Genetic control of chromosome synapsis in yeast meiosis

Both meiosis-specific and general recombination functions, recruited from the mitotic cell cycle, are required for elevated levels of recombination and for chromosome synapsis (assembly of the synaptonemal complex) during yeast meiosis. The meiosis-specific SPO11 gene (previously shown to be required for meiotic recombination) has been isolated and shown to be essential for synaptonemal complex formation but not for DNA metabolism during the vegetative cell cycle. In contrast, the RAD52 gene is required for mitotic and meiotic recombination but not for synaptonemal complex assembly. These data suggest that the synaptonemal complex may be necessary but is clearly not sufficient for meiotic recombination. Cytological analysis of spread meiotic nuclei demonstrates that chromosome behavior in yeast is comparable with that observed in larger eukaryotes. These spread preparations support the immunocytological localization of specific proteins in meiotic nuclei. This combination of genetic, molecular cloning, and cytological approaches in a single experimental system provides a means of addressing the role of specific gene products and nuclear structures in meiotic chromosome behavior.     

染色体展片法观察拟南芥雄性减数分裂过程中的染色体形态

减数分裂指DNA复制1次, 细胞核分裂2次, 产生染色体数目减半的单倍体配子, 是真核生物有性生殖所必需的环节。拟南芥(Arabidopsis thaliana)是分子遗传学研究的传统模式生物。近年来, 随着显微镜技术的快速发展, 利用细胞学方法观察拟南芥减数分裂过程中的染色体形态和同源染色体互作事件, 将有助于深入认识减数分裂的分子遗传机制。该文详细描述了染色体展片法观察拟南芥雄性减数分裂细胞中的染色体形态。     

染色体展片法观察拟南芥雄性减数分裂过程中的染色体形态

减数分裂指DNA复制1次, 细胞核分裂2次, 产生染色体数目减半的单倍体配子, 是真核生物有性生殖所必需的环节。拟南芥(Arabidopsis thaliana)是分子遗传学研究的传统模式生物。近年来, 随着显微镜技术的快速发展, 利用细胞学方法观察拟南芥减数分裂过程中的染色体形态和同源染色体互作事件, 将有助于深入认识减数分裂的分子遗传机制。该文详细描述了染色体展片法观察拟南芥雄性减数分裂细胞中的染色体形态。     

Genetic Analyses of Meiotic Recombination in Arabidopsis

Meiosis is essential for sexual reproduction and recombination is a critical step required for normal meiosis. Understanding the underlying molecular mechanisms that regulate recombination is important for medical, agricultural and ecological reasons. Readily available molecular and cytological tools make Arabidopsis an excellent system to study meiosis. Here we review recent developments in molecular genetic analyses on meiotic recombination. These include studies on plant homologs of yeast and animal genes, as well as novel genes that were first identified in plants. The characterizations of these genes have demonstrated essential functions from the initiation of recombination by double-strand breaks to repair of such breaks, from the formation of doubie-HoUiday junctions to possible resolution of these junctions, both of which are critical for crossover formation. The recent advances have ushered a new era in plant meiosis, in which the combination of genetics, genomics, and molecular cytology can uncover important gene functions.     

ASY1 coordinates early events in the plant meiotic recombination pathway

Meiosis is a fundamental and evolutionarily conserved process that is central to the life cycles of all sexually reproducing eukaryotes. An understanding of this process is critical to furthering research on reproduction, fertility, genetics and breeding. Plants have been used extensively in cytogenetic studies of meiosis during the last century. Until recently, our knowledge of the molecular and functional aspects of meiosis has emerged from the study of non-plant model organisms, especially budding yeast. However, the emergence of Arabidopsis thaliana as the model organism for plant molecular biology and genetics has enabled significant progress in the characterisation of key genes and proteins controlling plant meiosis. The development of molecular and cytological techniques in Arabidopsis, besides allowing investigation of the more conserved aspects of meiosis, are also providing insights into features of this complex process which may vary between organisms. This review highlights an example of this recent progress by focussing on ASY1, a meiosis-specific Arabidopsis protein which shares some similarity with the N-terminus region of the yeast axial core-associated protein, HOP1, a component of a multiprotein complex which acts as a meiosis-specific barrier to sister-chromatid repair in budding yeast. In the absence of ASY1, synapsis is interrupted and chiasma formation is dramatically reduced. ASY1 protein is initially detected during early meiotic G2 as numerous foci distributed over the chromatin. As G2 progresses the signal appears to be increasingly continuous and is closely associated with the axial elements. State-of-the-art cytogenetic techniques have revealed that initiation of recombination is synchronised with the formation of the chromosome axis. Furthermore, in the context of the developing chromosome axes, ASY1 plays a crucial role in co-ordinating the activity of a key member of the homologous recombination machinery, AtDMC1.     

The synaptonemal complex central region modulates crossover pathways and feedback control of meiotic double-strand break formation

The synaptonemal complex (SC) is a proteinaceous structure that mediates homolog engagement and genetic recombination during meiosis. In budding yeast, Zip-Mer-Msh (ZMM) proteins promote crossover (CO) formation and initiate SC formation. During SC elongation, the SUMOylated SC component Ecm11 and the Ecm11-interacting protein Gmc2 facilitate the polymerization of Zip1, an SC central region component. Through physical recombination, cytological, and genetic analyses, we found that ecm11 and gmc2 mutants exhibit chromosome-specific defects in meiotic recombination. CO frequencies on a short chromosome (chromosome III) were reduced, whereas CO and non-crossover frequencies on a long chromosome (chromosome VII) were elevated. Further, in ecm11 and gmc2 mutants, more double-strand breaks (DSBs) were formed on a long chromosome during late prophase I, implying that the Ecm11–Gmc2 (EG) complex is involved in the homeostatic regulation of DSB formation. The EG complex may participate in joint molecule (JM) processing and/or double-Holliday junction resolution for ZMM-dependent CO-designated recombination. Absence of the EG complex ameliorated the JM-processing defect in zmm mutants, suggesting a role for the EG complex in suppressing ZMM-independent recombination. Our results suggest that the SC central region functions as a compartment for sequestering recombination-associated proteins to regulate meiosis specificity during recombination.     

Nuclear architecture and chromosome dynamics in the search of the pairing partner in meiosis in plants

The formation of haploid gametes in organisms with sexual reproduction requires regular bivalent chromosome pairing in meiosis. In many species, homologous chromosomes occupy separate territories at the onset of meiosis. To be paired at metaphase I, they need to be brought into a close proximity for interactions that include homology recognition and the establishment of some form of bonds. How homologues find each other is one of the least understood meiotic events. Plant species with large or medium sized genomes, such as wheat or maize, are excellent materials for the cytological analysis of chromosome dynamics at early meiosis, but genes that control meiosis have been identified mainly in small genome species such as Arabidopsis thaliana. This review is focused on the contribution studies on plants are providing to the knowledge of the initial steps of the meiotic process.     

Meiotic restitution in amphihaploids in the tribe Triticeae

In haploid and diploid organisms of the plant kingdom, meiotic division of diploid cells proceeds in two consecutive stages, with DNA replicating only once. In amphihaploids (interspecific or intergeneric hybrids), where homologs are absent, the reduction of the chromosome number does not occur, meiosis is abnormal, and the plants are sterile. Gamete viability in F₁ hybrids is ensured by a single division when chromosomes are separated into sister chromatids in either the first or the second division. Such gametes ensure partial fertility of amphihaploids, thereby facilitating their survival and stabilization of the polygenome. The frequency of the formation of viable gametes varies from a few cases to 98.8% in different anthers of the hybrids. Here, studies on the cytological mechanisms and genetic control of chromosome unreduction or restitution in different amphihaploids of the tribe Triticeae are reviewed. The current notions on the control of formation of restitution nuclei based on the principles of a prolonged metaphase I and different types of meiocytes. The main terms used for systematization of restitution mechanisms are first-division restitution (FDR), single-division meiosis (SDM), and unreductional meiotic cell division (UMCD). It has been assumed that archesporial cells of wide hybrids may have two cell division programs, the meiotic and the mitoyic ones The possible approaches to the analysis of the genetic control of chromosome restitution in amphihaploids are discussed.     

Morphology of a human-derived YAC in yeast meiosis

In meiosis of human males DNA is packaged along pachytene chromosomes about 20 time more compactly than in meiosis of yeast. Nevertheless, a human-derived yeast artificial chromosome (YAC) shows the same degree of compaction of DNA as endogenous chromosomes in meiotic prophase nuclei of yeast. This suggests that in yeast meiosis, human and yeast DNA adopt a similar organization of chromatin along the pachytene chromosome cores. Therefore meiotic chromatin organization does not seem to be an inherent chromosomal property but is governed by the host-specific cellular environment. We suggest that there is a correlation between the less dense DNA packaging and the increased rate of recombination that has been reported for human-derived YACs as compared with human DNA in its natural environment.     

A Cis-Acting Locus That Promotes Crossing over between X Chromosomes in Caenorhabditis Elegans

This study reports the characterization of a cis-acting locus on the Caenorhabditis elegans X chromosome that is crucial for promoting normal levels of crossing over specifically between the X homologs and for ensuring their proper disjunction at meiosis I. The function of this locus is disrupted by the mutation me8, which maps to the extreme left end of the X chromosome within the region previously implicated by studies of X;A translocations and X duplications to contain a meiotic pairing site. Hermaphrodites homozygous for a deletion of the locus (Df/Df) or heterozygous for a deletion and the me8 mutation (me8/Df) exhibit extremely high levels of X chromosome nondisjunction at the reductional division; this is correlated with a sharp decrease in crossing over between the X homologs as evidenced both by reductions in genetic map distances and by the presence of achiasmate chromosomes in cytological preparations of oocyte nuclei. Duplications of the wild-type region that are unlinked to the X chromosome cannot complement the recombination and disjunction defects in trans, indicating that this region must be present in cis to the X chromosome to ensure normal levels of crossing over and proper homolog disjunction. me8 homozygotes exhibit an altered distribution of crossovers along the X chromosome that suggests a defect in processivity along the X chromosome of an event that initiates at the chromosome end. Models are discussed in which the cis-acting locus deleted by the Dfs functions as a meiotic pairing center that recruits trans-acting factors onto the chromosomes to nucleate assembly of a crossover-competent complex between the X homologs. This pairing center might function in the process of homolog recognition, or in the initiation of homologous synapsis.