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Meiosis
Kinds of Meiosis Terms modified by Meiosis Selected AbstractsMeiosis induced by inactivation of Pat1 kinase proceeds with aberrant nuclear positioning of centromeres in the fission yeast Schizosaccharomyces pombeGENES TO CELLS, Issue 8 2004Yuji Chikashige Nuclear organization of chromosomes proceeds with significant changes during meiosis. In the fission yeast Schizosaccharomyces pombe, centromeres are clustered at the spindle-pole body (SPB) during the mitotic cell cycle; however, during meiotic prophase telomeres become clustered to the SPB and centromeres dissociate from the SPB. We followed the movement of telomeres, centromeres and sister chromatids in living S. pombe cells that were induced to meiosis by inactivation of Pat1 kinase (a key negative regulator of meiosis). Time-course observation in living cells determined the temporal order of DNA synthesis, telomere clustering, centromere separation and meiotic chromosome segregation. When meiosis was induced by Pat1 inactivation at the G1 phase of mitosis, telomeres clustered to the SPB as per normal meiosis, but in most cells the centromeres remained partially associated with the SPB. When meiosis was initiated at the G2 phase by Pat1 inactivation, both telomeres and centromeres retained their mitotic nuclear positions in the majority of cells. These results indicate that the progression of meiosis induced by Pat1 inactivation is aberrant from normal meiosis in some events. As Pat1 inactivation is often useful to induce S. pombe cells synchronously into meiosis, the temporal order of chromosomal events determined here will provide landmarks for the progression of meiosis downstream the Pat1 inactivation. [source] A Resurgent Phoenix--A Hypothesis for the Origin of MeiosisIUBMB LIFE, Issue 1 2002Wei Li Abstract The origin and evolution of sex is a great puzzle in biology, and meiosis is the core of sex. Here, we propose a new hypothesis that meiosis may have evolved by blending features of two responses that cells have to deal with double-strand-DNA breaks, recombinational repair and apoptosis. This hypothesis can explain some unsolved problems and is in agreement with some known molecular mechanisms, cytological phenomena, and the fossil records. Furthermore, it can predict some testable features of meiosis. [source] Transitions in the evolution of meiosisJOURNAL OF EVOLUTIONARY BIOLOGY, Issue 3 2000Hurst Meiosis may have evolved gradually within the eukaryotes with the earliest forms having a one-step meiosis. It has been speculated that the putative transition from a one-step meiosis without recombination to one with recombination may have been stimulated by the invasion of Killer alleles. These imaginary selfish elements are considered to act prior to recombination. They prime for destruction (which occurs after cell division) the half of the cell on the opposite side of the meiotic spindle. Likewise the transition from one-step to two-step meiosis might have been stimulated by a subtly different sort of imaginary distorter allele, a SisterKiller. These are proposed to act after recombination. It has yet to be established that the presence of such distorter alleles could induce the transitions in question. To investigate these issues we have analysed the dynamics of a modifier (1) of recombination and (2) of the number of steps of meiosis, as they enter a population with one-step meiosis. For the modifier of recombination, we find that invasion conditions are very broad and that persistence of Killer and modifier is likely through most parameter space, even when the recombination rate is low. However, if we allow a Killer element to mutate into one that is self-tolerant, the modifier and the nonself-tolerant alleles are typically both lost from the population. The modifier of the number of steps can invade if the SisterKiller acts at meiosis II. However, a SisterKiller acting at meiosis I, far from promoting the modifier's spread, actually impedes it. In the former case the invasion is easiest if there is no recombination. The SisterKiller hypothesis therefore fails to provide a reasonable account of the evolution of two-step meiosis with recombination. As before, the evolution of self-tolerance on the part of the selfish element destroys the process. We conclude that the conditions under which SisterKillers promote the evolution of two-step meiosis are very much more limited than originally considered. We also conclude that there is no universal agreement between ESS and modifier analyses of the same transitions. [source] Genetic Analyses of Meiotic Recombination in ArabidopsisJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 8 2007Asela J. Wijeratne Abstract Meiosis is essential for sexual reproduction and recombination is a critical step required for normal meiosis. Understanding the underlying molecular mechanisms that regulate recombination is important for medical, agricultural and ecological reasons. Readily available molecular and cytological tools make Arabidopsis an excellent system to study meiosis. Here we review recent developments in molecular genetic analyses on meiotic recombination. These include studies on plant homologs of yeast and animal genes, as well as novel genes that were first identified in plants. The characterizations of these genes have demonstrated essential functions from the initiation of recombination by double-strand breaks to repair of such breaks, from the formation of double-Holliday junctions to possible resolution of these junctions, both of which are critical for crossover formation. The recent advances have ushered a new era in plant meiosis, in which the combination of genetics, genomics, and molecular cytology can uncover important gene functions. [source] The ,-1,3-glucanosyltransferase gas4p is essential for ascospore wall maturation and spore viability in Schizosaccharomyces pombeMOLECULAR MICROBIOLOGY, Issue 5 2008María De Medina-Redondo Summary Meiosis is the developmental programme by which sexually reproducing diploid organisms generate haploid gametes. In yeast, meiosis is followed by spore morphogenesis. The formation of the Schizosaccharomyces pombe ascospore wall requires the co-ordinated activity of enzymes involved in the biosynthesis and modification of its components, such as glucans. During sporogenesis, the ,-1,3-glucan synthase bgs2p synthesizes linear ,-1,3-glucans, which remain unorganized and alkali-soluble until covalent linkages are set up between ,-1,3-glucans and other cell wall components. Several proteins belonging to the glycoside hydrolase family 72 (GH72) with ,-1,3-glucanosyltransferase activity have been described in other organisms, such as the Saccharomyces cerevisiae Gas1p or the Aspergillus fumigatus Gel1p. Here we describe the characterization of gas4+, a new gene that encodes a protein of the GH72 family. Deletion of this gene does not lead to any apparent defect during vegetative growth, but homozygous gas4, diploids show a sporulation defect. Although meiosis occurs normally, ascospores are unable to mature or to germinate. The expression of gas4+ is strongly induced during sporulation and a yellow fluorescent protein (YFP),gas4p fusion protein localizes to the ascospore periphery during sporulation. We conclude that gas4p is required for ascospore maturation in S. pombe. [source] Consequences of Nitric Oxide Synthase Inhibition During Bovine Oocyte Maturation on Meiosis and Embryo DevelopmentREPRODUCTION IN DOMESTIC ANIMALS, Issue 1 2010KRL Schwarz Contents The importance of nitric oxide synthase (NOS) in bovine oocyte maturation was investigated. Oocytes were in vitro matured with the NOS inhibitor Nw - l -nitro-arginine methyl-ester (10,7, 10,5 and 10,3 m l -NAME) and metaphase II (MII) rates and embryo development and quality were assessed. The effect of l -NAME (10,7 m) during pre-maturation and/or maturation on embryo development and quality was also assessed. l -NAME decreased MII rates (78,82%, p < 0.05) when compared with controls without l -NAME (96%). Cleavage (77,88%, p > 0.05), Day 7 blastocyst rates (34,42%, p > 0.05) and total cell numbers in blastocysts were similar for all groups (146,171 cells, p > 0.05). Day 8 blastocyst TUNEL positive cells (3,4 cells) increased with l -NAME treatment (p < 0.05). For oocytes cultured with l -NAME during pre-maturation and/or maturation, Day 8 blastocyst development (26,34%) and Day 9 hatching rates (15,22%) were similar (p > 0.05) to controls pre-matured and matured without NOS inhibition (33 and 18%, respectively), while total cell numbers (Day 9 hatched blastocysts) increased (264,324 cells, p < 0.05) when compared with the controls (191 cells). TUNEL positive cells increased when NOS was inhibited only during the maturation period (8 cells, p < 0.05) when compared with the other groups (3,4 cells). NO may be involved in meiosis progression to MII and its deficiency during maturation increases apoptosis in embryos produced in vitro. Nitric oxide synthase inhibition during pre-maturation and/or maturation affects embryo quality. [source] Estimating the Frequency Distribution of Crossovers during Meiosis from Recombination DataBIOMETRICS, Issue 2 2001Kai Yu Summary. Estimation of tetrad crossover frequency distributions from genetic recombination data is a classic problem dating back to Weinstein (1936, Genetics21, 155,199). But a number of important issues, such as how to specify the maximum number of crossovers, how to construct confidence intervals for crossover probabilities, and how to obtain correct p -values for hypothesis tests, have never been adequately addressed. In this article, we obtain some properties of the maximum likelihood estimate (MLE) for crossover probabilities that imply guidelines for choosing the maximum number of crossovers. We give these results for both normal meiosis and meiosis with nondisjunction. We also develop an accelerated EM algorithm to find the MLE more efficiently. We propose bootstrap-based methods to find confidence intervals and p -values and conduct simulation studies to check the validity of the bootstrap approach. [source] Mechanism of malsegregations at meiosis: premature centromere separation and precocious division in female Chinese hamsters stimulated with gonadotropic hormonesCONGENITAL ANOMALIES, Issue 3 2000Shin-ichi Sonta ABSTRACT, Using female Chinese hamsters stimulated with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG), we investigated the influence of hormonal stimulation upon meiotic segregation in oocytes. In 1,576 oocytes ovulated spontaneously from 197 non-treated mature females, the number (percentage) of hyperhaploid oocytes with more than 12 (12,14) chromosomes was 16 (1.0%). These cells had no extra single chromatids, but all had extra chromosomes. Single chromatids were seen in 7 (0.4%) cells with a haploid chromosome set. On the other hand, a total of 1,329 and 1,198 second meiotic (MII) oocytes from 64 mature females and 61 immature females stimulated with PMSG and hCG, respectively, were subjected to chromosomal analysis. Single chromatids were seen in 34 (2.6%) and 62 (5.2%) of these oocytes, respectively. Since these chromatids were mostly paired and the sister chromatids existed near each other in many cells, they may have separated from some chromosomes of haploid cells. Compared with the non-treated females, the frequency of cells with single chromatids was significantly greater in oocytes from both mature and immature females stimulated with PMSG and hCG. The number (percentage) of hyperhaploid cells from mature and immature PMSG-hCG-stimulated females, respectively, was 15 (1.1%) and 14 (1.2%), which was not significantly greater than that in non-treated females. Most of these cells had extra whole chromosomes but one oocyte from mature females and one from immature females had an extra single chromatid. These findings indicate that such hormonal stimulation induces premature centromere separation in MII oocytes and precocious division at anaphase I, which can be assumed by the presence of MII cells with extra single chromatids. Considering that no or less hyperhaploid MII oocytes with an extra single chromatid were seen in oocytes from spontaneous ovulation and from artificial ovulation on hormonal stimulation, these findings suggest that the major mechanism of malsegregations at first meiotic (MI) division is not a precocious division but rather, errors such as nondisjunction of homologous chromosomes (dyads). [source] Arabidopsis thaliana protein, ATK1, is a minus-end directed kinesin that exhibits non-processive movementCYTOSKELETON, Issue 3 2002Adam I. Marcus Abstract The microtubule cytoskeleton forms the scaffolding of the meiotic spindle. Kinesins, which bind to microtubules and generate force via ATP hydrolysis, are also thought to play a critical role in spindle assembly, maintenance, and function. The A. thaliana protein, ATK1 (formerly known as KATA), is a member of the kinesin family based on sequence similarity and is implicated in spindle assembly and/or maintenance. Thus, we want to determine if ATK1 behaves as a kinesin in vitro, and if so, determine the directionality of the motor activity and processivity character (the relationship between molecular "steps" and microtubule association). The results show that ATK1 supports microtubule movement in an ATP-dependent manner and has a minus-end directed polarity. Furthermore, ATK1 exhibits non-processive movement along the microtubule and likely requires at least four ATK1 motors bound to the microtubule to support movement. Based on these results and previous data, we conclude that ATK1 is a non-processive, minus-end directed kinesin that likely plays a role in generating forces in the spindle during meiosis. Cell Motil. Cytoskeleton 52:144,150, 2002. © 2002 Wiley-Liss, Inc. [source] Regulation of oocyte maturation in fishDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 2008Yoshitaka Nagahama A period of oocyte growth is followed by a process called oocyte maturation (the resumption of meiosis) which occurs prior to ovulation and is a prerequisite for successful fertilization. Our studies using fish models have revealed that oocyte maturation is a three-step induction process involving gonadotropin (LH), maturation-inducing hormone (MIH), and maturation-promoting factor (MPF). LH acts on the ovarian follicle layer to produce MIH (17,, 20,-dihydroxy-4-pregnen-3-one, 17,, 20,-DP, in most fishes). The interaction of ovarian thecal and granulosa cell layers (two-cell type model), is required for the synthesis of 17,,20,-DP. The dramatic increase in the capacity of postvitellogenic follicles to produce 17,,20,-DP in response to LH is correlated with decreases in P450c17 (P450c17-I) and P450 aromatase (oP450arom) mRNA and increases in the novel form of P450c17 (P450c17-II) and 20,-hydroxysteroid dehydrogenase (20,-HSD) mRNA. Transcription factors such as Ad4BP/SF-1, Foxl2, and CREB may be involved in the regulation of expression of these steroidogenic enzymes. A distinct family of G-protein-coupled membrane-bound MIH receptors has been shown to mediate non-genomic actions of 17,, 20,-DP. The MIH signal induces the de novo synthesis of cyclin B from the stored mRNA, which activates a preexisting 35 kDa cdc2 kinase via phosphorylation of its threonine 161 by cyclin-dependent kinase activating kinase, thus producing the 34 kDa active cdc2 (active MPF). Upon egg activation, MPF is inactivated by degradation of cyclin B. This process is initiated by the 26S proteasome through the first cut in its NH2 terminus at lysine 57. [source] Hypotonic buffer induces meiosis and formation of anucleate cytoplasmic islands in the egg of the two-spotted cricket Gryllus bimaculatusDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 2 2003Isao Sarashina In insects, egg activation is known to occur in vivo and independently of fertilization, but its mechanisms are poorly understood. To gain understanding of these mechanisms, an attempt was made to activate the egg of Gryllus bimaculatus in vitro. It was found that meiosis resumed and was completed in unfertilized eggs treated with hypotonic buffer. Early developmental processes in activated, unfertilized eggs were investigated and compared with those in fertilized eggs. Mitosis did not progress, resulting in formation of anucleate cytoplasmic islands (pseudoenergids). Development in the activated, unfertilized eggs stopped at this stage and both yolk subdivision and cellularization did not occur. To elucidate the role of the nucleus in the developmental process to the syncytial stage in fertilized eggs, eggs were treated with aphidicolin to inhibit DNA polymerization. It was found that pseudoenergids also formed in these aphidicolin-treated fertilized eggs. These results demonstrate that pseudoenergids can increase in number independently of nuclei, suggesting that the cytoplasm rather than the nucleus plays the primary role in development to the syncytial stage in G. bimaculatus. [source] PRP-17 and the pre-mRNA splicing pathway are preferentially required for the proliferation versus meiotic development decision and germline sex determination in Caenorhabditis elegansDEVELOPMENTAL DYNAMICS, Issue 5 2010Jessica Amrozowicz Kerins Abstract In C. elegans, the decision between germline stem cell proliferation and entry into meiosis is controlled by GLP-1 Notch signaling, which promotes proliferation through repression of the redundant GLD-1 and GLD-2 pathways that direct meiotic entry. We identify prp-17 as another gene functioning downstream of GLP-1 signaling that promotes meiotic entry, largely by acting on the GLD-1 pathway, and that also functions in female germline sex determination. PRP-17 is orthologous to the yeast and human pre-mRNA splicing factor PRP17/CDC40 and can rescue the temperature-sensitive lethality of yeast PRP17. This link to splicing led to an RNAi screen of predicted C. elegans splicing factors in sensitized genetic backgrounds. We found that many genes throughout the splicing cascade function in the proliferation/meiotic entry decision and germline sex determination indicating that splicing per se, rather than a novel function of a subset of splicing factors, is necessary for these processes. Developmental Dynamics 239:1555,1572, 2010. © 2010 Wiley-Liss, Inc. [source] Erasure of the paternal transcription program during spermiogenesis: The first step in the reprogramming of sperm chromatin for zygotic developmentDEVELOPMENTAL DYNAMICS, Issue 5 2008Junke Zheng Abstract Male germ cells possess a unique epigenetic program and express a male-specific transcription profile. However, when its chromatin is passed onto the zygote, it expresses an transcription/epigenetic program characteristic of the zygote. The mechanism underlying this reprogramming process is not understood at present. In this study, we show that an extensive range of chromatin factors (CFs), including essential transcription factors and regulators, remodeling factors, histone deacetylases, heterochromatin-binding proteins, and topoisomerases, were removed from chromatin during spermiogenesis. This process will erase the paternal epigenetic program to generate a relatively naive chromatin, which is likely to be essential for installation of the zygotic developmental program after fertilization. We have also showed that transcription termination in male germ cells was temporally correlated with CF dissociation. A genome-wide CF dissociation will inevitably disassemble the transcription apparatus and regulatory mechanism and lead to transcription silence. Based on data presented in this and previous studies (Sun et al., Cell Research [2007] 17:117,134), we propose that paternal-zygotic transcription reprogramming begins with a genome-wide CF dissociation to erase the existing transcription program in later stages of spermatogenesis. This will be followed by assembling of the zygotic equivalent after fertilization. The transcription/epigenetic program of the male germ cell is transformed into a zygotic one using an erase-and-rebuild strategy similar to that used in the maternal-zygotic transition. It is also noted that transcription is terminated long after meiosis is completed and before chromatin becomes highly condensed during spermatogenesis. The temporal order of these events suggests that transcription silence does not have to be coupled to meiosis or chromatin condensation. Developmental Dynamics 237:1463-1476, 2008. © 2008 Wiley-Liss, Inc. [source] Nucleolar colocalization of TAF1 and testis-specific TAFs during Drosophila spermatogenesisDEVELOPMENTAL DYNAMICS, Issue 10 2007Chad E. Metcalf Abstract In Drosophila, testis-specific TBP-associated factors (tTAFs) predominantly localize to spermatocyte nucleoli and regulate the transcription of genes necessary for spermatocyte entry into meiosis. tTAFs are paralogs of generally expressed TAF subunits of transcription factor IID (TFIID). Our recent observation that the generally expressed TAF1 isoform TAF1-2 is greatly enriched in testes prompted us to explore the functional relationship between general TAFs and tTAFs during spermatogenesis. Analysis by immunofluorescence microscopy revealed that among the general TFIID subunits examined (TATA-box binding protein [TBP], TAF1, TAF4, TAF5, and TAF9), only TAF1 colocalized with the tTAF Mia in spermatocyte nucleoli. Nucleolar localization of TAF1, but not Mia, was disrupted in tTAF mutant flies, and TAF1 dissociated from DNA prior to Mia as spermatocytes entered meiosis. Taken together, our results suggest stepwise assembly of a testis-specific TFIID complex (tTFIID) whereby a TAF1 isoform, presumably TAF1-2, is recruited to a core subassembly of tTAFs in spermatocyte nucleoli. Developmental Dynamics 236:2836,2843, 2007. © 2007 Wiley-Liss, Inc. [source] Retinoids and spermatogenesis: Lessons from mutant mice lacking the plasma retinol binding proteinDEVELOPMENTAL DYNAMICS, Issue 6 2006Norbert B. Ghyselinck Abstract Using Rbp4 -null mice as models, we have established for the first time the kinetics of the spermatogenetic alterations during vitamin A deficiency (VAD). Our data demonstrate that the VAD-induced testicular degeneration arises through the normal maturation of germ cells in a context of spermatogonia differentiation arrest. They indicate that retinoic acid (RA) appears dispensable for the transition of premeiotic to meiotic spermatocytes, meiosis, and spermiogenesis. They confirm that RA plays critical roles in controlling spermatogonia differentiation, spermatid adhesion to Sertoli cells, and spermiation, and suggest that the VAD-induced arrest of spermatogonia differentiation results from simultaneous blocks in RA-dependent events mediated by RA receptor , (RAR,) in spermatogonia and by RAR, in Sertoli cells. They also provide evidence that expression of major RA-metabolizing enzymes is increased in mouse Sertoli cells upon VAD and that vitamin A-deficient A spermatogonia differ from their RA-sufficient counterparts by the expression of the Stra8 gene. Developmental Dynamics 235:1608,1622, 2006. © 2006 Wiley-Liss, Inc. [source] A novel mutant phenotype implicates dicephalic in cyst formation in the Drosophila ovaryDEVELOPMENTAL DYNAMICS, Issue 4 2006Ruth McCaffrey Abstract The establishment of polarity in Drosophila requires the correct specification of the oocyte in early stages of oogenesis, its positioning at the posterior of the egg chamber, and signalling events between the oocyte and the adjacent posterior follicle cells. As a consequence, the anterior-posterior and the dorsal-ventral axes are fixed. The posterior localisation of the oocyte depends on cadherin-mediated adhesion between the oocyte and the follicle cells. Here we show that dicephalic mutants affect the posterior positioning of the oocyte without interfering with oocyte specification in the germarium. Unlike other mutants that also affect the posterior placement of the oocyte, dicephalic mutants affect neither gurken expression nor karyosome formation during meiosis. By analysing in detail the mutant phenotypes of dicephalic, we find that cyst formation in mutant germaria is defective and that it shares some similarities with cysts that lack DE-cadherin in the germline cells. We propose a model in which dicephalic is involved in the proper adhesion between the oocyte and the somatic follicle cells. Developmental Dynamics 235:908,917, 2006. © 2005 Wiley-Liss, Inc. [source] REPRESSION OF COMPETITION AND THE EVOLUTION OF COOPERATIONEVOLUTION, Issue 4 2003Steven A. Frank Abstract Repression of competition within groups joins kin selection as the second major force in the history of life shaping the evolution of cooperation. When opportunities for competition against neighbors are limited within groups, individuals can increase their own success only by enhancing the efficiency and productivity of their group. Thus, characters that repress competition within groups promote cooperation and enhance group success. Leigh first expressed this idea in the context of fair meiosis, in which each chromosome has an equal chance of transmission via gametes. Randomized success means that each part of the genome can increase its own success only by enhancing the total number of progeny and thus increasing the success of the group. Alexander used this insight about repression of competition in fair meiosis to develop his theories for the evolution of human sociality. Alexander argued that human social structures spread when they repress competition within groups and promote successful group-against-group competition. Buss introduced a new example with his suggestion that metazoan success depended on repression of competition between cellular lineages. Maynard Smith synthesized different lines of thought on repression of competition. In this paper, I develop simple mathematical models to illustrate the main processes by which repression of competition evolves. With the concepts made clear, I then explain the history of the idea. I finish by summarizing many new developments in this subject and the most promising lines for future study. [source] PERSPECTIVE: SEX, RECOMBINATION, AND THE EFFICACY OF SELECTION,WAS WEISMANN RIGHT?EVOLUTION, Issue 2 2000Austin Burt Abstract., The idea that sex functions to provide variation for natural selection to act upon was first advocated by August Weismann and it has dominated much discussion on the evolution of sex and recombination since then. The goal of this paper is to further extend this hypothesis and to assess its place in a larger body of theory on the evolution of sex and recombination. A simple generic model is developed to show how fitness variation and covariation interact with selection for recombination and illustrate some important implications of the hypothesis: (1) the advantage of sex and recombination can accrue both to reproductively isolated populations and to modifiers segregating within populations, but the former will be much larger than the latter; (2) forces of degradation that are correlated across loci within an individual can reduce or reverse selection for increased recombination; and (3) crossing-over (which can occur at different places in different meioses) will create more variability than having multiple chromosomes and so will have more influence on the efficacy of selection. Several long-term selection experiments support Weismann's hypothesis, including those showing a greater response to selection in populations with higher rates of recombination and higher rates of recombination evolving as a correlated response to selection for some other character. Weismann's hypothesis is also consistent with the sporadic distribution of obligate asexuality, which indicates that clones have a higher rate of extinction than sexuals. Weismann's hypothesis is then discussed in light of other patterns in the distribution of sexuality versus asexuality. To account for variation in the frequency of obligate asexuality in different taxa, a simple model is developed in which this frequency is a function of three parameters: the rate of clonal origin, the initial fitness of clones when they arise, and the rate at which that fitness declines over time. Variation in all three parameters is likely to be important in explaining the distribution of obligate asexuality. Facultative asexuality also exists, and for this to be stable it seems there must be ecological differences between the sexual and asexual propagules as well as genetic differences. Finally, the timing of sex in cyclical parthenogens is most likely set to minimize the opportunity costs of sex. None of these patterns contradict Weismann's hypothesis, but they do show that many additional principles unrelated to the function of sex are required to fully explain its distribution. Weismann's hypothesis is also consistent with what we know about the mechanics and molecular genetics of recombination, in particular the tendency for chromatids to recombine with a homolog rather than a sister chromatid at meiosis, which is opposite to what they do during mitosis. However, molecular genetic studies have shown that cis -acting sites at which recombination is initiated are lost by gene conversion as a result, a factor that can be expected to affect many fine details in the evolution of recombination. In summary, although Weismann's hypothesis must be considered the leading candidate for the function of sex and recombination, nevertheless, many additional principles are needed to fully account for their evolution. [source] From meiosis to postmeiotic events: Homologous recombination is obligatory but flexibleFEBS JOURNAL, Issue 3 2010Lóránt Székvölgyi Sexual reproduction depends on the success of faithful chromosome transmission during meiosis to yield viable gametes. Central to meiosis is the process of recombination between paternal and maternal chromosomes, which boosts the genetic diversity of progeny and ensures normal homologous chromosome segregation. Imperfections in meiotic recombination are the source of de novo germline mutations, abnormal gametes, and infertility. Thus, not surprisingly, cells have developed a variety of mechanisms and tight controls to ensure sufficient and well-distributed recombination events within their genomes, the details of which remain to be fully elucidated. Local and genome-wide studies of normal and genetically engineered cells have uncovered a remarkable stochasticity in the number and positioning of recombination events per chromosome and per cell, which reveals an impressive level of flexibility. In this minireview, we summarize our contemporary understanding of meiotic recombination and its control mechanisms, and address the seemingly paradoxical and poorly understood diversity of recombination sites. Flexibility in the distribution of meiotic recombination events within genomes may reside in regulation at the chromatin level, with histone modifications playing a recently recognized role. [source] From meiosis to postmeiotic events: Uncovering the molecular roles of the meiosis-specific recombinase Dmc1FEBS JOURNAL, Issue 3 2010Wataru Kagawa In meiosis, the accurate segregation of maternal and paternal chromosomes is accomplished by homologous recombination. A central player in meiotic recombination is the Dmc1 recombinase, a member of the RecA/Rad51 recombinase superfamily, which is widely conserved from viruses to humans. Dmc1 is a meiosis-specific protein that functions with the ubiquitously expressed homolog, the Rad51 recombinase, which is essential for both mitotic and meiotic recombination. Since its discovery, it has been speculated that Dmc1 is important for unique aspects of meiotic recombination. Understanding the distinctive properties of Dmc1, namely, the features that distinguish it from Rad51, will further clarify the mechanisms of meiotic recombination. Recent structural, biochemical, and genetic findings are now revealing the molecular mechanisms of Dmc1-mediated homologous recombination and its regulation by various recombination mediators. [source] Interaction between Lim15/Dmc1 and the homologue of the large subunit of CAF-1 , a molecular link between recombination and chromatin assembly during meiosisFEBS JOURNAL, Issue 9 2008Satomi Ishii In eukaryotes, meiosis leads to genetically variable gametes through recombination between homologous chromosomes of maternal and paternal origin. Chromatin organization following meiotic recombination is critical to ensure the correct segregation of homologous chromosomes into gametes. However, the mechanism of chromatin organization after meiotic recombination is unknown. In this study we report that the meiosis-specific recombinase Lim15/Dmc1 interacts with the homologue of the largest subunit of chromatin assembly factor 1 (CAF-1) in the basidiomycete Coprinopsis cinerea (Coprinus cinereus). Using C. cinerea LIM15/DMC1 (CcLIM15) as the bait in a yeast two-hybrid screen, we have isolated the C. cinerea homologue of Cac1, the largest subunit of CAF-1 in Saccharomyces cerevisiae, and named it C. cinerea Cac1-like (CcCac1L). Two-hybrid assays confirmed that CcCac1L binds CcLim15 in vivo. ,-Galactosidase assays revealed that the N-terminus of CcCac1L preferentially interacts with CcLim15. Co-immunoprecipitation experiments showed that these proteins also interact in the crude extract of meiotic cells. Furthermore, we demonstrate that, during meiosis, CcCac1L interacts with proliferating cell nuclear antigen (PCNA), a component of the DNA synthesis machinery recently reported as an interacting partner of Lim15/Dmc1. Taken together, these results suggest a novel role of the CAF-1,PCNA complex in meiotic events. We propose that the CAF-1,PCNA complex modulates chromatin assembly following meiotic recombination. [source] A polyploid population of Saccharomyces cerevisiae with separate sexes (dioecy)FEMS YEAST RESEARCH, Issue 6 2010Rim Al Safadi Abstract Saccharomyces cerevisiae has proved to be an interesting model for studies of evolution, with whole-genome duplication shown to have played an important role in the evolution of this species. This phenomenon depends on the formation of a transient stable polyploid state. Previous studies have reported polyploidy to be an unstable state in yeast, but here, we describe a polyploid population of S. cerevisiae. The evolution of higher eukaryotes has also involved the development of different systems of sexual reproduction, the choice between self-fertilization and out-crossing becoming a key issue. Saccharomyces cerevisiae is a hermaphrodite eukaryote, despite the theoretical genetic disadvantages of this strategy, in which self-fertilization occurs. We describe, for the first time, a near-dioecious (with separate sexes) population in this species. Mating type and the MAT locus display complex segregations. Essentially, each strain produces, by meiosis, spores of only one mating type: mata or mat,. Moreover, strains are heterothallic, and diploid nonmating clones generated from a single spore do not sporulate. These three properties limit self-fertilization and strongly favour out-crossing. We suggest that the shift in sexual strategy, from hermaphroditism to dioecy, is specific to the brewing process, which overcomes the sexual isolation probably found in natural biotopes. [source] Glucose induction pathway regulates meiosis in Saccharomyces cerevisiae in part by controlling turnover of Ime2p meiotic kinaseFEMS YEAST RESEARCH, Issue 5 2008Misa Gray Abstract Several components of the glucose induction pathway, namely the Snf3p glucose sensor and the Rgt1p and Mth1p transcription factors, were shown to be involved in inhibition of sporulation by glucose. The glucose sensors had only a minor role in regulating transcript levels of the two key regulators of meiotic initiation, the Ime1p transcription factor and the Ime2p kinase, but a major role in regulating Ime2p stability. Interestingly, Rgt1p was involved in glucose inhibition of spore formation but not inhibition of Ime2p stability. Thus, the glucose induction pathway may regulate meiosis through both RGT1- dependent and RGT1- independent pathways. [source] Sex ratio, reproductive mode and genetic diversity in Triops cancriformisFRESHWATER BIOLOGY, Issue 7 2009THORID ZIEROLD Summary 1. Aquatic invertebrates display a wide array of alternative reproductive modes from apomixis to hermaphroditism and cyclical parthenogenesis. These have important effects on genetic diversity and population structure. Populations of the ,living fossil'Triops cancriformis display a range of sex ratios, and various reproductive modes are thought to underlie this variation. Using sex ratio information and histological analyses European populations have been inferred to be gonochoric (with separate males and females), selfing hermaphroditic and androdioecious, a rare reproductive mode in which selfing hermaphrodites coexist with variable proportions of males. In addition, some populations have been described as meiotic parthenogens. 2. Here we use population genetic analysis using microsatellite loci in populations with a range of sex ratios including a gonochoric population, and marker segregation patterns in heterozygote individuals reared in isolation, to clarify the reproductive mode in this species. 3. Our data show that populations in general have very low levels of genetic diversity. Non-gonochoric populations show lower genetic diversity, more heterozygote deficiencies, higher inbreeding coefficients and stronger linkage disequilibria than the gonochoric population. The maintenance of some heterozygosity in populations is consistent with some male influence in T. cancriformis populations, as would be expected from an androdioecious reproductive system. Results of marker segregation in eggs produced in isolation from non-gonochoric populations indicate that meiosis occurs and are consistent with two reproductive modes: selfing hermaphroditism and a type of ameiotic parthenogenesis. 4. Overall, our data indicate that androdioecy and selfing hermaphroditism are the most likely reproductive modes of non-gonochoric European Triops populations. Triops populations are strongly structured, suggesting high genetic drift and low levels of gene flow. [source] Meiosis induced by inactivation of Pat1 kinase proceeds with aberrant nuclear positioning of centromeres in the fission yeast Schizosaccharomyces pombeGENES TO CELLS, Issue 8 2004Yuji Chikashige Nuclear organization of chromosomes proceeds with significant changes during meiosis. In the fission yeast Schizosaccharomyces pombe, centromeres are clustered at the spindle-pole body (SPB) during the mitotic cell cycle; however, during meiotic prophase telomeres become clustered to the SPB and centromeres dissociate from the SPB. We followed the movement of telomeres, centromeres and sister chromatids in living S. pombe cells that were induced to meiosis by inactivation of Pat1 kinase (a key negative regulator of meiosis). Time-course observation in living cells determined the temporal order of DNA synthesis, telomere clustering, centromere separation and meiotic chromosome segregation. When meiosis was induced by Pat1 inactivation at the G1 phase of mitosis, telomeres clustered to the SPB as per normal meiosis, but in most cells the centromeres remained partially associated with the SPB. When meiosis was initiated at the G2 phase by Pat1 inactivation, both telomeres and centromeres retained their mitotic nuclear positions in the majority of cells. These results indicate that the progression of meiosis induced by Pat1 inactivation is aberrant from normal meiosis in some events. As Pat1 inactivation is often useful to induce S. pombe cells synchronously into meiosis, the temporal order of chromosomal events determined here will provide landmarks for the progression of meiosis downstream the Pat1 inactivation. [source] VDE-initiated intein homing in Saccharomyces cerevisiae proceeds in a meiotic recombination-like mannerGENES TO CELLS, Issue 7 2003Tomoyuki Fukuda Background: Inteins and group I introns found in prokaryotic and eukaryotic organisms occasionally behave as mobile genetic elements. During meiosis of the yeast Saccharomyces cerevisiae, the site-specific endonuclease encoded by VMA1 intein, VDE, triggers a single double-strand break (DSB) at an inteinless allele, leading to VMA1 intein homing. Besides the accumulating information on the in vitro activity of VDE, very little has been known about the molecular mechanism of intein homing in yeast nucleus. Results: We developed an assay to detect the product of VMA1 intein homing in yeast genome. We analysed mutant phenotypes of RecA homologs, Rad51p and Dmc1p, and their interacting proteins, Rad54p and Tid1p, and found that they all play critical roles in intein inheritance. The absence of DSB end processing proteins, Sae2p and those in the Mre11-Rad50-Xrs2 complex, also causes partial reduction in homing efficiency. As with meiotic recombination, crossover events are frequently observed during intein homing. We also observed that the absence of premeiotic DNA replication caused by hydroxyurea (HU) or clb5, clb6, mutation reduces VDE-mediated DSBs. Conclusion: The repairing system working in intein homing shares molecular machinery with meiotic recombination induced by Spo11p. Moreover, like Spo11p-induced DNA cleavage, premeiotic DNA replication is a prerequisite for a VDE-induced DSB. VMA1 intein thus utilizes several host factors involved in meiotic and recombinational processes to spread its genetic information and guarantee its progeny through establishment of a parasitic relationship with the organism. [source] Phosphatidylinositol 3-phosphate 5-kinase is required for the cellular response to nutritional starvation and mating pheromone signals in Schizosaccharomyces pombeGENES TO CELLS, Issue 2 2002Masayo Morishita Background: Phosphatidylinositol (3,5) bisphosphate, which is converted from phosphatidylinositol 3-phosphate by phosphatidylinositol 3-phosphate 5-kinase, is implicated in vacuolar functions and the sorting of cell surface proteins within endosomes in the endocytic pathway of budding yeast. A homologous protein, SpFab1p, has been found in the fission yeast Schizosaccharomyces pombe, but its role is not known. Results: Here we report that SpFab1p is encoded by ste12+ known as a fertility gene in S. pombe. The ste12 mutant grew normally under stress-free conditions, but was highly vacuolated and swelled at high temperatures and under starvation conditions. In nitrogen-free medium, ste12 cells were arrested in G1 phase, but partially defective in the expression of genes responsible for mating and meiosis. The ste12 mutant was defective both in the production of, and in the response to, mating pheromones. The amount of the pheromone receptor protein Map3p, was substantially decreased in ste12 cells. Map3p was transported to the cell surface, then internalized and eventually transported to the vacuolar lumen, even in the ste12 mutant. Conclusion: The results indicate that phosphatidylinositol(3,5)bisphosphate is essential for cellular responses to various stresses and for the mating pheromone signalling under starvation conditions. [source] Meu10 is required for spore wall maturation in Schizosaccharomyces pombeGENES TO CELLS, Issue 2 2002Takahiro Tougan Background: Many genes are meiosis and/or sporulation-specifically transcribed during this process. Isolation and analysis of these genes might help us to understand how meiosis and sporulation are regulated. For this purpose, we have isolated a large number of cDNA clones from Schizosaccharomyces pombe whose expression is up-regulated during meiosis. Results: We have isolated meu10+ gene, which encodes 416 amino acids and bears homology to SPS2 of Saccharomyces cerevisiae. A strain whose meu10+ gene has been deleted forms no viable spores. Thin-section electron micrographs showed that the meu10, strain has abnormally formed spore walls, and then they disrupt, allowing cytoplasmic material to escape. The Meu10-GFP fusion protein is localized to the spore periphery, thereafter returned to the cytoplasm after sporulation. Meu10-GFP localization to the spore wall was almost normal in the bgs2, or chs1, mutants that lack 1,3-,-glucan or chitin, respectively. In contrast, 1,3-,-glucan is abnormally localized in meu10, cells. Meu10 has an N-terminal domain with homology to the mammalian insulin receptor and a C-terminal domain with a transmembrane motif. Mutants whose N-terminal or C-terminal domain was truncated were severely defective for sporulation. Conclusions: Meu10 is a spore wall component and plays a pivotal role in the formation of the mature spore wall structure. [source] Polyploidy-Associated Genomic Instability in Arabidopsis thalianaGENESIS: THE JOURNAL OF GENETICS AND DEVELOPMENT, Issue 4 2010Yixing Wang Formation of polyploid organisms by fertilization of unreduced gametes in meiotic mutants is believed to be a common phenomenon in species evolution. However, not well understood is how species in nature generally exist as haploid and diploid organisms in a long evolutionary time while polyploidization must have repeatedly occurred via meiotic mutations. Here, we show that the ploidy increased for two consecutive generations due to unreduced but viable gametes in the Arabidopsis cyclin a1;2-2 (also named tardy asynchronousmeiosis-2) mutant, but the resultant octaploid plants produced progeny of either the same or reduced ploidy via genomic reductions during meiosis and pollen mitosis. Ploidy reductions through sexual reproduction were also observed in independently generated artificial octaploid and hexaploid Arabidopsis plants. These results demonstrate that octaploid is likely the maximal ploidy produced through sexual reproduction in Arabidopsis. The polyploidy-associated genomic instability may be a general phenomenon that constrains ploidy levels in species evolution. [source] Short rare MUC6 minisatellites-5 alleles influence susceptibility to gastric carcinoma by regulating gene,HUMAN MUTATION, Issue 8 2010Jeong-Ah Kwon Abstract The human MUC6 gene, which is reported to be expressed in the stomach and gall bladder, is clustered on chromosome 11p15.5 with other secreted mucins. In this study, the genomic structure of MUC6 has been analyzed and five VNTR (minisatellites; MS1,MS5) were identified. These minisatellites were analyzed in genomic DNA extracted from 1,103 controls, 470 gastric cancer patients, and multigenerational families. Five novel minisatellites were found to be polymorphic and transmitted through meiosis by Mendelian inheritance in families. We evaluated allelic variation in these minisatellites to determine if such variation affected the susceptibility to gastric cancer. A significant association (odds ratio [OR]=7.08) between short rare MUC6,MS5 alleles and relative risks were observed for gastric cancer (95% confidence interval [CI], 1.43,35.19; P=0.005). To investigate the function of minisatellite alleles of MUC6,MS5, we examined the effects on gene expression from luciferase reporters when inserted with minisatellites. Interestingly, when the shortest allele (7TR) was inserted in the promoter, the expression level decreased over 20-fold (P<0.001) in normal and cancer cell lines. Furthermore, the cancer-specific rare allele (TR8) also showed decreased expression levels in cancer cells. Therefore, we suggest that the short rare MUC6,MS5 alleles may be related to cancer development by the regulation of MUC6 expression. Hum Mutat 31:1,8, 2010. © 2010 Wiley-Liss, Inc. [source] | ||||||||||||||||||||||||||||||||||