Homologous Centromeres Research Articles

Paradox lost: Concerted evolution and centromeric instability: Centromeres are hospitable habitats for repeats that evolve adaptations for proliferation within the nucleus sometimes at organismal cost.: Centromeres are hospitable habitats for repeats that evolve adaptations for proliferation within the nucleus sometimes at organismal cost.

Homologous centromeres compete for segregation to the secondary oocyte nucleus at female meiosis I. Centromeric repeats also compete with each other to populate centromeres in mitotic cells of the germline and have become adapted to use the recombinational machinery present at centromeres to promote their own propagation. Repeats are not needed at centromeres, rather centromeres appear to be hospitable habitats for the colonization and proliferation of repeats. This is probably an indirect consequence of two distinctive features of centromeric DNA. Centromeres are subject to breakage by the mechanical forces exerted by microtubules and meiotic crossing-over is suppressed. Centromeric proteins acting in trans are under selection to mitigate the costs of centromeric repeats acting in cis. Collateral costs of mitotic competition at centromeres may help to explain the high rates of aneuploidy observed in early human embryos.

Genome architecture and tetrasomic inheritance of autotetraploid potato

Potato (Solanum tuberosum) is the most consumed non-cereal food crop. Most commercial potato cultivars are autotetraploids with highly heterozygous genomes, severely hampering genetic analyses and improvement. By leveraging the state-of-the-art sequencing technologies and polyploid graph binning, we achieved a chromosome-scale, haplotype-resolved genome assembly of a cultivated potato, Cooperation-88 (C88). Intra-haplotype comparative analyses revealed extensive sequence and expression differences in this tetraploid genome. We identified haplotype-specific pericentromeres on chromosomes, suggesting a distinct evolutionary trajectory of potato homologous centromeres. Furthermore, we detected double reduction events that are unevenly distributed on haplotypes in 1021 of 1034 selfing progeny, a feature of autopolyploid inheritance. By distinguishing maternal and paternal haplotype sets in C88, we simulated the origin of heterosis in cultivated tetraploid with a survey of 3110 tetra-allelic loci with deleterious mutations, which were masked in the heterozygous condition by two parents. This study provides insights into the genomic architecture of autopolyploids and will guide their breeding.

Chiasmata and the kinetochore component Dam1 are crucial for elimination of erroneous chromosome attachments and centromere oscillation at meiosis I.

Establishment of proper chromosome attachments to the spindle requires elimination of erroneous attachments, but the mechanism of this process is not fully understood. During meiosis I, sister chromatids attach to the same spindle pole (mono-oriented attachment), whereas homologous chromosomes attach to opposite poles (bi-oriented attachment), resulting in homologous chromosome segregation. Here, we show that chiasmata that link homologous chromosomes and kinetochore component Dam1 are crucial for elimination of erroneous attachments and oscillation of centromeres between the spindle poles at meiosis I in fission yeast. In chiasma-forming cells, Mad2 and Aurora B kinase, which provides time for attachment correction and destabilizes erroneous attachments, respectively, caused elimination of bi-oriented attachments of sister chromatids, whereas in chiasma-lacking cells, they caused elimination of mono-oriented attachments. In chiasma-forming cells, in addition, homologous centromere oscillation was coordinated. Furthermore, Dam1 contributed to attachment elimination in both chiasma-forming and chiasma-lacking cells, and drove centromere oscillation. These results demonstrate that chiasmata alter attachment correction patterns by enabling error correction factors to eliminate bi-oriented attachment of sister chromatids, and suggest that Dam1 induces elimination of erroneous attachments. The coincidental contribution of chiasmata and Dam1 to centromere oscillation also suggests a potential link between centromere oscillation and attachment elimination.

Spatial inter-centromeric interactions facilitated the emergence of evolutionary new centromeres.

Centromeres of Candida albicans form on unique and different DNA sequences but a closely related species, Candida tropicalis, possesses homogenized inverted repeat (HIR)-associated centromeres. To investigate the mechanism of centromere type transition, we improved the fragmented genome assembly and constructed a chromosome-level genome assembly of C. tropicalis by employing PacBio sequencing, chromosome conformation capture sequencing (3C-seq), chromoblot, and genetic analysis of engineered aneuploid strains. Further, we analyzed the 3D genome organization using 3C-seq data, which revealed spatial proximity among the centromeres as well as telomeres of seven chromosomes in C. tropicalis. Intriguingly, we observed evidence of inter-centromeric translocations in the common ancestor of C. albicans and C. tropicalis. Identification of putative centromeres in closely related Candida sojae, Candida viswanathii and Candida parapsilosis indicates loss of ancestral HIR-associated centromeres and establishment of evolutionary new centromeres (ENCs) in C. albicans. We propose that spatial proximity of the homologous centromere DNA sequences facilitated karyotype rearrangements and centromere type transitions in human pathogenic yeasts of the CUG-Ser1 clade.

The Cohesin Complex Subunit ZmSMC3 Participates in Meiotic Centromere Pairing in Maize.

Meiosis consists of two highly conserved nuclear divisions, which allow eukaryotes to maintain their chromosome number through sexual reproduction. The successful completion of meiosis depends on homologous chromosome pairing. Centromere interactions during early meiotic prophase I facilitate homologous chromosome pairing, but the underlying mechanism is unclear. Here, we performed chromatin immunoprecipitation-mass spectrometry analysis of maize (Zea mays) anthers during early meiotic prophase I using anti-centromeric histone H3 (CENH3) antibodies and determined that the cohesin subunit Structural Maintenance of Chromosome3 (SMC3) interacts with CENH3 during this period. SMC3 is enriched at centromeres and along chromosome arms in threads from leptotene to pachytene and might promote interactions between homologous centromeres. We observed dysfunctional SMC3 assembly in meiotic-specific maize mutants with defective centromere pairing. In SMC3 RNAi meiocytes, centromere pairing defects were observed during early meiotic prophase I, SMC3 was weakly associated with centromeres, and SMC3 did not localize to the chromosome arms. In wild-type mitosis, SMC3 is associated with chromatin and is enriched at centromeres from prophase to anaphase. CRISPR-Cas9-induced Zmsmc3 mutants showed premature loss of sister chromatid cohesion and mis-segregation of chromosomes in mitotic spreads. Our findings suggest that in addition to sister chromatid cohesion, ZmSMC3 participates in meiotic centromere pairing.

Distinct, Ethnic, Clinical, and Genetic Characteristics of Myelodysplastic Syndromes with Der(1;7)

der(1;7)(q10;p10) is a recurrent chromosomal abnormality found in a wide variety of myeloid neoplasms observed in as high as 6% of myelodysplastic syndromes (MDS) in Asian populations, while rarely observed in Caucasian populations. It is thought to be generated by a recombination between two highly homologous centromere alphoid sequences which lead to an unbalanced abnormality of monosomy of 7q and trisomy of 1q. However, despite the presence of -7q, der(1;7) has been associated with a better prognosis compared to monosomy 7 or other del(7q) (-7/del(7q)). In addition to its association with +8 and del(20q), frequent RUNX1 mutations and a paucity of mutated TP53 have been reported in der(1;7) tumors, but otherwise, the molecular features of this abnormality have been poorly characterized in the literature. This is most likely because it is very rare in Caucasians, even though it represents one of the most prevalent lesions among Asian populations. The purpose of our study is to clarify the frequency and mutational landscape of der(1;7) in myeloid neoplasms on the basis of targeted-capture sequencing. A total of 1,707 MDS cases, including 944 German and 763 Japanese cases, were enrolled, from which we identified 73 (4.0%) cases with der(1;7). The prevalence was >20 times higher in Japanese (9.0%) than German (0.43%) cohorts (p<0.0001). We also identified a strong male predominance in der(1;7)-positive cases (90.4%) compared to negative cases. Also including an additional 22 cases, somatic mutations and copy number abnormalities in der(1;7) were interrogated in a total of 95 cases, which included 84 (88.4%) with MDS, 9 (9.5%) with AML, and 2 (2.1%) with MPN. Among MDS patients, 29 were low-risk, 47 were high-risk, and the rest were not specified. In mutation analysis, at least one mutation was detected in 98% of der(1;7) cases, most frequently affecting RUNX1 (42%), followed by EZH2 (26%), and ETNK1 (25%). Copy number analysis showed a high frequency of del(20q) and trisomy 8 in der(1;7) cases: 27.4% and 18.9% respectively. On the basis of mutant cell fractions, most of these mutations were present in subclones acquired within the major population harboring der(1;7). In particular, most of the EZH2 (7q35-q36) mutations were thought to be secondary events in der(1;7)-positive cases, while representing initial events acquired before UPD(7q) or -7/del(7q) in der(1;7)-negative cases. Of interest, der(1;7) was associated with a low frequency of TP53 mutations, which were seen only in 3% of cases with der(1;7), whereas highly prevalent in non-der(1;7) cases with -7/del(7q) (52%), which is concordant with a better clinical outcome was observed in der(1;7) cases compared with non-der(1;7) cases with monosomy 7 or other del(7q). Another unique feature of der(1;7) positive MDS was an extremely high frequency of RUNX1 mutations. However, the most prominent finding with secondary mutations in der(1;7) cases is the frequent hot spot mutation in ETNK1, which were originally reported in 8.8% of myeloid neoplasms with MPN features, like SETBP1 mutations. ENTK1 mutations were found in as many as 25% (23/95) of der(1;7) cases, while rarely seen in -7/del(7q) (1/89) (p<0.0001) or amp(1q) (2/68) (p=0.0001). Despite the high frequency of trisomy 8 observed in der(1;7) cases, none were associated with ETNK1 mutations. In addition, all of the RAS pathway mutations (positive in 16 cases) were observed in der(1;7) cases with wild-type ETNK1, while none were in ETNK1-mutant cases. Morphologically, these ETNK1-mutated der(1;7) cases presented with an increased eosinophil count in peripheral blood (760.9/ul vs. 78.1/ul) (p<0.001), compared to those without EKNK1 mutations, suggesting that ENTK1-mutated der(1;7) cases represent a novel disease entity within der(1;7), characterized by unique genetic features and increased eosinophils. In conclusion, der(1;7) is a genetically and clinically distinct subset of myeloid neoplasms, which showed unique features that are distinct from MDS cases in -7 and other del(7q). Especially, ETNK1 mutations subdivided cases with der(1;7) into two groups of genetically distinct subsets as shown in Figure 1. In the future, inhibition of the kinase activity in ETNK1 could be a novel therapeutic strategy in such a previously unrecognized subset as characterized by der(1;7) and eosinophilia. Figure 1 Disclosures Kern: MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Baer:MLL Munich Leukemia Laboratory: Employment. Nadarajah:MLL Munich Leukemia Laboratory: Employment. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Atsuta:Janssen Paharmaceutical K.K.: Honoraria; Mochida Pharmaceutical Co. Ltd: Honoraria; Kyowa Kirin Co., Ltd: Honoraria; Chugai Pharmaceutical Co., Ltd.: Honoraria. Handa:Ono: Research Funding. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Ogawa:Qiagen Corporation: Patents & Royalties; Kan Research Laboratory, Inc.: Consultancy; ChordiaTherapeutics, Inc.: Consultancy, Equity Ownership; Dainippon-Sumitomo Pharmaceutical, Inc.: Research Funding; Asahi Genomics: Equity Ownership; RegCell Corporation: Equity Ownership.

Plasticity in Triticeae centromere DNA sequences: a wheat × tall wheatgrass (decaploid) model.

Centromeres mediate chromosome attachment to microtubules and maintain the integrity of chromosomes for proper segregation of the sister chromatids during cell division. Advances in the assembly of Triticeae genome sequences combined with the capacity to recover hybrid species derived from very distantly related species provides potential experimental systems for linking retrotransposon amplification and repositioning of centromeres via non-mendelian inheritance in partial amphiploid breeds. The decaploid tall wheatgrass (Thinopyrum ponticum) is one of the most successfully used perennial species in wheat breeding for generating translocation lines with valuable agronomic traits. We found that wheat centromere retrotransposons CRW and Quinta widely occur within the tall wheatgrass genome. In addition, one of the genome donors to Th. ponticum, Pseudoroegneria stipifolia (StSt), has been shown to have Abigail and a satellite repeat, CentSt. We also found two other centromeric retrotransposons, Abia and CL135 in Th. ponticum by ChIP-seq. Examination of partial amphiploid lines that were generated in the 1970s demonstrated extensive modification in centromere sequences using CentSt, Abigail and Abia as probes. We also detected that St-genome chromosomes were more enriched with Abigail and CentSt, whereas E-genome chromosomes were enriched with CRW and Quinta in tall wheatgrass and its closer relatives. It can be concluded that bursts of transposition of retrotransposons and repositioning of centromeres via non-mendelian segregation are common in partial amphiploids derived from interspecific hybrids. Practically speaking, our study reveals that the existence of homologous centromere functional sequences in both a donor and its receptor can substantially contribute to the successful transfer of alien genes into crop species. OPEN RESEARCH BADGES: This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://www.ncbi.nlm.nih.gov/sra/SRR9089557; https://www.ncbi.nlm.nih.gov/sra/SRR9089558; https://www.ncbi.nlm.nih.gov/sra/SRR9089559; https://www.ncbi.nlm.nih.gov/sra/SRR9089560; https://www.ncbi.nlm.nih.gov/sra/SRR9089561; https://www.ncbi.nlm.nih.gov/sra/SRR9089562; https://www.ncbi.nlm.nih.gov/sra/SRR9089563; https://www.ncbi.nlm.nih.gov/sra/SRR9089564; https://www.ncbi.nlm.nih.gov/nuccore/MK999394; https://www.ncbi.nlm.nih.gov/nuccore/MK999395; https://www.ncbi.nlm.nih.gov/nuccore/MK999396.

The pericentromeric heterochromatin of homologous chromosomes remains associated after centromere pairing dissolves in mouse spermatocyte meiosis

In meiosis, crossovers between homologous chromosomes link them together. This enables them to attach to microtubules of the meiotic spindle as a unit, such that the homologs will be pulled away from one another at anaphase I. Homologous pairs can sometimes fail to become linked by crossovers. In some organisms, these non-exchange partners are still able to segregate properly. In several organisms, associations between the centromeres of non-exchange partners occur in meiotic prophase. These associations have been proposed to promote segregation in meiosis I. But it is unclear how centromere pairing could promote subsequent proper segregation. Here we report that meiotic centromere pairing of chromosomes in mouse spermatocytes allows the formation of an association between chromosome pairs. We find that heterochromatin regions of homologous centromeres remain associated even after centromere-pairing dissolves. Our results suggest the model that, in mouse spermatocytes, heterochromatin maintains the association of homologous centromeres in the absence crossing-over.

HO Endonuclease-Initiated Recombination in Yeast Meiosis Fails To Promote Homologous Centromere Pairing and Is Not Constrained To Utilize the Dmc1 Recombinase.

Crossover recombination during meiosis is accompanied by a dramatic chromosome reorganization. In Saccharomyces cerevisiae, the onset of meiotic recombination by the Spo11 transesterase leads to stable pairwise associations between previously unassociated homologous centromeres followed by the intimate alignment of homologous axes via synaptonemal complex (SC) assembly. However, the molecular relationship between recombination and global meiotic chromosome reorganization remains poorly understood. In budding yeast, one question is why SC assembly initiates earliest at centromere regions while the DNA double strand breaks (DSBs) that initiate recombination occur genome-wide. We targeted the site-specific HO endonuclease to various positions on S. cerevisiae’s longest chromosome in order to ask whether a meiotic DSB’s proximity to the centromere influences its capacity to promote homologous centromere pairing and SC assembly. We show that repair of an HO-mediated DSB does not promote homologous centromere pairing nor any extent of SC assembly in spo11 meiotic nuclei, regardless of its proximity to the centromere. DSBs induced en masse by phleomycin exposure likewise do not promote homologous centromere pairing nor robust SC assembly. Interestingly, in contrast to Spo11, HO-initiated interhomolog recombination is not affected by loss of the meiotic kinase, Mek1, and is not constrained to use the meiosis-specific Dmc1 recombinase. These results strengthen the previously proposed idea that (at least some) Spo11 DSBs may be specialized in activating mechanisms that both 1) reinforce homologous chromosome alignment via homologous centromere pairing and SC assembly, and 2) establish Dmc1 as the primary strand exchange enzyme.

A ZIP1 separation-of-function allele reveals that centromere pairing drives meiotic segregation of achiasmate chromosomes in budding yeast.

In meiosis I, homologous chromosomes segregate away from each other–the first of two rounds of chromosome segregation that allow the formation of haploid gametes. In prophase I, homologous partners become joined along their length by the synaptonemal complex (SC) and crossovers form between the homologs to generate links called chiasmata. The chiasmata allow the homologs to act as a single unit, called a bivalent, as the chromosomes attach to the microtubules that will ultimately pull them away from each other at anaphase I. Recent studies, in several organisms, have shown that when the SC disassembles at the end of prophase, residual SC proteins remain at the homologous centromeres providing an additional link between the homologs. In budding yeast, this centromere pairing is correlated with improved segregation of the paired partners in anaphase. However, the causal relationship of prophase centromere pairing and subsequent disjunction in anaphase has been difficult to demonstrate as has been the relationship between SC assembly and the assembly of the centromere pairing apparatus. Here, a series of in-frame deletion mutants of the SC component Zip1 were used to address these questions. The identification of a separation-of-function allele that disrupts centromere pairing, but not SC assembly, has made it possible to demonstrate that centromere pairing and SC assembly have mechanistically distinct features and that the centromere pairing function of Zip1 drives disjunction of the paired partners in anaphase I.

Kdm5/Lid Regulates Chromosome Architecture in Meiotic Prophase I Independently of Its Histone Demethylase Activity.

During prophase of the first meiotic division (prophase I), chromatin dynamically reorganises to recombine and prepare for chromosome segregation. Histone modifying enzymes are major regulators of chromatin structure, but our knowledge of their roles in prophase I is still limited. Here we report on crucial roles of Kdm5/Lid, one of two histone demethylases in Drosophila that remove one of the trimethyl groups at Lys4 of Histone 3 (H3K4me3). In the absence of Kdm5/Lid, the synaptonemal complex was only partially formed and failed to be maintained along chromosome arms, while localisation of its components at centromeres was unaffected. Kdm5/Lid was also required for karyosome formation and homologous centromere pairing in prophase I. Although loss of Kdm5/Lid dramatically increased the level of H3K4me3 in oocytes, catalytically inactive Kdm5/Lid can rescue the above cytological defects. Therefore Kdm5/Lid controls chromatin architecture in meiotic prophase I oocytes independently of its demethylase activity.

Crosses Heterozygous for Hybrid Neurospora Translocation Strains Show Transmission Ratio Distortion Disfavoring Homokaryotic Ascospores Made Following Alternate Segregation.

By introgressing Neurospora crassa translocations into N. tetrasperma, we constructed heterokaryons bearing haploid nuclei of opposite mating types, and either the translocation and normal sequence chromosomes (i.e., [T + N]) or a duplication and its complementary deficiency (i.e., [Dp + Df]). The [T + N] heterokaryons result from alternate segregation of homologous centromeres, whereas adjacent-1 segregation generates [Dp + Df]. Self-cross of either heterokaryon produces [T + N] and [Dp + Df] progeny. Occasionally during N. tetrasperma ascus development, a pair of smaller homokaryotic ascospores replaces a heterokaryotic ascospore. Crosses with the Eight-spore mutant increase such replacement, and can generate asci with eight homokaryotic ascospores, either 4T + 4N from alternate segregation, or 4Dp + 4Df from adjacent-1 segregation. Crosses of some of the introgressed translocation strains with normal sequence N. tetrasperma produced more Dp than T or N homokaryotic progeny. We suggest this is due to an insufficiency for a presumptive ascospore maturation factor, which increases the chance that, in asci with > 4 viable ascospores, none properly mature. Since only four viable ascospores (Dp or [Dp + Df]) share the limiting factor following adjacent-1 segregation, whereas four to eight ascospores compete for it following alternate segregation, this would explain why Dp homokaryons outnumber T and N types, whereas the heterokaryons are not as affected. We believe that this novel form of transmission ratio distortion is caused by a Bateson–Dobzhansky–Muller Incompatibility (BDMI) triggered by an N. crassa gene in the N. tetrasperma background. Heterokaryons tend not to out-cross, and crosses of Dp strains are barren, thus the BDMI impedes interspecies gene flow.

The microtubule catastrophe promoter Sentin delays stable kinetochore-microtubule attachment in oocytes.

The critical step in meiosis is to attach homologous chromosomes to the opposite poles. In mouse oocytes, stable microtubule end-on attachments to kinetochores are not established until hours after spindle assembly, and phosphorylation of kinetochore proteins by Aurora B/C is responsible for the delay. Here we demonstrated that microtubule ends are actively prevented from stable attachment to kinetochores until well after spindle formation in Drosophila melanogaster oocytes. We identified the microtubule catastrophe-promoting complex Sentin-EB1 as a major factor responsible for this delay. Without this activity, microtubule ends precociously form robust attachments to kinetochores in oocytes, leading to a high proportion of homologous kinetochores stably attached to the same pole. Therefore, regulation of microtubule ends provides an alternative novel mechanism to delay stable kinetochore-microtubule attachment in oocytes.

Couples, pairs, and clusters: mechanisms and implications of centromere associations in meiosis

Observations of a wide range of organisms show that the centromeres form associations of pairs or small groups at different stages of meiotic prophase. Little is known about the functions or mechanisms of these associations, but in many cases, synaptonemal complex elements seem to play a fundamental role. Two main associations are observed: homology-independent associations very early in the meiotic program-sometimes referred to as centromere coupling-and a later association of homologous centromeres, referred to as centromere pairing or tethering. The later centromere pairing initiates during synaptonemal complex assembly, then persists after the dissolution of the synaptonemal complex. While the function of the homology-independent centromere coupling remains a mystery, centromere pairing appears to have a direct impact on the chromosome segregation fidelity of achiasmatic chromosomes. Recent work in yeast, Drosophila, and mice suggest that centromere pairing is a previously unappreciated, general meiotic feature that may promote meiotic segregation fidelity of the exchange and non-exchange chromosomes.

BRK1, a Bub1-related kinase, is essential for generating proper tension between homologous kinetochores at metaphase I of rice meiosis.

Bub1 (for budding uninhibited by benzimidazole 1), one of the main spindle checkpoint kinases, acts as a kinetochore scaffold for assembling other checkpoint proteins. Here, we identify a plant Bub1-related kinase 1 (BRK1) in rice (Oryza sativa). The brk1 mutants are sterile due to the precocious separation of sister chromatids at the onset of anaphase I. The centromeric recruitment of SHUGOSHIN1 and phosphorylation of histone H2A at Thr-134 (H2A-pT134) depend on BRK1. Although the homologs can faithfully separate from each other at the end of meiosis I, the uncorrected merotelic attachment of paired sister kinetochores at the early stage of metaphase I in brk1 reduces the tension across homologous kinetochores, causes the metaphase I spindle to be aberrantly shaped, and subsequently affects the synchronicity of homolog separation at the onset of anaphase I. In addition, the phosphorylation of inner centromeric histone H3 at Ser-10 (H3-pS10) during diakinesis depends on BRK1. Therefore, we speculate that BRK1 may be required for normal localization of Aurora kinase before the onset of metaphase I, which is responsible for correcting the merotelic attachment.

Synaptonemal Complex Components Persist at Centromeres and Are Required for Homologous Centromere Pairing in Mouse Spermatocytes

Recent studies in simple model organisms have shown that centromere pairing is important for ensuring high-fidelity meiotic chromosome segregation. However, this process and the mechanisms regulating it in higher eukaryotes are unknown. Here we present the first detailed study of meiotic centromere pairing in mouse spermatogenesis and link it with key events of the G2/metaphase I transition. In mouse we observed no evidence of the persistent coupling of centromeres that has been observed in several model organisms. We do however find that telomeres associate in non-homologous pairs or small groups in B type spermatogonia and pre-leptotene spermatocytes, and this association is disrupted by deletion of the synaptonemal complex component SYCP3. Intriguingly, we found that, in mid prophase, chromosome synapsis is not initiated at centromeres, and centromeric regions are the last to pair in the zygotene-pachytene transition. In late prophase, we first identified the proteins that reside at paired centromeres. We found that components of the central and lateral element and transverse filaments of the synaptonemal complex are retained at paired centromeres after disassembly of the synaptonemal complex along diplotene chromosome arms. The absence of SYCP1 prevents centromere pairing in knockout mouse spermatocytes. The localization dynamics of SYCP1 and SYCP3 suggest that they play different roles in promoting homologous centromere pairing. SYCP1 remains only at paired centromeres coincident with the time at which some kinetochore proteins begin loading at centromeres, consistent with a role in assembly of meiosis-specific kinetochores. After removal of SYCP1 from centromeres, SYCP3 then accumulates at paired centromeres where it may promote bi-orientation of homologous centromeres. We propose that, in addition to their roles as synaptonemal complex components, SYCP1 and SYCP3 act at the centromeres to promote the establishment and/or maintenance of centromere pairing and, by doing so, improve the segregation fidelity of mammalian meiotic chromosomes.

Rumors of Its Disassembly Have Been Greatly Exaggerated: The Secret Life of the Synaptonemal Complex at the Centromeres

Rumors of Its Disassembly Have Been Greatly Exaggerated: The Secret Life of the Synaptonemal Complex at the Centromeres

Differing Requirements for RAD51 and DMC1 in Meiotic Pairing of Centromeres and Chromosome Arms in Arabidopsis thaliana

During meiosis homologous chromosomes pair, recombine, and synapse, thus ensuring accurate chromosome segregation and the halving of ploidy necessary for gametogenesis. The processes permitting a chromosome to pair only with its homologue are not fully understood, but successful pairing of homologous chromosomes is tightly linked to recombination. In Arabidopsis thaliana, meiotic prophase of rad51, xrcc3, and rad51C mutants appears normal up to the zygotene/pachytene stage, after which the genome fragments, leading to sterility. To better understand the relationship between recombination and chromosome pairing, we have analysed meiotic chromosome pairing in these and in dmc1 mutant lines. Our data show a differing requirement for these proteins in pairing of centromeric regions and chromosome arms. No homologous pairing of mid-arm or distal regions was observed in rad51, xrcc3, and rad51C mutants. However, homologous centromeres do pair in these mutants and we show that this does depend upon recombination, principally on DMC1. This centromere pairing extends well beyond the heterochromatic centromere region and, surprisingly, does not require XRCC3 and RAD51C. In addition to clarifying and bringing the roles of centromeres in meiotic synapsis to the fore, this analysis thus separates the roles in meiotic synapsis of DMC1 and RAD51 and the meiotic RAD51 paralogs, XRCC3 and RAD51C, with respect to different chromosome domains.

Synaptonemal Complex-Dependent Centromeric Clustering and the Initiation of Synapsis in Drosophila Oocytes

The pairing of homologous chromosomes and the intimate synapsis of the paired homologs by the synaptonemal complex (SC) are essential for subsequent meiotic processes including recombination and chromosome segregation. Here we show that the centromere clustering plays an important role in initiating homolog synapsis during meiosis in Drosophila females. Although centromeres are not clustered prior to the onset of meiosis, all four pairs of centromeres are actively clustered into one or two masses during early meiotic prophase. Within the 16-cell cyst, centromeric clustering appears to define the first step in the initiation of synapsis. Clustering is restricted to the nuclei that form the SC and is dependent on all known SC proteins. Surprisingly, both centromeric clusters and the SC components associated with them persist long after the disassembly of the euchromatic SC at the end of pachytene. The initiation of homologous recombination through the formation of programmed double-strand breaks (DSBs) is not required for either the formation or the maintenance of the centromeric clusters. Our data support a view in which the SC-mediated clustering at the centromeres is the initiating event for meiotic synapsis.

The compact Brachypodium genome conserves centromeric regions of a common ancestor with wheat and rice

The evolution of five chromosomes of Brachypodium distachyon from a 12-chromosome ancestor of all grasses by dysploidy raises an interesting question about the fate of redundant centromeres. Three independent but complementary approaches were pursued to study centromeric region homologies among the chromosomes of Brachypodium, wheat, and rice. The genes present in pericentromeres of the basic set of seven chromosomes of wheat and the Triticeae, and the 80 rice centromeric genes spanning the CENH3 binding domain of centromeres 3, 4, 5, 7, and 8 were used as "anchor" markers to identify centromere locations in the B. distachyon chromosomes. A total of 53 B. distachyon bacterial artificial chromosome (BAC) clones anchored by wheat pericentromeric expressed sequence tags (ESTs) were used as probes for BAC-fluorescence in situ hybridization (FISH) analysis of B. distachyon mitotic chromosomes. Integrated sequence alignment and BAC-FISH data were used to determine the approximate positions of active and inactive centromeres in the five B. distachyon chromosomes. The following syntenic relationships of the centromeres for Brachypodium (Bd), rice (R), and wheat (W) were evident: Bd1-R6, Bd2-R5-W1, Bd3-R10, Bd4-R11-W4, and Bd5-R4. Six rice centromeres syntenic to five wheat centromeres were inactive in Brachypodium chromosomes. The conservation of centromere gene synteny among several sets of homologous centromeres of three species indicates that active genes can persist in ancient centromeres with more than 40 million years of shared evolutionary history. Annotation of a BAC contig spanning an inactive centromere in chromosome Bd3 which is syntenic to rice Cen8 and W7 pericentromeres, along with BAC FISH data from inactive centromeres revealed that the centromere inactivation was accompanied by the loss of centromeric retrotransposons and turnover of centromere-specific satellites during Bd chromosome evolution.