Chromosome Organization in Early Meiotic Prophase

One of the most fascinating aspects of meiosis is the extensive reorganization of the genome at the prophase of the first meiotic division (prophase I). The first steps of this reorganization are observed with the establishment of an axis structure, that connects sister chromatids, from which emanate arrays of chromatin loops. This axis structure, called the axial element, consists of various proteins, such as cohesins, HORMA-domain proteins, and axial element proteins. In many organisms, axial elements are required to set the stage for efficient sister chromatid cohesion and meiotic recombination, necessary for the recognition of the homologous chromosomes. Here, we review the different actors involved in axial element formation in Saccharomyces cerevisiae and in mouse. We describe the current knowledge of their localization pattern during prophase I, their functional interdependence, their role in sister chromatid cohesion, loop axis formation, homolog pairing before meiotic recombination, and recombination. We also address further challenges that need to be resolved, to fully understand the interplay between the chromosome structure and the different molecular steps that take place in early prophase I, which lead to the successful outcome of meiosis I.

EXPERIENCE OF STEEL PILE DRIVING BY PNEUMATIC HAMMERS IN UNDERGROUND CONSTRUCTION BY THE CUT-AND-COVER METHOD

The timeliness of application of pneumatic hammers in the deep bore tunneling in underground construction is justified. The penetration resistance forces in steel pipe driving in soil are determined. The analysis of long-term operation of adjustable capacity pneumatic hammers shows the conformity of the blow energy and the driven pipe diameter. The relationship between the unit blow energy and the open-ended pipe diameter is proposed. The main pneumatic hammer parameter to govern the machine performance is substantiated. The standard pneumatic hammers of adjustable capacity are characterized. The cases of vertical pipe driving at construction sites in Russia are described. The technological capability of construction of retention wall in soil using rolled steel, namely, I-beams, U-sections, piling bars, etc. is illustrated. The production tooling arrangements to ensure secure mounting of the hammer on the axial element driven in soil are described.

Repurposing of synaptonemal complex proteins for kinetochores in Kinetoplastida

Chromosome segregation in eukaryotes is driven by the kinetochore, a macromolecular complex that connects centromeric DNA to microtubules of the spindle apparatus. Kinetochores in well-studied model eukaryotes consist of a core set of proteins that are broadly conserved among distant eukaryotic phyla. By contrast, unicellular flagellates of the class Kinetoplastida have a unique set of 36 kinetochore components. The evolutionary origin and history of these kinetochores remain unknown. Here, we report evidence of homology between axial element components of the synaptonemal complex and three kinetoplastid kinetochore proteins KKT16-18. The synaptonemal complex is a zipper-like structure that assembles between homologous chromosomes during meiosis to promote recombination. By using sensitive homology detection protocols, we identify divergent orthologues of KKT16-18 in most eukaryotic supergroups, including experimentally established chromosomal axis components, such as Red1 and Rec10 in budding and fission yeast, ASY3-4 in plants and SYCP2-3 in vertebrates. Furthermore, we found 12 recurrent duplications within this ancient eukaryotic SYCP 2–3 gene family, providing opportunities for new functional complexes to arise, including KKT16-18 in the kinetoplastid parasite Trypanosoma brucei . We propose the kinetoplastid kinetochore system evolved by repurposing meiotic components of the chromosome synapsis and homologous recombination machinery that were already present in early eukaryotes.

ZmRAD17 Is Required for Accurate Double-Strand Break Repair During Maize Male Meiosis

RAD17, a replication factor C (RFC)-like DNA damage sensor protein, is involved in DNA checkpoint control and required for both meiosis and mitosis in yeast and mammals. In plant, the meiotic function of RAD17 was only reported in rice so far. Here, we identified and characterized the RAD17 homolog in maize. The Zmrad17 mutants exhibited normal vegetative growth but male was partially sterile. In Zmrad17 pollen mother cells, non-homologous chromosome entanglement and chromosome fragmentation were frequently observed. Immunofluorescence analysis manifested that DSB formation occurred as normal and the loading pattern of RAD51 signals was similar to wild-type at the early stage of prophase I in the mutants. The localization of the axial element ASY1 was normal, while the assembly of the central element ZYP1 was severely disrupted in Zmrad17 meiocytes. Surprisingly, no obvious defect in female sterility was observed in Zmrad17 mutants. Taken together, our results suggest that ZmRAD17 is involved in DSB repair likely by promoting synaptonemal complex assembly in maize male meiosis. These phenomena highlight a high extent of divergence from its counterpart in rice, indicating that the RAD17 dysfunction can result in a drastic dissimilarity in meiotic outcome in different plant species.

Repurposing of Synaptonemal Complex Proteins for Kinetochores in Kinetoplastida

Chromosome segregation in eukaryotes is driven by a macromolecular protein complex called the kinetochore that connects centromeric DNA to microtubules of the spindle apparatus. Kinetochores in well-studied model eukaryotes consist of a core set of proteins that are broadly conserved among distant eukaryotic phyla. In contrast, unicellular flagellates of the class Kinetoplastida have a unique set of kinetochore components. The evolutionary origin and history of these kinetochores remains unknown. Here, we report evidence of homology between three kinetoplastid kinetochore proteins KKT16–18 and axial element components of the synaptonemal complex, such as the SYCP2:SYCP3 multimers found in vertebrates. The synaptonemal complex is a zipper-like structure that assembles between homologous chromosomes during meiosis to promote recombination. Using a sensitive homology detection protocol, we identify divergent orthologues of SYCP2:SYCP3 in most eukaryotic supergroups including other experimentally established axial element components, such as Red1 and Rec10 in budding and fission yeast, and the ASY3:ASY4 multimers in land plants. These searches also identify KKT16–18 as part of this rapidly evolving protein family. The widespread presence of the SYCP2-3 gene family in extant eukaryotes suggests that the synaptonemal complex was likely present in the last eukaryotic common ancestor. We found at least twelve independent duplications of the SYCP2-3 gene family throughout the eukaryotic tree of life, providing opportunities for new functional complexes to arise, including KKT16–18 in Trypanosoma brucei. We propose that kinetoplastids evolved their unique kinetochore system by repurposing meiotic components of the chromosome synapsis and homologous recombination machinery that were already present in early eukaryotes.

A cohesin-independent mechanism modulates recombination activity along meiotic chromosomes

SummaryMeiotic chromosomes organize around a cohesin-dependent axial element, which promotes meiotic recombination and fertility. In the absence of cohesin, axial-element proteins instead accumulate in poorly understood genomic regions. Here, we show in S. cerevisiae that these regions are particularly enriched for axis proteins even on wild-type chromosomes and thus reflect a cohesin-independent recruitment mechanism. By contrast, other organizers of chromosome structure, including cohesin, condensin, and topoisomerases, are depleted from the same regions. This spatial patterning is observable before meiotic entry and therefore independent of meiotic recombination. Indeed, the regional density of gene-coding sequences is sufficient to predict a large fraction of cohesin-independent axis protein binding, suggesting that the gene-associated chromatin landscape plays a role in modulating axis protein deposition. The increased accumulation of axis proteins in these regions corresponds to a greater potential for initiation of recombination and progression to crossovers.

ZmRAD51C Is Essential for Double-Strand Break Repair and Homologous Recombination in Maize Meiosis

Radiation sensitive 51 (RAD51) recombinases play crucial roles in meiotic double-strand break (DSB) repair mediated by homologous recombination (HR) to ensure the correct segregation of homologous chromosomes. In this study, we identified the meiotic functions of ZmRAD51C, the maize homolog of Arabidopsis and rice RAD51C. The Zmrad51c mutants exhibited regular vegetative growth but complete sterility for both male and female inflorescence. However, the mutants showed hypersensitivity to DNA damage by mitomycin C. Cytological analysis indicated that homologous chromosome pairing and synapsis were rigorously inhibited, and meiotic chromosomes were often entangled from diplotene to metaphase I, leading to chromosome fragmentation at anaphase I. Immunofluorescence analysis showed that although the signals of the axial element absence of first division (AFD1) and asynaptic1 (ASY1) were normal, the assembly of the central element zipper1 (ZYP1) was severely disrupted. The DSB formation was normal in Zmrad51c meiocytes, symbolized by the regular occurrence of γH2AX signals. However, RAD51 and disrupted meiotic cDNA 1 (DMC1) signals were never detected at the early stage of prophase I in the mutant. Taken together, our results indicate that ZmRAD51C functions crucially for both meiotic DSB repair and homologous recombination in maize.

Transcription dynamically patterns the meiotic chromosome-axis interface

Meiotic chromosomes are highly compacted yet remain transcriptionally active. To understand how chromosome folding accommodates transcription, we investigated the assembly of the axial element, the proteinaceous structure that compacts meiotic chromosomes and promotes recombination and fertility. We found that the axial element proteins of budding yeast are flexibly anchored to chromatin by the ring-like cohesin complex. The ubiquitous presence of cohesin at sites of convergent transcription provides well-dispersed points for axis attachment and thus chromosome compaction. Axis protein enrichment at these sites directly correlates with the propensity for recombination initiation nearby. A separate modulating mechanism that requires the conserved axial-element component Hop1 biases axis protein binding towards small chromosomes. Importantly, axis anchoring by cohesin is adjustable and readily displaced in the direction of transcription by the transcriptional machinery. We propose that such robust but flexible tethering allows the axial element to promote recombination while easily adapting to changes in chromosome activity.