X-4 Translocations and Meiotic Drive in Drosophila melanogaster Males: Role of Sex Chromosome Pairing

Males carrying certain X-4 translocations exhibit strongly skewed sperm recovery ratios. The X^P4^D half of the translocation disjoins regularly from the Y chromosome and the 4^PX^D half disjoins regularly from the normal 4. Yet the smaller member of each bivalent is recovered in excess of its pairing partner, apparently due to differential gametic lethality. Chromosome recovery probabilities are multiplicative; the viability of each genotype is the product of the recovery probability of its component chromosomes. Meiotic drive can also be caused by deficiency for X heterochromatin. In(1)sc^4Lsc^8R males show the same size dependent chromosome recoveries and multiplicative recovery probabilities found in T(1;4)B^S males. Meiotic drive in In(1)sc^4Lsc^8R males has been shown to be due to X-Y pairing failure. Although pairing is regular in the T(X;4) males, the striking phenotypic parallels suggest a common explanation. The experiments described below show that the two phenomena are, in fact, one and the same. X-4 translocations are shown to have the same effect on recovery of independently assorting chromosomes as does In(1)sc^4Lsc^8R. Addition of pairing sites to the 4^PX^D half of the translocation eliminates drive. A common explanation—failure of the distal euchromatic portion of the X chromosome to participate in X:Y meiotic pairing—is suggested as the cause for drive. The effect of X chromosome breakpoint on X-4 translocation induced meiotic drive is investigated. It is found that translocations with breakpoints distal to 13C on the salivary map do not cause drive while translocations broken proximal to 13C cause drive. The level of drive is related to the position of the breakpoint—the more proximal the breakpoint the greater the drive.     

Interlock Formation and Coiling of Meiotic Chromosome Axes During Synapsis

The meiotic prophase chromosome has a unique architecture. At the onset of leptotene, the replicated sister chromatids are organized along an axial element. During zygotene, as homologous chromosomes pair and synapse, a synaptonemal complex forms via the assembly of a transverse element between the two axial elements. However, due to the limitations of light and electron microscopy, little is known about chromatin organization with respect to the chromosome axes and about the spatial progression of synapsis in three dimensions. Three-dimensional structured illumination microscopy (3D-SIM) is a new method of superresolution optical microscopy that overcomes the 200-nm diffraction limit of conventional light microscopy and reaches a lateral resolution of at least 100 nm. Using 3D-SIM and antibodies against a cohesin protein (AFD1/REC8), we resolved clearly the two axes that form the lateral elements of the synaptonemal complex. The axes are coiled around each other as a left-handed helix, and AFD1 showed a bilaterally symmetrical pattern on the paired axes. Using the immunostaining of the axial element component (ASY1/HOP1) to find unsynapsed regions, entangled chromosomes can be easily detected. At the late zygotene/early pachytene transition, about one-third of the nuclei retained unsynapsed regions and 78% of these unsynapsed axes were associated with interlocks. By late pachytene, no interlocks remain, suggesting that interlock resolution may be an important and rate-limiting step to complete synapsis. Since interlocks are potentially deleterious if left unresolved, possible mechanisms for their resolution are discussed in this article.MEIOSIS is a specialized cell division found in all organisms with a sexual life cycle. It requires the intricate coordination and precise timing of a series of cellular processes to ensure proper chromosome segregation and reduction in ploidy level. Meiotic prophase is initiated by the formation of cytologically characteristic leptotene chromosomes, which requires the installation of axial elements (AEs) onto the chromosomes. Cohesin complexes, required for sister chromatid cohesion during mitosis and meiosis, are an essential component of AE formation or maintenance (Klein et al. 1999). After the formation of double-strand breaks to initiate recombination, there is a global reorganization of chromosomes at the leptotene–zygotene transition as telomeres cluster on the nuclear envelope in the bouquet configuration (Harper et al. 2004). As zygotene proceeds, the close association of the paired homologs is stabilized by formation of the synaptonemal complex (SC). During synapsis, a transverse element is installed between the AEs, now called the lateral elements (LEs), to assemble the tripartite ladder-like SC (Page and Hawley 2003). On the basis of electron microscopy (EM) surveys, synapsis typically starts from the ends of chromosomes and works its way inward, although interstitial sites of synapsis initiation are also found, especially in organisms that have long chromosomes such as maize (Burnham et al. 1972; Zickler and Kleckner 1999). As homologous chromosomes synapse, their axes coil around each other. This feature has been called “relational coiling/twisting” of homologs (Moens 1972, 1974; Zickler and Kleckner 1999). The cause or possible function of this coiling is not known.As first described by Gelei, during zygotene chromosomes can become entangled within other synapsing pairs, forming interlocks (Gelei 1921). Either a bivalent (type I interlock) or one unsynapsed chromosome (type II interlock) can become trapped between unsynapsed AEs and caught by the formation of the SC on both sides of the loop. In one example described in maize, multiple chromosomes are trapped, forming a “complex interlock” (Gillies 1981). Since interlocks can be deleterious if left unresolved, mechanisms should be present in meiotic nuclei to prevent or resolve their occurrence (von Wettstein et al. 1984). To date, no mechanism has yet been identified. Interlocks could be resolved by coordinated breakage and rejoining of chromosomes (Holm et al. 1982; Rasmussen 1986; Moens 1990) or by chromosome movement and SC disassembly during zygotene and pachytene (Conrad et al. 2008; Koszul et al. 2008). For chromosomes to escape an interlock by this latter mechanism, one or more telomeres of the interlocking chromosomes either have to separate from each other on the nuclear envelope or be released from the nuclear envelope so that one interlocking chromosome can be pulled away from the other (Rasmussen and Holm 1980).Meiotic chromosomes are large, complex structures that can be tens of microns in length, yet the size of many structural elements of interest such as recombination nodules (50–200 nm diameter) and the LEs of the SC (spaced 100–200 nm apart) is just beyond the resolution of conventional wide-field microscopy (von Wettstein et al. 1984). In favorable light microscope preparations, DNA is organized into chromomeres of condensed chromatin; however, their organization with respect to AEs cannot be easily resolved by conventional light microscopy. Although analysis of spatial organization of AEs at a high resolution can be accomplished by three-dimensional (3D) EM reconstructions of entire nuclei (e.g., Gillies 1973; Zickler and Olson 1975; Goldstein and Moens 1976), 3D EM analysis of even one cell is arduous. It has been difficult to describe the spatial behavior of AEs during synapsis, which constrains our understanding of the kinetics of this dynamic process. Three-dimensional structured illumination microscopy (3D-SIM), a new method of superresolution light microscopy, can achieve a resolution of better than 100 nm in the lateral (xy) plane and 250 nm along the z-axis (Gustafsson 2008; Gustafsson et al. 2008; Schermelleh et al. 2008). The advantages of 3D-SIM over EM are the ability to use FISH and immunolocalizations with fluorescently labeled proteins to specifically and simultaneously detect DNA, RNA, and proteins on chromosomes and the ability to obtain three-dimensional information from thick specimens.Maize, a diploid (2n = 20) monocot grass of the Poaceae, is one of the few organisms with a large genome in which chromatin structure, homologous pairing, and synapsis are amenable to analysis by a combination of cytological, genetic, molecular, and biochemical techniques (Cande et al. 2009). Here, we use 3D-SIM to study chromomere and AE/LE organization of maize pachytene chromosomes. In maize, the cohesin REC8 alpha-kleisin homolog, encoded by absence of first division1 (afd1), is required for maintenance of AEs and proper leptotene chromosome structure (Golubovskaya et al. 2006). We used antibodies against AFD1/REC8 and ASYNAPTIC1 (ASY1), the Arabidopsis homolog of the yeast HOP1 gene (Armstrong et al. 2002), to delineate the prophase chromosome axis and show it changes during synapsis. This is the first exploration of chromosome behavior and synapsis at such a high resolution by light microscopy in any organism, and our findings allow us to quantitatively analyze key features of SC axial element behavior during zygotene and pachytene.     

人的Y染色体异染色质与形态生理学变异

对55名Y染色体具有形态上相同的、异染色质几乎全部缺失的男性和55名Y染色体正常的男性进行了80项形态生理学性状的研究。发现在这两个组之间大部分性状的平均值并无显著性差异,但有些血液学指标有显著差异。用模式认辨法确定出20个性状的组合可以区分这两个组,其认辨误差为4.6%。认辨系统包括的有价值的性状为心电图指标(占性状的25%)、某些人类学指标、血液学指标和个体年龄。结果表明,Y染色体异染色质可能在人的个体发育过程中对形态生理学性状间的表型关系起一种修饰作用。     

Prdm9 and Meiotic Cohesin Proteins Cooperatively Promote DNA Double-Strand Break Formation in Mammalian Spermatocytes

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X Chromosome Control of Meiotic Chromosome Synapsis in Mouse Inter-Subspecific Hybrids

Hybrid sterility (HS) belongs to reproductive isolation barriers that safeguard the integrity of species in statu nascendi. Although hybrid sterility occurs almost universally among animal and plant species, most of our current knowledge comes from the classical genetic studies on Drosophila interspecific crosses or introgressions. With the house mouse subspecies Mus m. musculus and Mus m. domesticus as a model, new research tools have become available for studies of the molecular mechanisms and genetic networks underlying HS. Here we used QTL analysis and intersubspecific chromosome substitution strains to identify a 4.7 Mb critical region on Chromosome X (Chr X) harboring the Hstx2 HS locus, which causes asymmetrical spermatogenic arrest in reciprocal intersubspecific F1 hybrids. Subsequently, we mapped autosomal loci on Chrs 3, 9 and 13 that can abolish this asymmetry. Combination of immunofluorescent visualization of the proteins of synaptonemal complexes with whole-chromosome DNA FISH on pachytene spreads revealed that heterosubspecific, unlike consubspecific, homologous chromosomes are predisposed to asynapsis in F1 hybrid male and female meiosis. The asynapsis is under the trans- control of Hstx2 and Hst1/Prdm9 hybrid sterility genes in pachynemas of male but not female hybrids. The finding concurred with the fertility of intersubpecific F1 hybrid females homozygous for the Hstx2^Mmm allele and resolved the apparent conflict with the dominance theory of Haldane''s rule. We propose that meiotic asynapsis in intersubspecific hybrids is a consequence of cis-acting mismatch between homologous chromosomes modulated by the trans-acting Hstx2 and Prdm9 hybrid male sterility genes.     

Sex Chromosome Meiotic Drive in Stalk-Eyed Flies

Meiotically driven sex chromosomes can quickly spread to fixation and cause population extinction unless balanced by selection or suppressed by genetic modifiers. We report results of genetic analyses that demonstrate that extreme female-biased sex ratios in two sister species of stalk-eyed flies, Cyrtodiopsis dalmanni and C. whitei, are due to a meiotic drive element on the X chromosome (X(d)). Relatively high frequencies of X(d) in C. dalmanni and C. whitei (13-17% and 29%, respectively) cause female-biased sex ratios in natural populations of both species. Sex ratio distortion is associated with spermatid degeneration in male carriers of X(d). Variation in sex ratios is caused by Y-linked and autosomal factors that decrease the intensity of meiotic drive. Y-linked polymorphism for resistance to drive exists in C. dalmanni in which a resistant Y chromosome reduces the intensity and reverses the direction of meiotic drive. When paired with X(d), modifying Y chromosomes (Y(m)) cause the transmission of predominantly Y-bearing sperm, and on average, production of 63% male progeny. The absence of sex ratio distortion in closely related monomorphic outgroup species suggests that this meiotic drive system may predate the origin of C. whitei and C. dalmanni. We discuss factors likely to be involved in the persistence of these sex-linked polymorphisms and consider the impact of X(d) on the operational sex ratio and the intensity of sexual selection in these extremely sexually dimorphic flies.     

Meiotic Chromosome Pairing in Triploid and Tetraploid Saccharomyces Cerevisiae

Meiotic chromosome pairing in isogenic triploid and tetraploid strains of yeast and the consequences of polyploidy on meiotic chromosome segregation are studied. Synaptonemal complex formation at pachytene was found to be different in the triploid and in the tetraploid. In the triploid, triple-synapsis, that is, the connection of three homologues at a given site, is common. It can even extend all the way along the chromosomes. In the tetraploid, homologous chromosomes mostly come in pairs of synapsed bivalents. Multiple synapsis, that is, synapsis of more than two homologues in one and the same region, was virtually absent in the tetraploid. About five quadrivalents per cell occurred due to the switching of pairing partners. From the frequency of pairing partner switches it can be deduced that in most chromosomes synapsis is initiated primarily at one end, occasionally at both ends and rarely at an additional intercalary position. In contrast to a considerably reduced spore viability (~40%) in the triploid, spore viability is only mildly affected in the tetraploid. The good spore viability is presumably due to the low frequency of quadrivalents and to the highly regular 2:2 segregation of the few quadrivalents that do occur. Occasionally, however, quadrivalents appear to be subject to 3:1 nondisjunction that leads to spore death in the second generation.     

植物减数分裂染色体配对与染色体组分析的研究进展

简要介绍了植物减数分裂染色体配对研究.综述了减数分裂染色体配对研究在鉴定异源易位系、确定多倍体物种类型、分析物种间亲缘关系和物种的染色体组来源及探讨杂种不育的细胞遗传学机制等诸多方面的应用进展.分析了影响染色体配对的主要因素,如配对控制体系、遗传背景和外界环境条件等,并展望了染色体配对研究与其他技术结合在染色体组分析中的应用前景.     

Alteration of Terminal Heterochromatin and Chromosome Rearrangements in Derivatives of Wheat-Rye Hybrids

Wheat-rye addition and substitution lines and their self progenies revealed variations in telomeric heterochromatin and centromeres.Furthermore,a mitotically unstable dicentric chromosome and stable mu...     

Smc5/6 Coordinates Formation and Resolution of Joint Molecules with Chromosome Morphology to Ensure Meiotic Divisions

During meiosis, Structural Maintenance of Chromosome (SMC) complexes underpin two fundamental features of meiosis: homologous recombination and chromosome segregation. While meiotic functions of the cohesin and condensin complexes have been delineated, the role of the third SMC complex, Smc5/6, remains enigmatic. Here we identify specific, essential meiotic functions for the Smc5/6 complex in homologous recombination and the regulation of cohesin. We show that Smc5/6 is enriched at centromeres and cohesin-association sites where it regulates sister-chromatid cohesion and the timely removal of cohesin from chromosomal arms, respectively. Smc5/6 also localizes to recombination hotspots, where it promotes normal formation and resolution of a subset of joint-molecule intermediates. In this regard, Smc5/6 functions independently of the major crossover pathway defined by the MutLγ complex. Furthermore, we show that Smc5/6 is required for stable chromosomal localization of the XPF-family endonuclease, Mus81-Mms4^Eme1. Our data suggest that the Smc5/6 complex is required for specific recombination and chromosomal processes throughout meiosis and that in its absence, attempts at cell division with unresolved joint molecules and residual cohesin lead to severe recombination-induced meiotic catastrophe.     

Genome-Wide Identification of Chromatin Transitional Regions Reveals Diverse Mechanisms Defining the Boundary of Facultative Heterochromatin

Due to the self-propagating nature of the heterochromatic modification H3K27me3, chromatin barrier activities are required to demarcate the boundary and prevent it from encroaching into euchromatic regions. Studies in Drosophila and vertebrate systems have revealed several important chromatin barrier elements and their respective binding factors. However, epigenomic data indicate that the binding of these factors are not exclusive to chromatin boundaries. To gain a comprehensive understanding of facultative heterochromatin boundaries, we developed a two-tiered method to identify the Chromatin Transitional Region (CTR), i.e. the nucleosomal region that shows the greatest transition rate of the H3K27me3 modification as revealed by ChIP-Seq. This approach was applied to identify CTRs in Drosophila S2 cells and human HeLa cells. Although many insulator proteins have been characterized in Drosophila, less than half of the CTRs in S2 cells are associated with known insulator proteins, indicating unknown mechanisms remain to be characterized. Our analysis also revealed that the peak binding of insulator proteins are usually 1–2 nucleosomes away from the CTR. Comparison of CTR-associated insulator protein binding sites vs. those in heterochromatic region revealed that boundary-associated binding sites are distinctively flanked by nucleosome destabilizing sequences, which correlates with significant decreased nucleosome density and increased binding intensities of co-factors. Interestingly, several subgroups of boundaries have enhanced H3.3 incorporation but reduced nucleosome turnover rate. Our genome-wide study reveals that diverse mechanisms are employed to define the boundaries of facultative heterochromatin. In both Drosophila and mammalian systems, only a small fraction of insulator protein binding sites co-localize with H3K27me3 boundaries. However, boundary-associated insulator binding sites are distinctively flanked by nucleosome destabilizing sequences, which correlates with significantly decreased nucleosome density and increased binding of co-factors.     

Evidence for Heterochromatin Involvement in Chromosome Breakage in Maize Callus Culture

ng from delayed separation of chromatids and typical bridgeswere observed in Feulgen preparations. The analysis of C-bandedanaphases showed that delayed chromatids were held togetherat heterochromatic knob sites (primary event), and the presenceof typical bridges with and without bands corresponding to knobs.These events suggest the occurrence of breakage-fusion-bridge(BFB) cycles initiated by chromosome arms broken during theprimary event. Additional evidence for such a mechanism wasthe presence of gross aberrations involving chromosome 7, detectedin several C-banded metaphases of some cultures. It is hypothesizedthat such aberrations are duplication deficiencies producedby BFB cycles and chromosome healing that would have occurredafter some cell divisions. Zea mays L.; maize; tissue culture; chromosome breakage; heterochromatin; C-banding