Maize, with its excellent forward genetics and male sterility screens, was used to identify >50 meiotic mutants representing at least 35 genes that affect key prophase processes such as pairing, ...synapsis, and homologous recombination. Most of these mutants were found by Inna Golubovskaya during the course of her remarkable career as a cytogeneticist. In addition to undertaking general cytological surveys to classify mutant phenotypes, Golubovskaya focused her efforts on characterizing several key regulatory mutants: ameiotic1 (am1), required to establish the meiotic cell cycle in maize; absence of first division (afd1), required for proper prophase chromosome morphology and for meiotic sister-chromatid cohesion leading to a reductive chromosome segregation at the first meiotic division; and plural abnormalities of meiosis (pam1), required for the clustering of telomeres on the nuclear envelope needed for pairing and synapsis. Her dramatic childhood in Leningrad during its siege in World War II, her fortuitous education in genetics at Leningrad State University, her continued research at the forward-looking Institute of Cytology and Genetics of the USSR Academy of Science Siberian branch, her plight at the fall of the Soviet Union, and her work in America helped engender a unique and valuable plant geneticist. Inna Golubovskaya related this personal history to the authors in conversation.
Protists account for the bulk of eukaryotic diversity. Through studies of gene and especially genome sequences the molecular basis for this diversity can be determined. Evident from genome sequencing ...are examples of versatile metabolism that go far beyond the canonical pathways described for eukaryotes in textbooks. In the last 2-3 years, genome sequencing and transcript profiling has unveiled several examples of heterotrophic and phototrophic protists that are unexpectedly well-equipped for ATP production using a facultative anaerobic metabolism, including some protists that can (Chlamydomonas reinhardtii) or are predicted (Naegleria gruberi, Acanthamoeba castellanii, Amoebidium parasiticum) to produce H2 in their metabolism. It is possible that some enzymes of anaerobic metabolism were acquired and distributed among eukaryotes by lateral transfer, but it is also likely that the common ancestor of eukaryotes already had far more metabolic versatility than was widely thought a few years ago. The discussion of core energy metabolism in unicellular eukaryotes is the subject of this review. Since genomic sequencing has so far only touched the surface of protist diversity, it is anticipated that sequences of additional protists may reveal an even wider range of metabolic capabilities, while simultaneously enriching our understanding of the early evolution of eukaryotes.
During meiosis, sequential release of sister chromatid cohesion (SSC) during two successive nuclear divisions allows the production of haploid gametes from diploid progenitor cells. Release of SSC ...along chromosome arms allows first a reductional segregation of homologs, and, subsequently, release of centromeric cohesion at anaphase II allows the segregation of chromatids
1–3. The Shugoshin (SGO) protein family plays a major role in the protection of centromeric cohesion in
Drosophila and yeast
4–12. We have isolated a maize mutant that displays premature loss of centromeric cohesion at anaphase I. We showed that this phenotype is due to the absence of ZmSGO1 protein, a maize shugoshin homolog. We also show that ZmSGO1 is localized to the centromeres. The ZmSGO1 protein is not found on mitotic chromosomes and has no obvious mitotic function. On the basis of these results, we propose that ZmSGO1 specifically maintains centromeric cohesion during meiosis I and therefore suggest that SGO1 core functions during meiosis are conserved across kingdoms and in large-genome species. However, in contrast to other Shugoshins, we observed an early and REC8-dependent recruitment of ZmSGO1 in maize, suggesting that control of SGO1 recruitment to chromosomes is different in plants than in other model organisms.
Background: Chromosomal behavior during mitosis and meiosis depends in part on heterochromatic modifications such as histone H3 lysine-9 methylation (H3K9me). In fission yeast, the Heterochromatin ...Protein 1 homolog Swi6 recognizes H3K9me, silences transcription, and retains cohesin at pericentromeric repeats. Heterochromatin formation also depends on processing of transcripts derived from centromeric repeats by the RNAi machinery. The DDB1 homolog, Rik1, and histone methyltransferase, Clr4, act in a complex to promote H3K9me. However, the mechanism underlying this interaction is poorly understood.
Results: Using a cytological screen, we have identified two novel genes, dos1+ and dos2+, which are required for localization of Swi6. Deletion of either of these genes results in mitotic and meiotic chromosome missegregation, defects in mitotic centromeric cohesion and meiotic telomere clustering, and loss of heterochromatic silencing. Dos1 is predominantly located in the nucleus in a Dos2-dependent manner and directly interacts with Rik1. Each of these genes is required for the association of H3K9me with centromeric repeats, as well as for the production of small interfering RNAs.
Conclusions: Dos1 and Dos2 are required for the formation of heterochromatin in fission yeast. We hypothesize that the physical interaction between Dos1 and Rik1 represents a role in regulating activity of the Rik1/Clr4 complex. Dos2 contributes to this role by regulating Dos1 localization. Our findings suggest a mechanism for recruitment of Clr4 in the RNAi-dependent heterochromatin pathway, in which Dos1 and Dos2 are essential.
During meiotic prophase homologous chromosomes find each other and pair. Then they synapse, as the linear protein core (axial element or lateral element) of each homologous chromosome is joined ...together by a transverse central element, forming the tripartite synaptonemal complex (SC). Ten uncloned Zea mays mutants in our collection were surveyed by transmission electron microscopy by making silver-stained spreads of SCs to identify mutants with non-homologous synapsis or improper synapsis. To analyse the mutants further, zyp1, the maize orthologue of the Arabidopsis central element component ZYP1 was cloned and an antibody was made against it. Using antibodies against ZYP1 and the lateral element components AFD1 and ASY1, it was found that most mutants form normal SCs but are defective in pairing. The large number of non-homologous synapsis mutants defective in pairing illustrates that synapsis and pairing can be uncoupled. Of the ten mutants studied, only dsy2 undergoes normal homologous chromosome recognition needed for homologous pairing. The dsy2 mutation fails to maintain the SC. ZYP1 elongation is blocked at zygotene, and only dots of ZYP1 are seen at prophase I. Another mutant, mei*N2415 showed incomplete but homologous synapsis and ASY1 and AFD1 have a normal distribution. Although installation of ZYP1 is initiated at zygotene, its progression is slowed down and not completed by pachytene in some cells and ZYP1 is not retained on pachytene chromosomes. The mutants described here are now available through the Maize Genetics Cooperation Stock Center (http://maizecoop.cropsci.uiuc.edu/).
Meiosis is a specialized type of cell division leading to the production of gametes. During meiotic prophase I, homologous chromosomes interact with each other and form bivalents (pairs of homologous ...chromosomes). Three major meiotic processes – chromosome pairing, synapsis and recombination – are involved in the formation of bivalents. Many recent reports have uncovered complex networks of interactions between these processes. Chromosome pairing is largely dependent on the initiation and progression of recombination in fungi, mammals and plants, but not in
Caenorhabditis elegans or
Drosophila. Synapsis and recombination are also tightly linked. Understanding the coordination between chromosome pairing, synapsis and recombination lends insight into many poorly explained aspects of meiosis, such as the nature of chromosome homology recognition.
Background Developmental cues to start meiosis occur late in plants. Ameiotic1 (Am1) encodes a plant-specific nuclear protein (AM1) required for meiotic entry and progression through early prophase ...I. Pollen mother cells (PMCs) remain mitotic in most am1 mutants including am1-489, while am1-praI permits meiotic entry but PMCs arrest at the leptotene/zygotene (L/Z) transition, defining the roles of AM1 protein in two distinct steps of meiosis. To gain more insights into the roles of AM1 in the transcriptional pre-meiotic and meiotic programs, we report here an in depth analysis of gene expression alterations in carefully staged anthers at 1 mm (meiotic entry) and 1.5 mm (L/Z) caused by each of these am1 alleles. Results 1.0 mm and 1.5 mm anthers of am1-489 and am1-praI were profiled in comparison to fertile siblings on Agilent® 4 × 44 K microarrays. Both am1-489 and am1-praI anthers are cytologically normal at 1.0 mm and show moderate transcriptome alterations. At the 1.5-mm stage both mutants are aberrant cytologically, and show more drastic transcriptome changes. There are substantially more absolute On/Off and twice as many differentially expressed genes (sterile versus fertile) in am1-489 than in am1-praI. At 1.5 mm a total of 4,418 genes are up- or down-regulated in either am1-489 or am1-praI anthers. These are predominantly stage-specific transcripts. Many putative meiosis-related genes were found among them including a small subset of allele-specific, mis-regulated genes specific to the PMCs. Nearly 60% of transcriptome changes in the set of transcripts mis-regulated in both mutants (N = 530) are enriched in PMCs, and only 1% are enriched in the tapetal cell transcriptome. All array data reported herein will be deposited and accessible at MaizeGDB http://www.maizegdb.org/ webcite. Conclusions Our analysis of anther transcriptome modulations by two distinct am1 alleles, am1-489 and am1-praI, redefines the role of AM1 as a modulator of expression of a subset of meiotic genes, important for meiotic progression and provided stage-specific insights into the genetic networks associated with meiotic entry and early prophase I progression.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Schizosaccharomyces pombe is an excellent organism for studying microtubule dynamics owing to the presence of well-defined microtubule arrays that undergo dramatic rearrangements during various ...stages of the cell cycle. Using sensitive time-lapse video microscopy and kymographic analysis, we have determined the polymerization/depolymerization kinetics of individual microtubules within these arrays throughout the fission yeast cell cycle. Interphase bundles are composed of 4-7 microtubules that act autonomously, demonstrating that individual microtubules are responsible for mediating the functions ascribed to these arrays. The nucleation and growth of cytoplasmic microtubules is inhibited upon cellular transition into mitosis, leading to their gradual disappearance. At the onset of mitosis, microtubules form on the nuclear face of the spindle pole body and exhibit dramatically increased dynamics. The presence of these intra-nuclear astral microtubules (INA) is reminiscent of spindle assembly and the search and chromosome capture mechanism observed in metazoan cells. Consistent with other in vivo studies, we do not observe microtubule flux in the anaphase B spindle. Finally, the depolymerization of individual microtubules alternates between each half-spindle, resulting in spindle collapse during telophase. On the basis of these observations, we conclude that microtubules in these diverse cytoskeletal arrays have autonomous behaviors that are an essential component of any model describing cell-cycle-dependent changes in the behavior and function of microtubule arrays.
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