуплоидия возникает вследствие нарушения расхождения хромосом, и большинство анеуплоидных эмбрионов погибают до 10 недели гестации, что приводит к потере на ранних сроках приблизительно 15% клинически диагностированных беременностей. Белок SYCP3 играет важную роль в конъюгации и рекомбинации гомологичных хромосом в первом мейотическом делении. Было показано, что отсутствие гена данного белка у мышей приводит к стерильности самцов и снижению фертильности самок вследствие нарушения расхождения хромосом. В этой связи SYCP3 можно рассматривать в качестве кандидатного гена для оценки гипотезы о фетальной анеуплоидии, возникающей в результате мутаций генов, продукты которых участвуют в мейозе, и ведущей к ранним репродуктивным потерям. Методы. В представленной работе был исследован полиморфизм Т657С гена SYCP3 у 100 женщин русской национальности с ранними репродуктивными потерями, подразделенных на две подгруппы: со спорадической потерей беременности (n=50) и привычным невынашиванием (n=50), а также у 56 фертильных женщин (контроль). Целью работы явилось изучение наличия ассоциации между данным полиморфизмом и ранними репродуктивными потерями в русской популяции. Геномная ДНК была выделена из периферической крови, генотипирование выполнялось с использованием аллель-специфической полимеразной цепной реакции. Для сравнения частот генов и генотипов в изучаемых группах применялись критерий Хи-квадрат и точный тест Фишера, для определения которых использовалось программное обеспечение SPSS Statistics, версия 22. Оно также было использовано для расчета отношения шансов (OR) и 95% доверительного интервала. Результаты. Частота гетерозиготного генотипа СТ была достоверно выше в группе женщин с ранними репродуктивными потерями и, в частности, в подгруппе со спорадической потерей беременности (р<0,05). Несмотря на тенденцию к увеличению частоты данного генотипа у женщин с привычным невынашиванием, статистически значимых отличий от контроля обнаружено не было. Заключение. Полученные результаты позволяют предполагать наличие ассоциации между полиморфизмом Т657С гена SYCP3 и спорадической потерей беременности на ранних сроках.

Chromosome-associated RNA\u2013protein complexes promote pairing of homologous chromosomes during meiosis in Schizosaccharomyces pombe

Pairing of homologous chromosomes in meiosis is essential for sexual reproduction. We have previously demonstrated that the fission yeast sme2 RNA, a meiosis-specific long noncoding RNA (lncRNA), accumulates at the sme2 chromosomal loci and mediates their robust pairing in meiosis. However, the mechanisms underlying lncRNA-mediated homologous pairing have remained elusive. In this study, we identify conserved RNA-binding proteins that are required for robust pairing of homologous chromosomes. These proteins accumulate mainly at the sme2 and two other chromosomal loci together with meiosis-specific lncRNAs transcribed from these loci. Remarkably, the chromosomal accumulation of these lncRNA–protein complexes is required for robust pairing. Moreover, the lncRNA–protein complexes exhibit phase separation properties, since 1,6-hexanediol treatment reversibly disassembled these complexes and disrupted the pairing of associated loci. We propose that lncRNA–protein complexes assembled at specific chromosomal loci mediate recognition and subsequent pairing of homologous chromosomes.

SPO16 binds SHOC1 to promote homologous recombination and crossing-over in meiotic prophase I.

Segregation of homologous chromosomes in meiosis I is tightly regulated by their physical links, or crossovers (COs), generated from DNA double-strand breaks (DSBs) through meiotic homologous recombination. In budding yeast, three ZMM (Zip1/2/3/4, Mer3, Msh4/5) proteins, Zip2, Zip4, and Spo16, form a "ZZS" complex, functioning to promote meiotic recombination via a DSB repair pathway. Here, we identified the mammalian ortholog of Spo16, termed SPO16, which interacts with the mammalian ortholog of Zip2 (SHOC1/MZIP2), and whose functions are evolutionarily conserved to promote the formation of COs. SPO16 localizes to the recombination nodules, as SHOC1 and TEX11 do. SPO16 is required for stabilization of SHOC1 and proper localization of other ZMM proteins. The DSBs formed in SPO16-deleted meiocytes were repaired without COs formation, although synapsis is less affected. Therefore, formation of SPO16-SHOC1 complex-associated recombination intermediates is a key step facilitating meiotic recombination that produces COs from yeast to mammals.

Versatility of multivalent orientation, inverted meiosis, and rescued fitness in holocentric chromosomal hybrids

Chromosomal rearrangements (e.g., fusions/fissions) have the potential to drive speciation. However, their accumulation in a population is generally viewed as unlikely, because chromosomal heterozygosity should lead to meiotic problems and aneuploid gametes. Canonical meiosis involves segregation of homologous chromosomes in meiosis I and sister chromatid segregation during meiosis II. In organisms with holocentric chromosomes, which are characterized by kinetic activity distributed along almost the entire chromosome length, this order may be inverted depending on their metaphase I orientation. Here we analyzed the evolutionary role of this intrinsic versatility of holocentric chromosomes, which is not available to monocentric ones, by studying F1 to F4 hybrids between two chromosomal races of the Wood White butterfly (Leptidea sinapis), separated by at least 24 chromosomal fusions/fissions. We found that these chromosomal rearrangements resulted in multiple meiotic multivalents, and, contrary to the theoretical prediction, the hybrids displayed relatively high reproductive fitness (42% of that of the control lines) and regular behavior of meiotic chromosomes. In the hybrids, we also discovered inverted meiosis, in which the first and critical stage of chromosome number reduction was replaced by the less risky stage of sister chromatid separation. We hypothesize that the ability to invert the order of the main meiotic events facilitates proper chromosome segregation and hence rescues fertility and viability in chromosomal hybrids, potentially promoting dynamic karyotype evolution and chromosomal speciation.

Sexual dimorphism in the width of the mouse synaptonemal complex.

Sexual dimorphism has been used to describe morphological differences between the sexes, but can be extended to any biologically related process that varies between males and females. The synaptonemal complex (SC) is a tripartite structure that connects homologous chromosomes in meiosis. Here, aided by super-resolution microscopy techniques, we show that the SC is subject to sexual dimorphism, in mouse germ cells. We have identified a significantly narrower SC in oocytes and have established that this difference does not arise from a different organization of the lateral elements nor from a different isoform of transverse filament protein SYCP1. Instead, we provide evidence for the existence of a narrower central element and a different integration site for the C-termini of SYCP1, in females. In addition to these female-specific features, we speculate that post-translation modifications affecting the SYCP1 coiled-coil region could render a more compact conformation, thus contributing to the narrower SC observed in females.

Analysis of Meiotic Double-Strand Break Initiation in Mammals.

The repair of programmed DNA double-strand breaks (DSBs) physically tethers homologous chromosomes in meiosis to allow for accurate segregation through meiotic cell divisions. This process, known as recombination, also results in the exchange of alleles between parental chromosomes and contributes to genetic diversity. In mammals, meiotic DSBs occur predominantly in a small fraction of the genome, at sites known as hotspots. Studies of the formation and repair of meiotic DSBs in mammals are challenging, because few cells undergo meiotic DSB formation at a given time. To better understand the initiation and control of meiotic recombination in mammals, we have devised a highly sensitive method to map the sites of meiotic DSBs genome wide. Our method first isolates DNA bound to DSB repair proteins and then specifically sequences the associated single-stranded DNA. This protocol has generated the first meiotic DSB maps in several mammals and the only map of meiotic DSBs in humans.

Chromosomal Studies of the Hybrid between Female <i>Rhodeus ocellatus ocellatus</i> and Male <i>Rhodeus atremius fangi</i> in Bitterlings (Teleostei: Cypriniformes: Acheilognathinae)

The chromosome analysis of the masculinized hybrid between female Rhodeus ocellatus ocellatus and male R. atremius fangi in bitterlings (Acheilognathinae) was done. It was presumed that they had intermediate karyotype between the parents, and formed sperms with heteroploidy resulting from the incomplete pairing of homologous chromosomes in meiosis. Due to the abundance of species and the ease of artificial fertilization, the study of the factor of the hybrid sterility in bitterlings would lead to the clarification of the mechanism about species differentiation and karyotype differentiation, and also develop a new variety. And also, it would also be important to make the hybrid various natures clear in environmental preservation.

The proteasome enters the meiotic prophase fray.

The segregation of homologous chromosomes in meiosis depends on their ability to locate one another in the nucleus and establish a physical association through crossing over. A tightly regulated number of crossovers (COs) emerges following repair of induced DNA double-strand breaks by homologous recombination (HR), but the process of how HR intermediates transition into COs is still poorly understood. Two recent studies by Ahuja et al. and Rao et al. have revealed a role for chromosomally localized proteasomes in choreographing both homologous chromosome pairing and the evolution of HR intermediates into segregation-competent COs. Using chemical inhibition of the proteasome and mutant analysis, the collective data reveal conserved functions for both the proteasome and a family of E3 ligases that can direct or compete with its activity in ensuring CO formation. Here, we review these findings and the impact of the discovery that protein modification dynamics and proteasomal activity cooperate to regulate key meiotic processes.

Reappearance from Obscurity: Mammalian Rad52 in Homologous Recombination.

Homologous recombination (HR) plays an important role in maintaining genomic integrity. It is responsible for repair of the most harmful DNA lesions, DNA double-strand breaks and inter-strand DNA cross-links. HR function is also essential for proper segregation of homologous chromosomes in meiosis, maintenance of telomeres, and resolving stalled replication forks. Defects in HR often lead to genetic diseases and cancer. Rad52 is one of the key HR proteins, which is evolutionarily conserved from yeast to humans. In yeast, Rad52 is important for most HR events; Rad52 mutations disrupt repair of DNA double-strand breaks and targeted DNA integration. Surprisingly, in mammals, Rad52 knockouts showed no significant DNA repair or recombination phenotype. However, recent work demonstrated that mutations in human RAD52 are synthetically lethal with mutations in several other HR proteins including BRCA1 and BRCA2. These new findings indicate an important backup role for Rad52, which complements the main HR mechanism in mammals. In this review, we focus on the Rad52 activities and functions in HR and the possibility of using human RAD52 as therapeutic target in BRCA1 and BRCA2-deficient familial breast cancer and ovarian cancer.

Chromosomal Studies of Masculinized Hybrids in Bitterlings (Teleostei: Cypriniformes: Acheilognathinae)

The chromosome analysis of the masculinized hybrid between female Tanakia limbata and male T. signifer in bitterlings (Acheilognathinae) was done. It was presumed that they had intermediate karyotype between the parents, and formed sperms with heteroploidy resulting from the incomplete pairing of homologous chromosomes in meiosis. Due to the abundance of species and the ease of artificial fertilization, the study of the factor of the hybrid sterility in bitterlings would lead to the clarification of the mechanism about species differentiation and karyotype differentiation, and also to developing a new variety.

The T657C polymorphism on the SYCP3 gene is associated with recurrent pregnancy loss.

SYCP3 (Sinaptonemal complex protein 3) plays a critical role in pairing and recombination of homologous chromosomes in meiosis 1. It has been shown that lack of this gene leads to infertility in male and weakened fertility in female mice. In a case-control study, we investigated the SYCP3T657C polymorphism in the genome of 100 Iranian women with recurrent pregnancy losses of unknown causes as well as 100 control samples of normal fertile women having at least one healthy child. The general aim of our study was to determine whether there is a relationship between genetic changes in the SYCP3 gene and recurrent pregnancy loss in human or not. Frequency of the heterozygous genotype and mutated allele C were significantly higher in women with recurrent pregnancy losses (P-value < 0.005). Our findings suggest that the T657C polymorphism of the SYCP3 gene is possibly associated with recurrent pregnancy loss of unknown cause in human.

BILATERAL SUBCLAVIAN STENOSIS AS A CAUSE OF UNEXPLAINED HYPOTENSION

The centromere is the DNA region of the eukaryotic chromosome that determines kinetochore formation and sister chromatid cohesion. Centromeres interact with spindle microtubules to ensure the segregation of chromatids during mitosis and of homologous chromosomes in meiosis. The origin of centromeres, therefore, is inseparable from the evolution of cytoskeletal components that distribute chromosomes to offspring cells. Although the origin of the nucleus has been debated, no explanation for the evolutionary appearance of centromeres is available. We propose an evolutionary scenario: The centromeres originated from telomeres. The breakage of the ancestral circular genophore activated the transposition of retroelements at DNA ends that allowed the formation of telomeres by a recombination-dependent replication mechanism. Afterward, the modification of the tubulin-based cytoskeleton that allowed specific subtelomeric repeats to be recognized as new cargo gave rise to the first centromere. This switch from actin-based genophore partition to a tubulin-based mechanism generated a transition period during which both types of cytoskeleton contributed to fidelity of chromosome segregation. During the transition, pseudodicentric chromosomes increased the tendency toward chromosomal breakage and instability. This instability generated multiple telocentric chromosomes that eventually evolved into metacentric or holocentric chromosomes.

Centromere Pairing in Early Meiotic Prophase Requires Active Centromeres and Precedes Installation of the Synaptonemal Complex in Maize

Pairing of homologous chromosomes in meiosis is critical for their segregation to daughter cells. In most eukaryotes, clustering of telomeres precedes and facilitates chromosome pairing. In several species, centromeres also form pairwise associations, known as coupling, before the onset of pairing. We found that, in maize (Zea mays), centromere association begins at the leptotene stage and occurs earlier than the formation of the telomere bouquet. We established that centromere pairing requires centromere activity and the sole presence of centromeric repeats is not sufficient for pairing. In several species, homologs of the ZIP1 protein, which forms the central element of the synaptonemal complex in budding yeast (Saccharomyces cerevisiae), play essential roles in centromere coupling. However, we found that the maize ZIP1 homolog ZYP1 installs in the centromeric regions of chromosomes after centromeres form associations. Instead, we found that maize structural maintenance of chromosomes6 homolog forms a central element of the synaptonemal complex, which is required for centromere associations. These data shed light on the poorly understood mechanism of centromere interactions and suggest that this mechanism may vary somewhat in different species.

Meiotic Recombination in Arabidopsis Is Catalysed by DMC1, with RAD51 Playing a Supporting Role

Recombination establishes the chiasmata that physically link pairs of homologous chromosomes in meiosis, ensuring their balanced segregation at the first meiotic division and generating genetic variation. The visible manifestation of genetic crossing-overs, chiasmata are the result of an intricate and tightly regulated process involving induction of DNA double-strand breaks and their repair through invasion of a homologous template DNA duplex, catalysed by RAD51 and DMC1 in most eukaryotes. We describe here a RAD51-GFP fusion protein that retains the ability to assemble at DNA breaks but has lost its DNA break repair capacity. This protein fully complements the meiotic chromosomal fragmentation and sterility of Arabidopsis rad51, but not rad51 dmc1 mutants. Even though DMC1 is the only active meiotic strand transfer protein in the absence of RAD51 catalytic activity, no effect on genetic map distance was observed in complemented rad51 plants. The presence of inactive RAD51 nucleofilaments is thus able to fully support meiotic DSB repair and normal levels of crossing-over by DMC1. Our data demonstrate that RAD51 plays a supporting role for DMC1 in meiotic recombination in the flowering plant, Arabidopsis.