The morphology and development of Nosema carpocapsae,a microsporidian pathogen of the codling moth,Cydia pomonella (Lepidoptera: Tortricidae) in New Zealand

The morphology of Nosema carpocapsae and its development in experimentally infected codling moth larvae are described. Spherical uninucleate meronts were the first stages. Nuclear division produced binucleate meronts which were the most abundant vegetative stage, although additional uninucleate and a few tetranucleate meronts were also observed at this time. All meronts were spherical and ranged from 2.8 to 5.8 μm in diameter. Uninucleate and binucleate fusiform sporonts then appeared followed by some tetranucleate and dividing forms. Oval sporoblasts developed after these and did not divide before maturing into spores. Sporonts were approximately 5.0 to 7.9 × 2.4 to 3.0 μm. Spores developed in all host tissues except the nervous tissue. The binucleate spores showed considerable variation in spore size, 2.4 to 3.9 × 1.3 to 3.1 μm (alcohol fixed, Giemsa stained). The polar filament was usually coiled 11 times (range 9 to 13) at an angle of 53° to the long axis of the spore. Its maximum observed length was 75 μm.     

Life Cycle of Culicosporella lunata (Hazard & Savage, 1970) Weiser, 1977 (Microspora) as Revealed in the Light Microscope with a Redescription of the Genus and Species1

Light microscopy studies of Culicosporella lunata (Hazard & Savage), a parasite of the mosquito Culex pilosus (Dyar & Knab), revealed two sporogonial sequences. One sequence begins with diplokaryotic meronts that undergo repeated nuclear divisions to produce sporogonial plasmodia with nuclei in diplokaryotic arrangement. These plasmodia form rosette-like clusters of sporoblasts during incomplete cytokinesis and, eventually, binucleate spores. These spores initiate infections in healthy larvae when they ingest spores. The second sequence begins with diplokaryotic meronts that undergo karyogamy and meiosis to form Thelohania-like sporonts and haploid spores. Anomalies are often observed in these sporonts which result in aberrant spores, usually fewer than eight, in an accessory (pansporoblastic) membrane. Normal haploid spores are morphologically similar to those of species of Amblyospora. The genus and the type species are redefined based on new information presented here and it and the type species are placed in the family Amblyosporidae.     

Meiosis-like reduction during somatic embryogenesis of Arabidopsis thaliana

Summary Somatic meiosis-like reduction was observed in some cells of the embryogenic callus of Arabidopsis thaliana. Two types were identified. One type was somatic chromosome reductional grouping, in wich the chromesomes in a cell were separated direetly at either prophase or metaphase. Chromosome reductional grouping happened more frequently in polyploid cells, and the morphology of the chromosomes did not show the role of the spindle fibers. The other type was somatic meiosis which was analogous to the process of gametogenesis, characterized by the pairing and synapsis of homologous chromosomes. The roles of somatic meiosis-like reduction in somatic embryogenesis and somaclonal variations are discussed     

Chromosome Axis Defects Induce a Checkpoint-Mediated Delay and Interchromosomal Effect on Crossing Over during Drosophila Meiosis

Crossovers mediate the accurate segregation of homologous chromosomes during meiosis. The widely conserved pch2 gene of Drosophila melanogaster is required for a pachytene checkpoint that delays prophase progression when genes necessary for DSB repair and crossover formation are defective. However, the underlying process that the pachytene checkpoint is monitoring remains unclear. Here we have investigated the relationship between chromosome structure and the pachytene checkpoint and show that disruptions in chromosome axis formation, caused by mutations in axis components or chromosome rearrangements, trigger a pch2-dependent delay. Accordingly, the global increase in crossovers caused by chromosome rearrangements, known as the “interchromosomal effect of crossing over,” is also dependent on pch2. Checkpoint-mediated effects require the histone deacetylase Sir2, revealing a conserved functional connection between PCH2 and Sir2 in monitoring meiotic events from Saccharomyces cerevisiae to a metazoan. These findings suggest a model in which the pachytene checkpoint monitors the structure of chromosome axes and may function to promote an optimal number of crossovers.     

Increased variability in nuclear DNA content of testis cells and spermatozoa from mice with irregular meiotic segregation.

Variability in DNA content to testis cells and sperm from F₁ hybrids between the laboratory mouse (M. muscullus) and the tobacco mouse (M. poschiavinus), has been determined by flow cytometry (FMC). The F₁ hybrid mouse is known to be heterozygous for seven metacentric chromosomes produced by Robertsonian fusion. Enriched populations of nuclei from late pachytene spermatocytes and round spermatids were obtained by velocity sedimentation. These nuclei, as well as epididymal sperm nuclei and spleen cells, were stained by the acriflavin-Feulgen technique for DNA and measured by FCM. Peaks in the fluorescence intensity frequency distributions resulting from these measurements were analyzed to determine their mean fluorescence intensities and their widths (coefficients of variation). Because mean intensities of corresponding cell types from M. musculus and the F₁ hybrids were identical, the average DNA contents were taken to be the same. The average coefficients of variation of the peaks to fluorescence from the pachytene, spermatid, and sperm nuclei and spleen cells from M. muscullus animals were about 5%. While the peaks of fluorescence from spleen cells and pachytene nuclei from f₁ hybrids also had average coefficients of variation of 5%, post-meiotic nuclei from spermatids and spermatozoa had coefficients of variationof 8%. From these results we conclude that, in these F₁ hybrids, abnormal meiotic segregation causes an increased variability of 6% in the amount of DNA in the spermatozoa.     

Synaptonemal complexes and chromosome chains in the rodent Ellobius talpinus heterozygous for ten Robertsonian translocations

Synaptonemal complexes (SC) in four Ellobius talpinus males heterozygous for ten Robertsonian translocations were examined with an electron microscope using a surface-spreading technique. A total of 136 late zygotene and pachytene spermatocytes were examined. From one to three completely paired SC trivalents were found in each early pachytene spermatocyte. The lateral elements of the short arms of the acrocentric chromosomes in these trivalents were joined with an SC thus forming the third arm of the SC trivalent. At the same stage a few SC trivalents did not contain lateral elements in the pericentromeric region of the metacentric chromosomes and remained unpaired in this region up to mid pachytene. At zygotene and pachytene from two to eight SC trivalents were joined into chains due to formation of SCs between the short arms of acrocentrics of other SC trivalents. These chains are frequent at late zygotene, but are resolved during pachytene into individual trivalents. It is proposed that pairing and SC formation between the short arms of the acrocentric chromosomes results from the monosomy of the short arms and partial DNA homology between these heterochromatic regions. Since crossing over probably does not take place in these segments, the chromosomal chains may subsequently be corrected into trivalents by a dissolution of the SCs combining adjacent trivalents. The correction and disjoining of chains may not be effective in all cells. The cells in which the chains are retained are assumed to be arrested at the pachytene stage.     

拟南芥（Arabidopsis thaliana）体细胞胚胎发生体系中的体细胞类减数分裂研究

在拟南芥生态型LandsbergErecta体细胞胚胎发生体系的胚性愈伤组织中观察到2种类型的体细胞减数分裂现象。一种是体细胞染色体减数分组,其中,处于前期或中期的细胞染色体分为2个或2个以上的组。其共同特点是,染色体直接分开,未观察到纺锤体,从染色体的形态也看不出纺锤体的作用。染色体减数分组较多发生于多倍体细胞中。另一种类型是体细胞减数分裂,这种类型类似于大小孢子发生过程的减数分裂,如第一次分裂前期也有染色体的联会和配对。在脱分化培养基上的胚性愈伤组织中,单倍体细胞约占3%,四倍体细胞约占4%。经体细胞类减数分裂产生的细胞都发生染色体重组。     

XX/XY System of Sex Determination in the Geophilomorph Centipede Strigamia maritima

We show that the geophilomorph centipede Strigamia maritima possesses an XX/XY system of sex chromosomes, with males being the heterogametic sex. This is, to our knowledge, the first report of sex chromosomes in any geophilomorph centipede. Using the recently assembled Strigamia genome sequence, we identified a set of scaffolds differentially represented in male and female DNA sequence. Using quantitative real-time PCR, we confirmed that three candidate X chromosome-derived scaffolds are present at approximately twice the copy number in females as in males. Furthermore, we confirmed that six candidate Y chromosome-derived scaffolds contain male-specific sequences. Finally, using this molecular information, we designed an X chromosome-specific DNA probe and performed fluorescent in situ hybridization against mitotic and meiotic chromosome spreads to identify the Strigamia XY sex-chromosome pair cytologically. We found that the X and Y chromosomes are recognizably different in size during the early pachytene stage of meiosis, and exhibit incomplete and delayed pairing.     

The Yeast Red1 Protein Localizes to the Cores of Meiotic Chromosomes

Mutants in the meiosis-specific RED1 gene of S. cerevisiae fail to make any synaptonemal complex (SC) or any obvious precursors to the SC. Using antibodies that specifically recognize the Red1 protein, Red1 has been localized along meiotic pachytene chromosomes. Red1 also localizes to the unsynapsed axial elements present in a zip1 mutant, suggesting that Red1 is a component of the lateral elements of mature SCs. Anti-Red1 staining is confined to the cores of meiotic chromosomes and is not associated with the loops of chromatin that lie outside the SC. Analysis of the spo11 mutant demonstrates that Red1 localization does not depend upon meiotic recombination. The localization of Red1 has been compared with two other meiosisspecific components of chromosomes, Hop1 and Zip1; Zip1 serves as a marker for synapsed chromosomes. Double labeling of wild-type meiotic chromosomes with anti-Zip1 and anti-Red1 antibodies demonstrates that Red1 localizes to chromosomes both before and during pachytene. Double labeling with anti-Hop1and anti-Red1 antibodies reveals that Hop1 protein localizes only in areas that also contain Red1, and studies of Hop1 localization in a red1 null mutant demonstrate that Hop1 localization depends on Red1 function. These observations are consistent with previous genetic studies suggesting that Red1 and Hop1 directly interact. There is little or no Hop1 protein on pachytene chromosomes or in synapsed chromosomal regions.     

Mouse pachytene checkpoint 2 (trip13) is required for completing meiotic recombination but not synapsis

In mammalian meiosis, homologous chromosome synapsis is coupled with recombination. As in most eukaryotes, mammalian meiocytes have checkpoints that monitor the fidelity of these processes. We report that the mouse ortholog (Trip13) of pachytene checkpoint 2 (PCH2), an essential component of the synapsis checkpoint in Saccharomyces cerevisiae and Caenorhabditis elegans, is required for completion of meiosis in both sexes. TRIP13-deficient mice exhibit spermatocyte death in pachynema and loss of oocytes around birth. The chromosomes of mutant spermatocytes synapse fully, yet retain several markers of recombination intermediates, including RAD51, BLM, and RPA. These chromosomes also exhibited the chiasmata markers MLH1 and MLH3, and okadaic acid treatment of mutant spermatocytes caused progression to metaphase I with bivalent chromosomes. Double mutant analysis demonstrated that the recombination and synapsis genes Spo11, Mei1, Rec8, and Dmc1 are all epistatic to Trip13, suggesting that TRIP13 does not have meiotic checkpoint function in mice. Our data indicate that TRIP13 is required after strand invasion for completing a subset of recombination events, but possibly not those destined to be crossovers. To our knowledge, this is the first model to separate recombination defects from asynapsis in mammalian meiosis, and provides the first evidence that unrepaired DNA damage alone can trigger the pachytene checkpoint response in mice.     

Die Entstehung multipler Oocytennukleolen aus akzessorischen DNS-Körpern bei Gryllus domesticus

The early stages of female and male germ cells have been investigated in Feulgen squash preparations, in unfixed state with phase contrast optics and in the electron microscope. The DNA axes of the ring-shaped multiple nucleoli in the growing oocytes of Gryllus arise from compact DNA bodies which are found in oogonia of young larvae and in oocytes prior to the growth period. The nuclei of the early oogonia contain several little DNA bodies whereas young oocytes at leptotene, zygotene and pachytene have only one body which is bigger than at earlier stages (Pig. 3). At metaphase and anaphase during oogonial mitosis the DNA body has a filamentous shape distinguishable from the compact chromosomes (Fig. 5). In oogonia as well as at leptotene and zygotene stages, nucleoli are produced in the peripheral, uncoiled parts of each DNA body whereas the compact interior is completely free of nucleolar material (Figs. 4, 12). At pachytene, the whole DNA body begins to despiralize, and single DNA strands are released into the nucleoplasm. These strands form hundreds of multiple nucleoli which finally are dispersed in the germinal vesicle (Fig. 11). — Incorporation studies with radio-active thymidine have shown that DNA synthesis in the DNA body is not synchronous with the S-phase of the chromosomes (Fig. 7). — The DNA body is an own formation distinct from the sex chromosomes (in contrast to the opinion of Sotelo and Wettstein, 1964). Although the positive heteropycnotic X-chromosome in the germ cells of the male cricket is very similar to the DNA body of the female (Fig. 8), there is no regular contact between sex chromosome and nucleolus neither in spermatogonia nor in spermatocytes (Figs. 9, 14). In all probability, the site of the nucleolar organizer is autosomal. — It is suggested that the amplification of the nucleolar genes in Gryllus oocytes results in an accumulation of ribosomal RNA for use during the early cleavage stages of the embryo     

Dynamics of DNA Replication during Premeiosis and Early Meiosis in Wheat

Meiosis is a specialised cell division that involves chromosome replication, two rounds of chromosome segregation and results in the formation of the gametes. Meiotic DNA replication generally precedes chromosome pairing, recombination and synapsis in sexually developing eukaryotes. In this work, replication has been studied during premeiosis and early meiosis in wheat using flow cytometry, which has allowed the quantification of the amount of DNA in wheat anther in each phase of the cell cycle during premeiosis and each stage of early meiosis. Flow cytometry has been revealed as a suitable and user-friendly tool to detect and quantify DNA replication during early meiosis in wheat. Chromosome replication was detected in wheat during premeiosis and early meiosis until the stage of pachytene, when chromosomes are associated in pairs to further recombine and correctly segregate in the gametes. In addition, the effect of the Ph1 locus, which controls chromosome pairing and affects replication in wheat, was also studied by flow cytometry. Here we showed that the Ph1 locus plays an important role on the length of meiotic DNA replication in wheat, particularly affecting the rate of replication during early meiosis in wheat.     

Evidence for Emergence of Sex-Determining Gene(s) in a Centromeric Region in Vasconcellea parviflora

Sex chromosomes have been studied in many plant and animal species. However, few species are suitable as models to study the evolutionary histories of sex chromosomes. We previously demonstrated that papaya (Carica papaya) (2n = 2x = 18), a fruit tree in the family Caricaceae, contains recently emerged but cytologically heteromorphic X/Y chromosomes. We have been intrigued by the possible presence and evolution of sex chromosomes in other dioecious Caricaceae species. We selected a set of 22 bacterial artificial chromosome (BAC) clones that are distributed along the papaya X/Y chromosomes. These BACs were mapped to the meiotic pachytene chromosomes of Vasconcellea parviflora (2n = 2x = 18), a species that diverged from papaya ∼27 million years ago. We demonstrate that V. parviflora contains a pair of heteromorphic X/Y chromosomes that are homologous to the papaya X/Y chromosomes. The comparative mapping results revealed that the male-specific regions of the Y chromosomes (MSYs) probably initiated near the centromere of the Y chromosomes in both species. The two MSYs, however, shared only a small chromosomal domain near the centromere in otherwise rearranged chromosomes. The V. parviflora MSY expanded toward the short arm of the chromosome, whereas the papaya MSY expanded in the opposite direction. Most BACs mapped to papaya MSY were not located in V. parviflora MSY, revealing different DNA compositions in the two MSYs. These results suggest that mutation of gene(s) in the centromeric region may have triggered sex chromosome evolution in these plant species.     

Super-stretched pachytene chromosomes for fluorescence in situ hybridization mapping and immunodetection of DNA methylation

Meiotic pachytene chromosome-based fluorescence in situ hybridization (FISH) mapping is one of the most important tools in plant molecular cytogenetic research. Here we report a simple technique that allows stretching of pachytene chromosomes of maize to up to at least 20 times their original size. A modified Carnoy's II fixative (6:1:3 ethanol:chloroform:acetic acid) was used in the procedure, and proved to be key for super-stretching of pachytene chromosomes. We demonstrate that super-stretched pachytene chromosomes provide unprecedented resolution for chromosome-based FISH mapping. DNA probes separated by as little as 50 kb can be resolved on super-stretched chromosomes. A combination of FISH with immunofluorescent detection of 5-methyl cytosine on super-stretched pachytene chromosomes provides a powerful tool to reveal DNA methylation of specific chromosomal domains, especially those associated with highly repetitive DNA sequences.     

Why plant chromosomes do not show G-bands

Summary Giemsa techniques have refused to reveal G-banding patterns in plant chromosomes. Whatever has been differentially stained so far in plant chromosomes by various techniques represents constitutive heterochromatin (redefined in this paper). Patterns of this type must not be confused with the G-banding patterns of higher vertebrates which reveal an additional chromosome segmentation beyond that due to constitutive heterochromatin. The absence of G-bands in plants is explained as follows: 1) Plant chromosomes in metaphase contain much more DNA than G-banding vertebrate chromosomes of comparable length. At such a high degree of contraction vertebrate chromosomes too would not show G-bands, simply for optical reasons. 2) The striking correspondence of pachytene chromomeres and mitotic G-bands in higher vertebrates suggests that pachytene chromomeres are G-band equivalents, and that this may also be the case in plants. G-banded vertebrate chromosomes are on the average only 2.3 times shorter in mitosis than in pachytene; the chromomeric pattern therefore still can be shown. In contrast, plant chromosomes are approximately 10 times shorter at mitotic metaphase; their pachytene-like arrangement of chromomeres is therefore no longer demonstrable.     

Genetic recombination in Coprinus: III. Inlluence of gamma-irradiation and temperature treatments on meiotic recombination

Non-lethal doses of gamma-irradiation (5 krad) increased meiotic recombination in Coprinus lagopus when treatments were given at the beginning of karyogamy. The division stage at this time was judged to be late leptotene and the duration of the sensitive period was assessed to be 3–4 h. In C. lagopus the radiation-sensitive stage is distinct from the cold-sensitive stage (pachytene). The additive effect of irradiation at early karyogamy followed by cold treatment in pachytene suggested that the two factors influenced different steps in the recombination process. On the other hand, irradiation followed by heat treatment did not significantly alter recombination frequency as compared to single treatments. It was surmised that radiation and high temperature act on the same factor(s) or at the same steps to bring about a similar net result. It was suggested that irradiation at leptotene may cause single-strand breaks in DNA which eventually participate in exchange.     

Meiotic Structures in the Animal-Parasitic Nematode Ascaris megalocephala: Synaptonemal Complexes,Recombination Nodules,and Centrioles

Two synaptonemal complexes (SCs) were present in the pachytene nuclei of Ascaris megalocephala. The SC was tripartite and comprised of two lateral elements (25 nm) with a striated central element (25 nm) and a central region of 65 nm. Spherical recombination nodules were observed to be associated only with the central element, although they are non-existent in the related A. lumbricoides var. suum (Goldstein, 1977). The SCs were attached to the nuclear envelope at only one end, while the other end was free in the nucleoplasm. This lack of bouquet formation of the chromosomes is consistent with all other nematodes studied. Morphologically distinct sex chromosomes were not observed, which differs from the presence of five Y-chromosomes present in A. lumbricoides var. suum (Goldstein and Moens, 1976). Centrioles (0.2 µm wide) reproduced by budding off the parental centriole. The centrioles consisted of nine singlet microtubules connected by an electron-dense proteinaceous ring. This structure is consistent with centrioles described in other nematodes, yet distinctly different from the centriole structure observed in most organisms in which it consists of nine triplet microtubules without any connecting ring. Multiple synaptonemal complexes, or polycomplexes, are found in A. megalocephala and A. lumbriocoides var. suum. They appear as stacked SC and are present inside the nucleus during zygotene and in the cytoplasm at pachytene.