抑制植物减数分裂重组的分子机理

减数分裂重组不仅保证了真核生物有性生殖过程中染色体数量的稳定,还通过父母亲本间遗传物质的互换在后代中产生遗传变异。因此,减数分裂重组是遗传多样性形成的重要途径,也是生物多样性和物种进化的主要动力。在绝大多数真核生物中,不管染色体数目的多少或基因组的大小,减数分裂重组的形成都受到严格的调控,但抑制减数分裂重组的分子机理目前仍不清楚。近年来,通过正向遗传学筛选鉴定出多个减数分裂重组抑制基因,揭示了抑制基因的功能和调控途径。本文基于拟南芥中减数分裂重组抑制基因的研究现状,综述了植物减数分裂重组抑制基因研究取得的突破性进展,并结合基因功能与其调控网络阐述了抑制植物减数分裂重组的分子机理。     

Role of ubiquitination in meiotic recombination repair

Programmed and unprogrammed double-strand breaks (DSBs) often arise from such physiological requirements as meiotic recombination, and exogenous insults, such as ionizing radiation (IR). Due to deleterious impacts on genome stability, DSBs must be appropriately processed and repaired in a regulatory manner. Recent investigations have indicated that ubiquitination is a critical factor in DNA damage response and meiotic recombination repair. This review summarizes the effects of proteins and complexes associa...     

Hotspots of meiotic recombination in the mouse major histocompatibility complex

Meiotic recombination is not random in the proximal region of the mouse major histocompatibility complex (MHC). It is clustered at four restricted positions, so-called hotspots. Some of the MHC haplotypes derived from Asian wild mice enhance recombination at the hotspots in genetic crosses with standard MHC haplotypes of laboratory mouse strains. In particular, the wm7 haplotype derived from Japanese wild mouse indicated an approximately 2% recombination frequency within a 1.2 kb fragment of DNA in the interval between the Pb and Ob genes. Interestingly, this enhancement of recombination was observed only in female meiosis but not in male meiosis. Mating experiments demonstrated that the wm7 haplotype carries a genetic factor in the region proximal to the hotspot, which instigates recombination. In addition, the wm7 haplotype has a genetic factor located in the region distal to the hotspot, which suppresses recombination. From the molecular characterization of the two hotspots located in the Eb gene and the Pb-Ob interval, it appeared that there are several common molecular elements, the consensus of the middle repetitive MT-family, TCTG or CCTG tetramer repeats, and the solitary long terminal repeat (LTR) of mouse retrovirus.     

A new seed-based assay for meiotic recombination in Arabidopsis thaliana

Meiotic recombination is a fundamental biological process that plays a central role in the evolution and breeding of plants. We have developed a new seed-based assay for meiotic recombination in Arabidopsis. The assay is based on the transformation of green and red fluorescent markers expressed under a seed-specific promoter. A total of 74 T-DNA markers were isolated, sequenced and mapped both physically and genetically. Lines containing red and green markers that map 1-20 cM apart were crossed to produce tester lines with the two markers linked in cis yielding seeds that fluoresced both in red and green. We show that these lines can be used for efficient scoring of recombinant types (red only or green only fluorescing seeds) in a seed population derived from a test cross (backcross) or self-pollination. Two tester lines that were characterized during several generations of backcross and self-pollination, one in the background of ecotype Landsberg and one in the ecotype Columbia, are described. We discuss the number of plants and seeds to be scored in order to obtain reliable and reproducible crossing over rate values. This assay offers a relatively high-throughput method, with the benefit of seed markers (similar to the maize classical genetic markers) combined with the advantages of Arabidopsis. It advances the prospect to better understand the factors that affect the rate of meiotic crossover in plants and to stimulate this process for more efficient breeding and mapping.     

利用作图函数和实验方法讲解减数分裂重组值的教学方法

减数分裂教学中重组值的教学,是高等医药院校遗传学中教学的关键点和难点。作者利用泊松分布的作图函数和蝗虫的精子形成过程装片制作、观察实验教学相结合的方法,不仅使学生掌握了减数分裂的基本知识,同时也使学生掌握了减数分裂最大重组值不超过50%的教学难点。     

Histone H3 lysine 4 trimethylation marks meiotic recombination initiation sites

The function of histone modifications in initiating and regulating the chromosomal events of the meiotic prophase remains poorly understood. In Saccharomyces cerevisiae, we examined the genome‐wide localization of histone H3 lysine 4 trimethylation (H3K4me3) along meiosis and its relationship to gene expression and position of the programmed double‐strand breaks (DSBs) that initiate interhomologue recombination, essential to yield viable haploid gametes. We find that the level of H3K4me3 is constitutively higher close to DSB sites, independently of local gene expression levels. Without Set1, the H3K4 methylase, 84% of the DSB sites exhibit a severely reduced DSB frequency, the reduction being quantitatively correlated with the local level of H3K4me3 in wild‐type cells. Further, we show that this differential histone mark is already established in vegetative cells, being higher in DSB‐prone regions than in regions with no or little DSB. Taken together, our results demonstrate that H3K4me3 is a prominent and preexisting mark of active meiotic recombination initiation sites. Novel perspectives to dissect the various layers of the controls of meiotic DSB formation are discussed.     

人精母细胞重组频率和年龄相关性的分析

减数分裂遗传重组对同源染色体的正确分离和单倍体的正确形成起至关重要的作用,但人们对人精母细胞减数分裂遗传重组机制了解的还很少。通过免疫荧光染色技术标记减数分裂I联会复合体上的MLH1(DNA错配修复蛋白)位点可以检测人精母细胞的重组。文章对10例可育男性进行分析,发现每个细胞中重组位点数平均为49.4士4.4,范围为33~63,具有显著的个体差异,只有0.4%(1/220)的常染色体SC上缺少MLH1位点。进一步通过Spearman相关性分析,分析了年龄因素与个体间重组位点差异的相关性,结果提示年龄因素对常染色体及性染色体的重组均无影响。     

Immunofluorescent analysis of meiotic recombination in the domestic cat

The paper presents the analysis of the frequency, density, and distribution of recombination sites in the male meiosis of the domestic cat (Felis silvestris catus). The study was carried out using immunofluorescent staining of synaptonemal complex (SC) proteins, centromeric proteins and mismatch repair protein MLH1, a reliable marker of crossingover sites. We mapped 2633 sites of crossing over in 1098 individual autosomes. Based on these data, we estimated the total length of the genetic map of the domestic cat to be 2176 centimorgans. Positive correlation between the length of SC and the number of recombination sites common for mammalians was also found in the domestic cat. It was shown that this species was characterized by the highest density of recombination and the lowest interference in mammals.     

Molecular mechanisms for environmentally induced and evolutionarily rapid redistribution (plasticity) of meiotic recombination

It has long been known (circa 1917) that environmental conditions, as well as speciation, can affect dramatically the frequency distribution of Spo11/Rec12-dependent meiotic recombination. Here, by analyzing DNA sequence-dependent meiotic recombination hotspots in the fission yeast Schizosaccharomyces pombe, we reveal a molecular basis for these phenomena. The impacts of changing environmental conditions (temperature, nutrients, and osmolarity) on local rates of recombination are mediated directly by DNA site-dependent hotspots (M26, CCAAT, and Oligo-C). This control is exerted through environmental condition-responsive signal transduction networks (involving Atf1, Pcr1, Php2, Php3, Php5, and Rst2). Strikingly, individual hotspots modulate rates of recombination over a very broad dynamic range in response to changing conditions. They can range from being quiescent to being highly proficient at promoting activity of the basal recombination machinery (Spo11/Rec12 complex). Moreover, each different class of hotspot functions as an independently controlled rheostat; a condition that increases the activity of one class can decrease the activity of another class. Together, the independent modulation of recombination rates by each different class of DNA site-dependent hotspots (of which there are many) provides a molecular mechanism for highly dynamic, large-scale changes in the global frequency distribution of meiotic recombination. Because hotspot-activating DNA sites discovered in fission yeast are conserved functionally in other species, this process can also explain the previously enigmatic, Prdm9-independent, evolutionarily rapid changes in hotspot usage between closely related species, subspecies, and isolated populations of the same species.     

Arabidopsis SPO11-2 functions with SPO11-1 in meiotic recombination

The Spo11 protein is a eukaryotic homologue of the archaeal DNA topoisomerase VIA subunit (topo VIA). In archaea it is involved, together with its B subunit (topo VIB), in DNA replication. However, most eukaryotes, including yeasts, insects and vertebrates, instead have a single gene for Spo11/topo VIA and no homologues for topo VIB. In these organisms, Spo11 mediates DNA double-strand breaks that initiate meiotic recombination. Many plant species, in contrast to other eukaryotes, have three homologues for Spo11/topo VIA and one for topo VIB. The homologues in Arabidopsis, AtSPO11-1, AtSPO11-2 and AtSPO11-3, all share 20-30% sequence similarity with other Spo11/topo VIA proteins, but their functional relationship during meiosis or other processes is not well understood. Previous genetic evidence suggests that AtSPO11-1 is a true orthologue of Spo11 in other eukaryotes and is required for meiotic recombination, whereas AtSPO11-3 is involved in DNA endo-reduplication as a part of the topo VI complex. In this study, we show that plants homozygous for atspo11-2 exhibit a severe sterility phenotype. Both male and female meiosis are severely disrupted in the atspo11-2 mutant, and this is associated with severe defects in synapsis during the first meiotic division and reduced meiotic recombination. Further genetic analysis revealed that AtSPO11-1 and AtSPO11-2 genetically interact, i.e. plants heterozygous for both atspo11-1 and atspo11-2 are also sterile, suggesting that AtSPO11-1 and AtSPO11-2 have largely overlapping functions. Thus, the three Arabidopsis Spo11 homologues appear to function in two discrete processes, i.e. AtSPO11-1 and AtSPO11-2 in meiotic recombination and AtSPO11-3 in DNA replication.     

Yeast axial-element protein,Red1, binds SUMO chains to promote meiotic interhomologue recombination and chromosome synapsis

The synaptonemal complex (SC) is a tripartite protein structure consisting of two parallel axial elements (AEs) and a central region. During meiosis, the SC connects paired homologous chromosomes, promoting interhomologue (IH) recombination. Here, we report that, like the CE component Zip1, Saccharomyces cerevisiae axial-element structural protein, Red1, can bind small ubiquitin-like modifier (SUMO) polymeric chains. The Red1–SUMO chain interaction is dispensable for the initiation of meiotic DNA recombination, but it is essential for Tel1- and Mec1-dependent Hop1 phosphorylation, which ensures IH recombination by preventing the inter-sister chromatid DNA repair pathway. Our results also indicate that Red1 and Zip1 may directly sandwich the SUMO chains to mediate SC assembly. We suggest that Red1 and SUMO chains function together to couple homologous recombination and Mec1–Tel1 kinase activation with chromosome synapsis during yeast meiosis.     

Pericentromeric chromatin reorganisation follows the initiation of recombination and coincides with early events of synapsis in cereals

The reciprocal exchange of genetic information between homologous chromosomes during meiotic recombination is essential to secure balanced chromosome segregation and to promote genetic diversity. The chromosomal position and frequency of reciprocal genetic exchange shapes the efficiency of breeding programmes and influences crop improvement under a changing climate. In large genome cereals, such as wheat and barley, crossovers are consistently restricted to subtelomeric chromosomal regions, thus preventing favourable allele combinations being formed within a considerable proportion of the genome, including interstitial and pericentromeric chromatin. Understanding the key elements driving crossover designation is therefore essential to broaden the regions available for crossovers. Here, we followed early meiotic chromatin dynamism in cereals through the visualisation of a homologous barley chromosome arm pair stably transferred into the wheat genetic background. By capturing the dynamics of a single chromosome arm at the same time as detecting the undergoing events of meiotic recombination and synapsis, we showed that subtelomeric chromatin of homologues synchronously transitions to an open chromatin structure during recombination initiation. By contrast, pericentromeric and interstitial regions preserved their closed chromatin organisation and become unpackaged only later, concomitant with initiation of recombinatorial repair and the initial assembly of the synaptonemal complex. Our results raise the possibility that the closed pericentromeric chromatin structure in cereals may influence the fate decision during recombination initiation, as well as the spatial development of synapsis, and may also explain the suppression of crossover events in the proximity of the centromeres.     

FLP recombinase-mediated site-specific recombination in rice

The feasibility of using the FLP/ FRT site-specific recombination system in rice for genome engineering was evaluated. Transgenic rice plants expressing the FLP recombinase were crossed with plants harbouring the kanamycin resistance gene ( neomycin phosphotransferase II , nptII ) flanked by FRT sites, which also served to separate the corn ubiquitin promoter from a promoterless gusA . Hybrid progeny were tested for excision of the nptII gene and the positioning of the ubiquitin promoter proximal to gusA . While the hybrid progeny from various crosses exhibited β-glucuronidase (GUS) expression, the progeny of selfed parental rice plants did not show detectable GUS activity. Despite the variable GUS expression and incomplete recombination displayed in hybrids from some crosses, uniform GUS staining and complete recombination were observed in hybrids from other crosses. The recombined locus was shown to be stably inherited by the progeny. These data demonstrate the operation of FLP recombinase in catalysing excisional DNA recombination in rice, and confirm that the FLP/ FRT recombination system functions effectively in the cereal crop rice. Transgenic rice lines expressing active FLP recombinase generated in this study provide foundational stock material, thus facilitating the future application and development of the FLP/ FRT system in rice genetic improvement.     

The evolution of meiotic sex and its alternatives

Meiosis is an ancestral, highly conserved process in eukaryotic life cycles, and for all eukaryotes the shared component of sexual reproduction. The benefits and functions of meiosis, however, are still under discussion, especially considering the costs of meiotic sex. To get a novel view on this old problem, we filter out the most conserved elements of meiosis itself by reviewing the various modifications and alterations of modes of reproduction. Our rationale is that the indispensable steps of meiosis for viability of offspring would be maintained by strong selection, while dispensable steps would be variable. We review evolutionary origin and processes in normal meiosis, restitutional meiosis, polyploidization and the alterations of meiosis in forms of uniparental reproduction (apomixis, apomictic parthenogenesis, automixis, selfing) with a focus on plants and animals. This overview suggests that homologue pairing, double-strand break formation and homologous recombinational repair at prophase I are the least dispensable elements, and they are more likely optimized for repair of oxidative DNA damage rather than for recombination. Segregation, ploidy reduction and also a biparental genome contribution can be skipped for many generations. The evidence supports the theory that the primary function of meiosis is DNA restoration rather than recombination.     

Retinoblastoma protein is essential for early meiotic events in Arabidopsis

We have analysed the role of RBR (retinoblastoma related), the Arabidopsis homologue of the tumour suppressor Retinoblastoma protein (pRb), during meiosis. We characterise the rbr-2 mutation, which causes a loss of RBR in male meiocytes. The rbr-2 plants exhibit strongly reduced fertility, while vegetative growth is generally unaffected. The reduced fertility is due to a meiotic defect that results in reduced chiasma formation and subsequent errors in chromosome disjunction. Immunolocalisation studies in wild-type meiocytes reveal that RBR is recruited as foci to the chromosomes during early prophase I in a DNA double-strand-break-dependent manner. In the absence of RBR, expression of several meiotic genes is reduced. The localisation of the recombinases AtRAD51 and AtDMC1 is normal. However, localisation of the MutS homologue AtMSH4 is compromised. Additionally, polymerisation of the synaptonemal complex protein AtZYP1 is abnormal. Together, these data indicate that loss of RBR during meiosis results in a reduction of crossover formation and an associated failure in chromosome synapsis. Our results indicate that RBR has an important role in meiosis affecting different aspects of this complex process.     

水稻中的无融合生殖研究及问题

本文综述了植物无融合生殖概念的发展历史。简述了目前报道的裟上主要无融合生殖材料的特点特征以及近年来的一些新进展。同时指出了水稻无融合生殖研究中存在的问题和注意事项。     

Sex-specific variation in the genome-wide recombination rate

In most species that reproduce sexually, successful gametogenesis requires recombination during meiosis. The number and placement of crossovers (COs) vary among individuals, with females and males often presenting the most striking contrasts. Despite the recognition that the sexes recombine at different rates (heterochiasmy), existing data fail to answer the question of whether patterns of genetic variation in recombination rate are similar in the two sexes. To fill this gap, we measured the genome-wide recombination rate in both sexes from a panel of wild-derived inbred strains from multiple subspecies of house mice (Mus musculus) and from a few additional species of Mus. To directly compare recombination rates in females and males from the same genetic backgrounds, we applied established methods based on immunolocalization of recombination proteins to inbred strains. Our results reveal discordant patterns of genetic variation in the two sexes. Whereas male genome-wide recombination rates vary substantially among strains, female recombination rates measured in the same strains are more static. The direction of heterochiasmy varies within two subspecies, Mus musculus molossinus and Mus musculus musculus. The direction of sex differences in the length of the synaptonemal complex and CO positions is consistent across strains and does not track sex differences in genome-wide recombination rate. In males, contrasts between strains with high recombination rate and strains with low recombination rate suggest more recombination is associated with stronger CO interference and more double-strand breaks. The sex-specific patterns of genetic variation we report underscore the importance of incorporating sex differences into recombination research.     

The role of OsCOM1 in homologous chromosome synapsis and recombination in rice meiosis

COM1/SAE2 is a highly conserved gene from yeast to higher eukaryotes. Its orthologs, known to cooperate with the MRX complex (Mre11/Rad50/Xrs2), are required for meiotic DNA double‐strand break (DSB) ends resection and specific mitotic DSB repair events. Here, the rice (Oryza sativa, 2n = 2x = 24) COM1/SAE2 homolog was identified through positional cloning, termed OsCOM1. Four independent mutants of OsCOM1 were isolated and characterized. In Oscom1 mutants, synaptonemal complex (SC) formation, homologous pairing and recombination were severely inhibited, whereas aberrant non‐homologous chromosome entanglements occurred constantly. Several key meiotic proteins, including ZEP1 and OsMER3, were not loaded normally onto chromosomes in Oscom1 mutants, whereas the localization of OsREC8, PAIR2 and PAIR3 seemed to be normal. Moreover, OsCOM1 was loaded normally onto meiotic chromosomes in Osrec8, zep1 and Osmer3 mutants, but could not be properly loaded in Osam1, pair2 and OsSPO11‐1^RNAi plants. These results provide direct evidence for the functions of OsCOM1 in promoting homologous synapsis and recombination in rice meiosis.     

Male-mediated effects on female meiotic recombination

Recombination rates vary owing to an individual's genetic composition and/or its environmental condition. Yet, the effects of mating partner on recombination rates have not been considered. Here, I document a previously undescribed, male-mediated effect on female recombination rates. After crossing females to males from different genetic backgrounds, I observed a significant difference in proportion of recombinant offspring based on the genetic background of the father (P= 0.0292; df = 3; F= 3.07). Genetic variation in male ability to affect recombination rate in their mates suggests the potential for sexual conflict on optimal proportion of recombinant offspring, perhaps leading to changes in population-level recombination rates with varying levels of sexual selection.