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。     

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.     

党参的体细胞胚发生及不同发育阶段几种同工酶的分析

以党参为材料,在附加0.1 mg·L-1 2,4-D、0.3 mg·L-1 6-BA和3%蔗糖的MS培养基上获得大量发育良好的体细胞胚. 附加6-BA可以使胚性愈伤组织进行芽的分化,而添加0.1%活性炭后仅有根的分化.利用梯度凝胶电泳进行同工酶的研究表明在胚性愈伤组织和体细胞胚之间,酶谱差异比较大;而在不同发育时期的体细胞胚之间,差异较小.并讨论了同工酶酶谱和酶活性变化与体细胞胚发生、发育的关系.     

The Multiple Roles of Cohesin in Meiotic Chromosome Morphogenesis and Pairing

Sister chromatid cohesion, mediated by cohesin complexes, is laid down during DNA replication and is essential for the accurate segregation of chromosomes. Previous studies indicated that, in addition to their cohesion function, cohesins are essential for completion of recombination, pairing, meiotic chromosome axis formation, and assembly of the synaptonemal complex (SC). Using mutants in the cohesin subunit Rec8, in which phosphorylated residues were mutated to alanines, we show that cohesin phosphorylation is not only important for cohesin removal, but that cohesin's meiotic prophase functions are distinct from each other. We find pairing and SC formation to be dependent on Rec8, but independent of the presence of a sister chromatid and hence sister chromatid cohesion. We identified mutations in REC8 that differentially affect Rec8's cohesion, pairing, recombination, chromosome axis and SC assembly function. These findings define Rec8 as a key determinant of meiotic chromosome morphogenesis and a central player in multiple meiotic events.     

Isozyme Profile During Somatic Embryogenesis and In Vitro Flowering of Bambusa vulgaris

Peroxidase and esterase isozymes were investigated during plant regeneration via somatic embryogenesis in Bambusa vulgaris, The transition of non-embryogenic calli to embryogenic calli, somatic embryo development, germination and subsequent flowering of somatic embryo derived shoots were associated with selective expression or repression of isoforms of peroxidase and esterase. Non-embryogenic callus showed six peroxidase and four esterase bands. During somatic embryogenesis and germination of somatic embryos, some bands were suppressed and new isoforms of peroxidase and esterase appeared. During flowering, in addition to four peroxidase bands, a new unique esterase band ‘a’ appeared. Each developmental stage was thus associated with a definite isozyme profile.     

Two Types of Sites Required for Meiotic Chromosome Pairing in Caenorhabditis Elegans

Previous studies have shown that isolated portions of Caenorhabditis elegans chromosomes are not equally capable of meiotic exchange. These results led to the proposal that a homolog recognition region (HRR), defined as the region containing those sequences enabling homologous chromosomes to pair and recombine, is localized near one end of each chromosome. Using translocations and duplications we have localized the chromosome I HRR to the right end. Whereas the other half of chromosome I did not confer any ability for homologs to pair and recombine, deficiencies in this region dominantly suppressed recombination to the middle of the chromosome. These deletions may have disrupted pairing mechanisms that are secondary to and require an HRR. Thus, the processes of pairing and recombination appear to utilize at least two chromosomal elements, the HRR and other pairing sites. For example, terminal sequences from other chromosomes increase the ability of free duplications to recombine with their normal homologs, suggesting that telomere-associated sequences, homologous or nonhomologous, play a role in facilitating meiotic exchange. Recombination can also initiate at internal sites separated from the HRR by chromosome rearrangement, such as deletions of the unc-54 region of chromosome I. When crossing over was suppressed in a region of chromosome I, compensatory increases were observed in other regions. Thus, the presence of the HRR enabled recombination to occur but did not determine the distribution of the crossover events. It seems most likely that there are multiple initiation sites for recombination once homolog recognition has been achieved.     

Morphological Alterations in Sterile Mutant of Pisum Sativum Obtained Via Somatic Embryogenesis

A sterile mutant of pea (Pisum sativum L. line HM-6) with a number of morphological alterations was found after plant regeneration via somatic embryogenesis. Embryogenic callus was derived from the whole immature zygotic embryo on medium with 2.26 M 2,4-dichlorophenoxyacetic acid. Morphological changes included altered leaflet shape, one pair of leaflets only, altered stipule morphology, shortened internodia, irregular or opposite leaf position on the stem, shortened flower stalk, and aborted flowers resulting in complete sterility. If the isolation of the shoot apex and axillary buds from evidently sterile plant and their culture in vitro resulted in morphologically normal and fertile regenerated plants, the chimaeric nature of R₀ mutant is considered.     

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.     

The Rec102 Mutant of Yeast Is Defective in Meiotic Recombination and Chromosome Synapsis

A mutation at the REC102 locus was identified in a screen for yeast mutants that produce inviable spores. rec102 spore lethality is rescued by a spo13 mutation, which causes cells to bypass the meiosis I division. The rec102 mutation completely eliminates meiotically induced gene conversion and crossing over but has no effect on mitotic recombination frequencies. Cytological studies indicate that the rec102 mutant makes axial elements (precursors to the synaptonemal complex), but homologous chromosomes fail to synapse. In addition, meiotic chromosome segregation is significantly delayed in rec102 strains. Studies of double and triple mutants indicate that the REC102 protein acts before the RAD52 gene product in the meiotic recombination pathway. The REC102 gene was cloned based on complementation of the mutant defect and the gene was mapped to chromosome XII between CDC25 and STE11.     

Morewright (Mwr), a New Meiotic Mutant of Drosophila Melanogaster Affecting Nonexchange Chromosome Segregation

A new meiotic mutation, morewright (mwr) was identified by screening for new mutations that act as dominant enhancers of the dosage-sensitive Drosophila melanogaster female meiotic mutant, nod(DTW). mwr is a recessive meiotic mutant, specifically impairing the segregation of nonexchange chromosomes. Cytological evidence suggests that the meitoic defect in mwr/mwr females is in homologue recognition because the chromosomes appear to be misaligned on an intact spindle. The mwr mutation was recovered during a screen of random P-element insertions on a chromosome with a single insertion located at 50C. The P-element insertion is a recessive female-sterile mutation. While excision of the P element from the mwr-bearing chromosome partially relieves the female sterility, the excisions retain the dominant nod(DTW)-enhancing activity. The mwr meiotic phenotype maps very close to the female-sterile P insertion. Thus the mwr locus appears to encode a function required for partner recognition in meiosis, although its relationship to the neighboring female-sterile mutation remains to be elucidated.     

Roles of Hop1 and Mek1 in Meiotic Chromosome Pairing and Recombination Partner Choice in Schizosaccharomyces pombe

Synaptonemal complex (SC) proteins Hop1 and Mek1 have been proposed to promote homologous recombination in meiosis of Saccharomyces cerevisiae by establishment of a barrier against sister chromatid recombination. Therefore, it is interesting to know whether the homologous proteins play a similar role in Schizosaccharomyces pombe. Unequal sister chromatid recombination (USCR) was found to be increased in hop1 and mek1 single and double deletion mutants in assays for intrachromosomal recombination (ICR). Meiotic intergenic (crossover) and intragenic (conversion) recombination between homologous chromosomes was reduced. Double-strand break (DSB) levels were also lowered. Notably, deletion of hop1 restored DSB repair in rad50S meiosis. This may indicate altered DSB repair kinetics in hop1 and mek1 deletion strains. A hypothesis is advanced proposing transient inhibition of DSB processing by Hop1 and Mek1 and thus providing more time for repair by interaction with the homologous chromosome. Loss of Hop1 and Mek1 would then result in faster repair and more interaction with the sister chromatid. Thus, in S. pombe meiosis, where an excess of sister Holliday junction over homologous Holliday junction formation has been demonstrated, Hop1 and Mek1 possibly enhance homolog interactions to ensure wild-type level of crossover formation rather than inhibiting sister chromatid interactions.Sexual reproduction in eukaryotes involves formation of haploid gametes from diploid cells by one round of DNA replication, pairing of the homologous chromosomes, and recombination and then by the two meiotic divisions (53). In fungi the gametes differentiate into haploid spores, which germinate to form vegetative cells. Crossover (CO) formation between homologous chromosomes and DNA repair processes between sister chromatids are required for spore viability (10, 55, 58).In vegetative cells homologous recombination (HR) is important for repair of DNA damage and stalled replication forks, with the sister chromatid as the preferred partner (28). Many of the enzymes involved in mitotic HR also contribute to meiotic recombination. In addition, meiosis-specific cytological structures and enzymes enhance recombination frequency (meiotic induction) and shift partner preference from sister chromatids to homologous chromosomes (3, 47, 64, 74). In detail the steps of HR vary between different types of sequence organization (allelic versus sister versus ectopic), between different types of DNA damage, between meiotic and mitotic cells, and between species (10, 55, 58).Meiotic recombination, including CO formation, is initiated by DNA double-strand breaks (DSBs). In Saccharomyces cerevisiae and other eukaryotes, DSBs are formed by Spo11. Many cofactors are required (29). The Schizosaccharomyces pombe homolog is Rec12, also requiring auxiliary factors whose elimination leads to loss of meiotic DSB formation (12). The 5′ single-strand ends at DSBs are processed by nucleases. In S. cerevisiae the MRX complex made up by the proteins Rad50, Mre11, and Xrs2 is required for this resection, as well as for DSB formation. The corresponding MRN complex of S. pombe (Rad50, Rad32, and Nbs1) is not required for DSB formation but is essential for DSB repair (43, 72). Deletion of rad50, rad32, or ctp1 (homologous to SAE2/COM1 in S. cerevisiae and CtIP in humans) leads to very low spore viability. These proteins are also essential for DSB processing (23, 24, 32, 43, 60, 62).Free DNA 3′ ends at DSBs are recruited for invasion of a sister or homologous chromatid by the strand transfer proteins Rad51 and Dmc1, again involving many accessory proteins (16). This results in the central intermediates of HR: heteroduplex DNA consisting of single strands originating from different chromatids and Holliday junctions (HJs). In S. cerevisiae HJs form preferably between homologs with a two- to sixfold excess over intersister HJs (64). Surprisingly, meiotic HJs form with about a fourfold excess between sisters in S. pombe (11). Eventually the intermediates are resolved into crossover (CO) and noncrossover (NCO) events. COs show exchange of the flanking sequences of the two chromatids involved and usually carry a patch of conversion (unilateral transfer of DNA sequences from one chromatid to its interacting partner) near the DSB site. NCOs are conversion events without associated COs (22). In S. pombe loss of core HR functions leads to very low spore viability: deletion of rad51 but not of dmc1 (20), double mutation of rad54 and rdh54 (7), inactivation of the endonuclease activity encoded by mus81 and eme1 (5, 52), and combined deletion of rad22 and rti1 (homologs of RAD52 of S. cerevisiae). But, differently from the other core functions, Rad22 and Rti1 are not required for CO and NCO (50).Early in meiotic prophase of many eukaryotes, axial elements (called lateral elements in later stages) form along sister chromatids, and pairing of homologous chromosomes is initiated, leading to juxtaposition of the homologous chromosomes along their whole length in the synaptonemal complex (SC) (54). In S. pombe no SC is formed, but linear elements (LEs), resembling axial elements of other eukaryotes, are formed. LEs do not form continuously along the chromosomes (1) but load the proteins Rec10, Hop1, and Mek1 (36, 44, 57), which are homologs of, or at least related, to the S. cerevisiae proteins Red1, Hop1, and Mek1, respectively, localizing to axial/lateral elements (2, 67). Hop1 carries a HORMA domain, also present in proteins associating with axial elements and regulating the progress of recombination in higher eukaryotes: Arabidopsis thaliana (61), Caenorhabditis elegans (9, 41), and mammals (18).In S. cerevisiae localization of Hop1 and Mek1 (meiosis-specific protein kinase) to axial elements is dependent on Red1 (2, 67). Mutation of the three S. cerevisiae genes results in reduction of DSB formation, CO and conversion frequencies, and spore viability (26, 31, 59). Direct comparison of unequal sister chromatid recombination (USCR) frequencies in an assay excluding the scoring of intrachromatid recombination (ICR) revealed no increase in the hop1 null mutant but about fourfold increases in the red1 and mek1 null mutants (69). The S. cerevisiae Hop1, Red1, and Mek1 proteins are involved in biasing meiotic DSB repair to occur between homologous chromosomes rather than between sister chromatids (47). Activated Mek1 kinase is required for the inhibition of sister chromatid-mediated DSB repair by Rad51, when the DMC1 gene is deleted and the meiotic recombination checkpoint is activated (4, 27, 38, 47). For Mek1 activation, phosphorylation of Hop1 by the Mec1/Tel1 kinases is also required (6).Less is known about the S. pombe proteins. Hop1 of S. pombe was identified as a nonsignificant hit by sequence comparison with full-length S. cerevisiae Hop1 and contains an N-terminal HORMA domain and a central zinc finger motif like Hop1 in S. cerevisiae. In addition they share a short homology block toward the C terminus (36). The Mek1 protein of S. pombe shares 34% identity and 54% similarity with its S. cerevisiae counterpart along the whole sequence. It contains an FHA domain in the N-terminal part like the other members of its family of checkpoint kinases and is involved in regulation of the meiotic cell cycle (57). Hop1 and Mek1 are strongly expressed in meiosis but not expressed or only slightly expressed in vegetative cells (42, 57). In prophase both proteins localize to LEs as defined by colocalization with the LE component Rec10 (36). Deletion of the distant RED1 homolog rec10 abolishes LE formation (36, 44) and strongly reduces meiotic recombination (17, 70). Rec10, but not Hop1 and Mek1, is required for localization of Rec7 (a distant homolog of S. cerevisiae Rec114) to meiotic chromosomes (34). Rec7 and Rec10 are required for Rec12 activity (12, 29).Obtaining information on the functions of Hop1 and Mek1 in S. pombe was the aim of the work presented here, especially on their possible roles in homolog versus sister discrimination for DSB repair. Deletion mutants have been studied with respect to spore viability and the frequencies of CO and conversion. They have also been assessed for genetic recombination events between sister chromatids in the known PS1 assay (63) and the newly developed VL1 assay (for details, see Fig. ​Fig.3).3). Physical analysis of DSB formation and repair has been performed in meiotic time course experiments. It is proposed that S. pombe Hop1 and Mek1 are promoting interactions between homologous chromosomes rather than inhibiting interactions between sister chromatids.Open in a separate windowFIG. 3.PS1 and VL1 assay systems for intrachromosomal recombination. Strains with constructs carrying repeated DNA sequences have been assayed for prototroph formation either by intrachromatid recombination (ICR, yielding prototrophs only in PS1) or by unequal sister chromatid recombination (USCR, in PS1 and VL1). Crosses of the constructs were performed with strains carrying a deletion of the ade6 gene to exclude other homologous recombination events. (A) The PS1 assay involves copies of the ade6 gene inactivated by either the hot spot mutation M26 or the mutation 469. The repeated sequences are separated by the ura4⁺ marker (63). ICR (left) or USCR (right) between the repeated sequences can lead to formation of adenine prototrophs that have lost the ura4⁺ marker by crossover (CO) or single-strand annealing (SSA) events. Adenine prototrophs maintaining the ura4⁺ marker can derive from noncrossover (NCO) events. Both types of pairing may lead to CO or NCO products. (B) The newly constructed VL1 assay (see the supplemental material) involves different truncations of the ade6 gene separated by the hyg^R marker (also called hphMX6), conferring hygromycin resistance. The left truncation carries a 3′ portion of ade6; the right truncation carries a 5′ portion of ade6. While the gray parts of the truncations are not overlapping, the white sections of 500-bp length are of almost identical sequence, allowing for homologous pairing. CO and SSA products resulting from ICR retain only the central portion of ade6 and remain auxotrophic. Adenine prototrophic CO and NCO products resulting from USCR both retain hygromycin resistance. Note that NCO events may arise through loop formation of one sister chromatid and pairing with a single block (500 bp) of the repeated ade6 sequence (39).     

Use of a Flowering Mutant to Investigate Changes in Carbohydrates during Floral Transition in Red Clover

A mutant red clover is used to investigate changes in the carbohydratestatus of leaves and apical zones of red clover (Trifolium pratenseL.) during the transition to flowering. Mutant plants will notflower during normally inductive photoperiods unless suppliedwith exogenous gibberellin. Changes in carbohydrates are describedin normal-flowering plants following transfer to inductive long-daysas well as in mutant plants with and without gibberellin. Theeffects of gibberellin on mutants under short-days—whenstem elongation occurs but flowering does not ensue—arealso investigated. Carbohydrate concentrations are virtuallyidentical in the leaves of normal and mutant plants under short-daysalthough significant differences exist at the apex. Transferfrom short to long-days results in major alterations in thecarbohydrate status, but changes specifically associated withthe onset of flowering were not apparent. These results arediscussed in relation to currently held views on the triggerinvolved in the initiation of flowering. Key words: Flowering, carbohydrates, mutants, gibberellins     

The Relation between the Axial Complex of Meiotic Prophase Chromosomes and Chromosome Pairing in a Salamander (Plethodon cinereus)

An investigation of the structure of meiotic chromosomes from primary spermatocytes of two salamanders, Plethodon cinereus and Desmognathus fusca, has been made using correlated light and electron microscopy. Feulgen squashes were compared with stained sections and these related to adjacent thin sections in the electron microscope. A transition from the familiar cytological preparation to the electron image was thus effected. A linear complex consisting of three parallel strands has been observed with the electron microscope, passing along the central axis of primary spermatocyte chromosomes. The complex is similar to that found in comparable chromosomes from at least a dozen animal species. The structure in Plethodon is described in detail. Synapsis has been positively identified as the stage of meiotic prophase at which the complex occurs. Thus the complex is a part of bivalent chromosomes. It has not been seen in other stages or other divisions and is thus thought to be exclusively of synaptic occurrence. The term synaptinemal complex is suggested for the entire structure. By virtue of the material condensed around it, the complex is also seen in the light microscope where it appears as a fine, densely Feulgen-positive central core along the chromosome. The complex is thus closely associated with DNA, if not at least in part, composed of it. In the stages studied, homologous chromosomes are not always completely paired. The lateral elements of the complex separate and follow the single chromosome axes at these points. The central element disappears and thus may be a phenomenon of pairing. It is concluded that the lateral elements of the synaptinemal complex may more correctly be a "core" of the single meiotic prophase chromosome, possibly being concerned with its linear organization.     

Meiotic Disjunction of Homologs in Saccharomyces Cerevisiae Is Directed by Pairing and Recombination of the Chromosome Arms but Not by Pairing of the Centromeres

We explored the behavior of meiotic chromosomes in Saccharomyces cerevisiae by examining the effects of chromosomal rearrangements on the pattern of disjunction and recombination of chromosome III during meiosis. The segregation of deletion chromosomes lacking part or all (telocentric) of one arm was analyzed in the presence of one or two copies of a normal chromosome III. In strains containing one normal and any one deletion chromosome, the two chromosomes disjoined in most meioses. In strains with one normal chromosome and both a left and right arm telocentric chromosome, the two telocentrics preferentially disjoined from the normal chromosome. Homology on one arm was sufficient to direct chromosome disjunction, and two chromosomes could be directed to disjoin from a third. In strains containing one deletion chromosome and two normal chromosomes, the two normal chromosomes preferentially disjoined, but in 4-7% of the tetrads the normal chromosomes cosegregated, disjoining from the deletion chromosome. Recombination between the two normal chromosomes or between the deletion chromosome and a normal chromosome increased the probability that these chromosomes would disjoin, although cosegregation of recombinants was observed. Finally, we observed that a derivative of chromosome III in which the centromeric region was deleted and CEN5 was integrated at another site on the chromosome disjoined from a normal chromosome III with fidelity. These studies demonstrate that it is not pairing of the centromeres, but pairing and recombination along the arms of the homologs, that directs meiotic chromosome segregation.     

落叶松体胚发育中5个miRNA前体与成熟体的表达

利用同源比对或RACE克隆了5个落叶松(Larix leptolepis) miRNA前体。结果显示, 在各物种miRNA前体间, 成熟序列高度相似, 但其它序列相似度差异大, 序列相似度与亲缘关系有关。采用qRT-PCR分析了5个miRNA、前体和靶基因在落叶松体胚8个发育阶段的表达变化。结果显示, miRNA表达最高峰出现在后期子叶胚, 暗示与促进胚胎休眠有关; 表达次高峰出现时期不同, 表现为miR397和miR408在PEMIII, miR398在早期单胚, miR156和miR166在早期子叶胚, 表明其与保持薄细胞壁、质子传递、顶端分生组织形成等调控有关。miRNA成熟体表达与前体含量不呈线性相关, 可能受多重调控。研究结果对于阐明MIR基因进化、表达调控及在体胚发育中的调控功能具有重要理论意义。     

Regional Bivalent-Univalent Pairing versus Trivalent Pairing of a Trisomic Chromosome in Saccharomyces Cerevisiae

In meiosis I, homologous chromosomes pair, recombine and segregate to opposite poles. These events and subsequent meiosis II ensure that each of the four meiotic products has one complete set of chromosomes. In this study, the meiotic pairing and segregation of a trisomic chromosome in a diploid (2n + 1) yeast strain was examined. We find that trivalent pairing and segregation is the favored arrangement. However, insertions near the centromere in one of the trisomic chromosomes leads to preferential pairing and segregation of the ``like' centromeres of the remaining two chromosomes, suggesting that bivalent-univalent pairing and segregation is favored for this region.     

Integration of Linkage and Chromosome Maps of Red Clover ( Trifolium pratense L.)

Red clover (Trifolium pratense L.) is a forage legume and an allogamous diploid plant (2n = 14; 440 Mb). Here, we examine the 7 prometaphase chromosomes of red clover using fluorescence in situ hybridization (FISH) with ribosomal RNA sequences, pericentromeric and telomeric repeats, as well as bacterial artificial chromosome (BAC) clones. Position of hybridization signals and chromosome condensation patterns were quantified by the help of the chromosome image analysis system ver. 4.0 (CHIAS IV). Fourteen BAC clones belonging to linkage groups (LG) 1-7 hybridized to individual chromosomes 4, 2, 6, 5, 1, 7, and 3, respectively. Quantitative analysis of FISH mapping and chromosome analysis using CHIAS IV allowed us to construct a quantitative idiogram that constitutes the comprehensive chromosome map of red clover. Chromosomal positions of the 26S rDNA locus were detected at a heterozygous locus on chromosome 6 in the variety HR, and polymorphisms of rDNA loci were observed in other varieties, although chromosomal positions of some BAC clones did not vary among HR and other varieties. These results demonstrate chromosomal collinearity among allogamous red clover varieties. This integration of genetic linkage and quantitative chromosome maps should provide valuable insight into allogamous legume genetics.     

A Defective Meiotic Outcome of a Failure in Homologous Pairing and Synapsis Is Masked by Meiotic Quality Control

Successful gamete production is ensured by meiotic quality control, a process in which germ cells that fail in bivalent chromosome formation are eliminated during meiotic prophase. To date, numerous meiotic mutants have been isolated in a variety of model organisms, using defects associated with a failure in bivalent formation as hallmarks of the mutant. Presumably, the meiotic quality control mechanism in those mutants is overwhelmed. In these mutants, all germ cells fail in bivalent formation, and a subset of cells seem to survive the elimination process and develop into gametes. It is possible that mutants that are partially defective in bivalent formation were missed in past genetic screens, because no evident meiotic defects associated with failure in bivalent formation would have been detectable. Meiotic quality control effectively eliminates most failed germ cells, leaving predominately successful ones. Here, we provide evidence supporting this possibility. The Caenorhabditis elegans mrg-1 loss-of-function mutant does not appear to be defective in bivalent formation in diakinesis oocytes. However, defects in homologous chromosome pairing and synapsis during the preceding meiotic prophase, prerequisites for successful bivalent formation, were observed in most, but not all, germ cells. Failed bivalent formation in the oocytes became evident once meiotic quality control was abrogated in the mrg-1 mutant. Both double-strand break repair and synapsis checkpoints are partly responsible for eliminating failed germ cells in the mrg-1 mutant. Interestingly, removal of both checkpoint activities from the mrg-1 mutant is not sufficient to completely suppress the increased germline apoptosis, suggesting the presence of a novel meiotic checkpoint mechanism.