Faithful meiotic segregation requires pairwise alignment of the homologous chromosomes and their synaptonemal complex (SC) mediated stabilization. Here, we investigate factors that promote and coordinate these events during C. elegans meiosis. We identify BRA-2 (BMP Receptor Associated family member 2) as an interactor of HIM-17, previously shown to promote double-strand break formation. We found that loss of bra-2 impairs synapsis elongation without affecting homolog recognition, chromosome movement or SC maintenance. Epistasis analyses reveal previously unrecognized activities for HIM-17 in regulating homolog pairing and SC assembly in a partially overlapping manner with BRA-2. We show that removing bra-2 or him-17 restores nuclear clustering, recruitment of PLK-2 at the nuclear periphery, and abrogation of ectopic synapsis in htp-1 mutants, suggesting intact CHK-2-mediated signaling and presence of a barrier that prevents SC polymerization in the absence of homology. Our findings shed light on the regulatory mechanisms ensuring faithful pairing and synapsis.

• MeSH
• Caenorhabditis elegans * genetics   metabolism   MeSH
• Checkpoint Kinase 2 MeSH
• DNA Breaks, Double-Stranded MeSH
• Meiosis * MeSH
• Mutation MeSH
• Chromosome Pairing * MeSH
• Polo-like Kinases MeSH
• Protein Serine-Threonine Kinases metabolism   genetics   MeSH
• Cell Cycle Proteins metabolism   genetics   MeSH
• Caenorhabditis elegans Proteins * metabolism   genetics   MeSH
• Chromosome Segregation MeSH
• Synaptonemal Complex * metabolism   genetics   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Checkpoint Kinase 2 MeSH
• CHK-2 protein, C elegans MeSH Browser
• Htp-1 protein, C elegans MeSH Browser
• polo-like kinase 2, C elegans MeSH Browser
• Polo-like Kinases MeSH
• Protein Serine-Threonine Kinases MeSH
• Cell Cycle Proteins MeSH
• Caenorhabditis elegans Proteins * MeSH

Replication stress, particularly in hard-to-replicate regions such as telomeres and centromeres, leads to the accumulation of replication intermediates that must be processed to ensure proper chromosome segregation. In this study, we identify a critical role for the interaction between RECQ4 and MUS81 in managing such stress. We show that RECQ4 physically interacts with MUS81, targeting it to specific DNA substrates and enhancing its endonuclease activity. Loss of this interaction, results in significant chromosomal segregation defects, including the accumulation of micronuclei, anaphase bridges, and ultrafine bridges (UFBs). Our data further demonstrate that the RECQ4-MUS81 interaction plays an important role in ALT-positive cells, where MUS81 foci primarily colocalise with telomeres, highlighting its role in telomere maintenance. We also observe that a mutation associated with Rothmund-Thomson syndrome, which produces a truncated RECQ4 unable to interact with MUS81, recapitulates these chromosome instability phenotypes. This underscores the importance of RECQ4-MUS81 in safeguarding genome integrity and suggests potential implications for human disease. Our findings demonstrate the RECQ4-MUS81 interaction as a key mechanism in alleviating replication stress at hard-to-replicate regions and highlight its relevance in pathological conditions such as RTS.

• MeSH
• Chromosomal Instability MeSH
• DNA-Binding Proteins * metabolism   genetics   MeSH
• Endonucleases * metabolism   genetics   MeSH
• RecQ Helicases * metabolism   genetics   MeSH
• Telomere Homeostasis MeSH
• Humans MeSH
• Mutation MeSH
• DNA Replication * MeSH
• Rothmund-Thomson Syndrome * genetics   metabolism   MeSH
• Chromosome Segregation MeSH
• Telomere * metabolism   genetics   MeSH
• Protein Binding MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• DNA-Binding Proteins * MeSH
• Endonucleases * MeSH
• RecQ Helicases * MeSH
• MUS81 protein, human MeSH Browser
• RECQL4 protein, human MeSH Browser

UNLABELLED: Transmission of genetic material from one generation to the next is a fundamental feature of all living cells. In eukaryotes, a macromolecular complex called the kinetochore plays crucial roles during chromosome segregation by linking chromosomes to spindle microtubules. Little is known about this process in evolutionarily diverse protists. Within the supergroup Discoba, Euglenozoa forms a speciose group of unicellular flagellates-kinetoplastids, euglenids, and diplonemids. Kinetoplastids have an unconventional kinetochore system, while euglenids have subunits that are conserved among most eukaryotes. For diplonemids, a group of extremely diverse and abundant marine flagellates, it remains unclear what kind of kinetochores are present. Here, we employed deep homology detection protocols using profile-versus-profile Hidden Markov Model searches and AlphaFold-based structural comparisons to detect homologies that might have been previously missed. Interestingly, we still could not detect orthologs for most of the kinetoplastid or canonical kinetochore subunits with few exceptions including a putative centromere-specific histone H3 variant (cenH3/CENP-A), the spindle checkpoint protein Mad2, the chromosomal passenger complex members Aurora and INCENP, and broadly conserved proteins like CLK kinase and the meiotic synaptonemal complex proteins SYCP2/3 that also function at kinetoplastid kinetochores. We examined the localization of five candidate kinetochore-associated proteins in the model diplonemid, Paradiplonema papillatum. PpCENP-A shows discrete dots in the nucleus, implying that it is likely a kinetochore component. PpMad2, PpCLKKKT10/19, PpSYCP2L1KKT17/18, and PpINCENP reside in the nucleus, but no clear kinetochore localization was observed. Altogether, these results point to the possibility that diplonemids evolved a hitherto unknown type of kinetochore system. IMPORTANCE: A macromolecular assembly called the kinetochore is essential for the segregation of genetic material during eukaryotic cell division. Therefore, characterization of kinetochores across species is essential for understanding the mechanisms involved in this key process across the eukaryotic tree of life. In particular, little is known about kinetochores in divergent protists such as Euglenozoa, a group of unicellular flagellates that includes kinetoplastids, euglenids, and diplonemids, the latter being a highly diverse and abundant component of marine plankton. While kinetoplastids have an unconventional kinetochore system and euglenids have a canonical one similar to traditional model eukaryotes, preliminary searches detected neither unconventional nor canonical kinetochore components in diplonemids. Here, we employed state-of-the-art deep homology detection protocols but still could not detect orthologs for the bulk of kinetoplastid-specific nor canonical kinetochore proteins in diplonemids except for a putative centromere-specific histone H3 variant. Our results suggest that diplonemids evolved kinetochores that do not resemble previously known ones.

• Keywords
• Diplonemea, Kinetoplastea, Paradiplonema, cell division, cenH3/CENP-A, kinetochore,
• MeSH
• Euglenozoa * genetics   metabolism   MeSH
• Phylogeny MeSH
• Kinetochores * metabolism   MeSH
• Protozoan Proteins metabolism   genetics   MeSH
• Chromosome Segregation MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Protozoan Proteins MeSH

Microtubules, built from heterodimers of α- and β-tubulins, control cell shape, mediate intracellular transport, and power cell division. The concentration of αβ-tubulins is tightly controlled through a posttranscriptional mechanism involving selective and regulated degradation of tubulin-encoding mRNAs. Degradation is initiated by TTC5, which recognizes tubulin-synthesizing ribosomes and recruits downstream effectors to trigger mRNA deadenylation. Here, we investigate how cells regulate TTC5 activity. Biochemical and structural proteomic approaches reveal that under normal conditions, soluble αβ-tubulins bind to and sequester TTC5, preventing it from engaging nascent tubulins at translating ribosomes. We identify the flexible C-terminal tail of TTC5 as a molecular switch, toggling between soluble αβ-tubulin-bound and nascent tubulin-bound states. Loss of sequestration by soluble αβ-tubulins constitutively activates TTC5, leading to diminished tubulin mRNA levels and compromised microtubule-dependent chromosome segregation during cell division. Our findings provide a paradigm for how cells regulate the activity of a specificity factor to adapt posttranscriptional regulation of gene expression to cellular needs.

• MeSH
• Humans MeSH
• RNA, Messenger * metabolism   genetics   MeSH
• Microtubules * metabolism   MeSH
• Microtubule-Associated Proteins metabolism   genetics   MeSH
• Ribosomes metabolism   MeSH
• Chromosome Segregation MeSH
• RNA Stability * MeSH
• Tubulin * metabolism   genetics   MeSH
• Protein Binding MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• RNA, Messenger * MeSH
• Microtubule-Associated Proteins MeSH
• Tubulin * MeSH

The genomes of many plants, animals, and fungi frequently comprise dispensable B chromosomes that rely upon various chromosomal drive mechanisms to counteract the tendency of non-essential genetic elements to be purged over time. The B chromosome of rye - a model system for nearly a century - undergoes targeted nondisjunction during first pollen mitosis, favouring segregation into the generative nucleus, thus increasing their numbers over generations. However, the genetic mechanisms underlying this process are poorly understood. Here, using a newly-assembled, ~430 Mb-long rye B chromosome pseudomolecule, we identify five candidate genes whose role as trans-acting moderators of the chromosomal drive is supported by karyotyping, chromosome drive analysis and comparative RNA-seq. Among them, we identify DCR28, coding a microtubule-associated protein related to cell division, and detect this gene also in the B chromosome of Aegilops speltoides. The DCR28 gene family is neo-functionalised and serially-duplicated with 15 B chromosome-located copies that are uniquely highly expressed in the first pollen mitosis of rye.

• MeSH
• Aegilops genetics   metabolism   MeSH
• Chromosomes, Plant * genetics   MeSH
• Karyotyping MeSH
• Mitosis * genetics   MeSH
• Nondisjunction, Genetic MeSH
• Pollen genetics   MeSH
• Gene Expression Regulation, Plant MeSH
• Genes, Plant MeSH
• Plant Proteins genetics   metabolism   MeSH
• Secale * genetics   MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Plant Proteins MeSH

Most eukaryotes maintain the stability of their cellular genome sizes to ensure genome transmission to offspring through sexual reproduction. However, some alter their genome size by selectively eliminating parts or increasing ploidy at specific developmental stages. This phenomenon of genome elimination or whole genome duplication occurs in animal hybrids reproducing asexually. Such genome alterations occur during gonocyte development ensuring successful reproduction of these hybrids. Although multiple examples of genome alterations are known, the underlying molecular and cellular processes involved in selective genome elimination and duplication remain largely unknown. Here, we uncovered the process of selective genome elimination and genome endoreplication in hemiclonal fish hybrids from the genus Hypseleotris. Specifically, we examined parental sexual species H. bucephala and hybrid H. bucephala × H. gymnocephala (HB × HX). We observed micronuclei in the cytoplasm of gonial cells in the gonads of hybrids, but not in the parental sexual species. We also observed misaligned chromosomes during mitosis which were unable to attach to the spindle. Moreover, we found that misaligned chromosomes lag during anaphase and subsequently enclose in the micronuclei. Using whole mount immunofluorescent staining, we showed that chromatid segregation has failed in lagging chromosomes. We also performed three-dimensional comparative genomic hybridization (3D-CGH) using species-specific probes to determine the role of micronuclei in selective genome elimination. We repeatedly observed that misaligned chromosomes of the H. bucephala genome were preferentially enclosed in micronuclei of hybrids. In addition, we detected mitotic cells without a mitotic spindle as a potential cause of genome duplication. We conclude that selective genome elimination in the gonads of hybrids occurs through gradual elimination of individual chromosomes of one parental genome. Such chromosomes, unable to attach to the spindle, lag and become enclosed in micronuclei.

• Keywords
• Asexual, Carp gudgeon, Gonocytes, Histone modification, Hybridogenesis, Micronucleus,
• MeSH
• Chromosomes genetics   MeSH
• Genome Size MeSH
• Genome * MeSH
• Gonads metabolism   MeSH
• Hybridization, Genetic MeSH
• Mitosis genetics   MeSH
• Fishes genetics   MeSH
• Chromosome Segregation genetics   MeSH
• Animals MeSH
• Check Tag
• Male MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH

The B chromosomes exhibit diverse behaviour compared with conventional genetic models. The capacity of the B chromosome either to accumulate or to be eliminated in a tissue-specific manner is dependent on biological processes related to aberrant cell division(s), but here yet remains compatible with normal development. We studied B chromosome elimination in Sorghum purpureosericeum embryos through cryo-sections and demonstrated the B chromosome instability during plant growth using flow cytometry, molecular markers and fluorescent in situ hybridization techniques. Consequently, using B chromosome-specific probes we revealed the non-Mendelian inheritance of B chromosomes in developing pollen. We disclosed that the occurrence of the B chromosome is specific to certain tissues or organs. The distribution pattern is mainly caused by an extensive elimination that functions primarily during embryo development and persists throughout plant development. Furthermore, we described that B chromosome accumulation can occur either by nondisjunction at first pollen mitosis (PMI) or the initiation of extra nuclear division(s) during pollen development. Our study demonstrates the existence of a not-yet-fully described B chromosome drive process, which is likely under the control of the B chromosome.

• Keywords
• B chromosome, accumulation mechanism, chromosome drive, chromosome elimination, extra pollen mitosis, nondisjunction, pollen mitosis, polymitosis,
• MeSH
• Chromosomes, Plant * genetics   MeSH
• Mitosis * MeSH
• Nondisjunction, Genetic * MeSH
• Pollen * genetics   cytology   MeSH
• Seeds genetics   growth & development   MeSH
• Sorghum * genetics   MeSH
• Publication type
• Journal Article MeSH

Heat stress is a major threat to global crop production, and understanding its impact on plant fertility is crucial for developing climate-resilient crops. Despite the known negative effects of heat stress on plant reproduction, the underlying molecular mechanisms remain poorly understood. Here, we investigated the impact of elevated temperature on centromere structure and chromosome segregation during meiosis in Arabidopsis thaliana. Consistent with previous studies, heat stress leads to a decline in fertility and micronuclei formation in pollen mother cells. Our results reveal that elevated temperature causes a decrease in the amount of centromeric histone and the kinetochore protein BMF1 at meiotic centromeres with increasing temperature. Furthermore, we show that heat stress increases the duration of meiotic divisions and prolongs the activity of the spindle assembly checkpoint during meiosis I, indicating an impaired efficiency of the kinetochore attachments to spindle microtubules. Our analysis of mutants with reduced levels of centromeric histone suggests that weakened centromeres sensitize plants to elevated temperature, resulting in meiotic defects and reduced fertility even at moderate temperatures. These results indicate that the structure and functionality of meiotic centromeres in Arabidopsis are highly sensitive to heat stress, and suggest that centromeres and kinetochores may represent a critical bottleneck in plant adaptation to increasing temperatures.

• Keywords
• A. thaliana, cell biology, centremeres, centromeric histone, chromosomes, gene expression, meiosis, micronuclei, spindle assembly checkpoint,
• MeSH
• Arabidopsis * genetics   metabolism   MeSH
• Centromere metabolism   MeSH
• Histones metabolism   MeSH
• Kinetochores metabolism   MeSH
• Meiosis MeSH
• Heat-Shock Response MeSH
• Plants genetics   MeSH
• Chromosome Segregation MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Histones MeSH

Centromere is the chromosomal site of kinetochore assembly and microtubule attachment for chromosome segregation. Given its importance, markers that allow specific labeling of centromeric chromatin throughout the cell cycle and across all chromosome types are sought for facilitating various centromere studies. Antibodies against the N-terminal region of CENH3 are commonly used for this purpose, since CENH3 is the near-universal marker of functional centromeres. However, because the N-terminal region of CENH3 is highly variable among plant species, antibodies directed against this region usually function only in a small group of closely related species. As a more versatile alternative, we present here antibodies targeted to the conserved domains of two outer kinetochore proteins, KNL1 and NDC80. Sequence comparison of these domains across more than 350 plant species revealed a high degree of conservation, particularly within a six amino acid motif, FFGPVS in KNL1, suggesting that both antibodies would function in a wide range of plant species. This assumption was confirmed by immunolabeling experiments in angiosperm (monocot and dicot) and gymnosperm species, including those with mono-, holo-, and meta-polycentric chromosomes. In addition to centromere labeling on condensed chromosomes during cell division, both antibodies detected the corresponding regions in the interphase nuclei of most species tested. These results demonstrated that KNL1 and NDC80 are better suited for immunolabeling centromeres than CENH3, because antibodies against these proteins offer incomparably greater versatility across different plant species which is particularly convenient for studying the organization and function of the centromere in non-model species.

• Keywords
• CENH3, Centromere, KNL1, NDC80, immunolabeling, kinetochore,
• MeSH
• Centromere * MeSH
• Chromatin MeSH
• Kinetochores * MeSH
• Plant Proteins * genetics   MeSH
• Chromosome Segregation MeSH
• Amino Acid Sequence MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Chromatin MeSH
• Plant Proteins * MeSH

Chromosome segregation in female germ cells and early embryonic blastomeres is known to be highly prone to errors. The resulting aneuploidy is therefore the most frequent cause of termination of early development and embryo loss in mammals. And in specific cases, when the aneuploidy is actually compatible with embryonic and fetal development, it leads to severe developmental disorders. The main surveillance mechanism, which is essential for the fidelity of chromosome segregation, is the Spindle Assembly Checkpoint (SAC). And although all eukaryotic cells carry genes required for SAC, it is not clear whether this pathway is active in all cell types, including blastomeres of early embryos. In this review, we will summarize and discuss the recent progress in our understanding of the mechanisms controlling chromosome segregation and how they might work in embryos and mammalian embryos in particular. Our conclusion from the current literature is that the early mammalian embryos show limited capabilities to react to chromosome segregation defects, which might, at least partially, explain the widespread problem of aneuploidy during the early development in mammals.

• Keywords
• CDK1, aneuploidy, cell size, chromosome division, embryo, segregation errors, spindle, spindle assembly checkpoint,
• MeSH
• Aneuploidy MeSH
• Chromosomes MeSH
• Embryonic Development * genetics   MeSH
• Humans MeSH
• Mammals genetics   MeSH
• Chromosome Segregation * MeSH
• Cell Size MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Review MeSH