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Advances in oyster farming have been achieved with biotechnological applications, such as chromosome manipulations aimed at polyploidy. The bivalve Anomalocardia brasiliana is of considerable importance to artisanal fishing activities and is a potential organism for aquaculture in Brazil. The cytogenetic behavior of polar bodies during meiosis provides essential information for chromosome manipulation directed at the production of triploid organisms. The objective of the present study was to identify postfertilization times in which the polar bodies are expelled and determine the most frequent number of chromosomes in A. brasiliana. Individuals were caught on the coast of the state of Pernambuco, in northeast Brazil, and subsequently induced to release the gametes. Samples of the oocyte solution were taken before and every 2 min after fertilization. The material was fixed in Carnoy's solution, stained with 4',6-diamidino-2-phenylindole and photographed under an epifluorescence microscope. Among the 50 oocytes analyzed in metaphase I, 19 bivalents were found. The release of the first polar body was detected 10 min after fertilization among 70% of the eggs, whereas the second polar body was released at 16 min among 62% of the eggs. This work provides relevant information on the time of initiation of shock treatments for chromosome set manipulation, aiming triploids of this important marine fishery resource in Brazil. KEY WORDS: Anomalocardia brasiliana, polar bodies release timing, number of chromosomes, polyploid, INTRODUCTION Chromosome manipulation is a strategy that has been adopted in aquaculture activities around the world since the 1980s to accelerate the growth of bivalves (Guo et al. 2009, Maldonado-Amparo [...]

We sought to elucidate the mechanisms that regulate the spatial patterning of meiotic DSBs. The conserved DNA damage response (DDR) kinase ataxia-telangiectasia mutated (ATM) inhibits excessive DSB formation in a [...], Meiotic recombination is a critical step in gametogenesis for many organisms, enabling the creation of genetically diverse haploid gametes. In each meiotic cell, recombination is initiated by numerous DNA double-strand breaks (DSBs) created by Spo11, the evolutionarily conserved topoisomerase-like protein (1), but how these DSBs are distributed relatively uniformly across the four chromatids that make up each chromosome pair is poorly understood. Here we employ Saccharomyces cerevisiae to demonstrate distance-dependent DSB interference in cis (in which the occurrence of a DSB suppresses adjacent DSB formation)--a process that is mediated by the conserved DNA damage response kinase, [Tel1.sup.ATM]. The inhibitory function of Tel1 acts on a relatively local scale, while over large distances DSBs have a tendency to form independently of one another even in the presence of Tel1. Notably, over very short distances, loss of Tel1 activity causes DSBs to cluster within discrete zones of concerted DSB activity. Our observations support a hierarchical view of recombination initiation where [Tel1.sup.ATM] prevents clusters of DSBs, and further suppresses DSBs within the surrounding chromosomal region. Such collective negative regulation will help to ensure that recombination events are dispersed evenly and arranged optimally for genetic exchange and efficient chromosome segregation.

During meiosis, crossovers promote genetic diversity and create physical connections between homologues that ensure their accurate segregation (reviewed in refs 1-3). Crossovers arise stochastically from a larger set of undifferentiated [...], Spatial patterning is a ubiquitous feature of biological systems. Meiotic crossovers provide an interesting example, defined by the classic phenomenon of crossover interference. Here we identify a molecular pathway for interference by analysing crossover patterns in budding yeast. Topoisomerase II plays a central role, thus identifying a new function for this critical molecule. SUMOylation (of topoisomerase II and axis component Red1) and ubiquitin-mediated removal of SUMOylated proteins are also required. The findings support the hypothesis that crossover interference involves accumulation, relief and redistribution of mechanical stress along the protein/DNA meshwork of meiotic chromosome axes, with topoisomerase II required to adjust spatial relationships among DNA segments.

Meiosis in Sphagnum lescurii The sphagnum mosses (Sphagnopsida) are an early divergent and distinct group. They are hydrophytes of worldwide distribution, but more abundant in boreal regions of the world [...]

Polar Organizers in Meiosis of Sphaerocarpos texanus Whereas POs are the only MTOCs known in liverwort mitosis, their occurrence in meiosis is sporadic. POs are usually associated with meiotic quadripolarity [...]

Meiosis in bryophytes retains unusual features that provide clues to the innovation of sporogenesis in early land plants. Sporocytes are typically quadrilobed before nuclear division and the meiotic spindle is quadripolar with poles in the four future spore domains. Whereas seed plants consistently have anastral spindles arising from γ-tubulin in the perinuclear area, bryophytes have spindles organized at POs, plastids, or nuclear envelope. All of these MTOCs are significantly different from centrosomes of the algal ancestors. Mosses and hornworts have quadrilobed sporocytes with meiotic spindles organized at plastids. Meiosis in liverworts is extremely varied. Sporocytes of Jungermanniopsida are deeply quadrilobed and have microtubule bands marking division planes prior to cytoplasmic shaping. Spindles are organized at POs or nuclear envelope. Sporocytes of Marchantiopsida are quadrilobed to apolar with spindles organized at plastids, POs, or nuclear envelope. Premeiotic bands have been reported in only one marchantiod, the early divergent Blasia. An atlas of cytological data on 13 liverworts, 3 mosses and 2 homworts is presented and analyzed. Keywords Cell division * Meiosis * Microtubules * MTOC * Mitosis * QMS. Quadripolarity, Introduction Meiosis is an essential event in the life cycle of all sexually reproducing plants. It occurs in specialized cells (sporocytes) of the sporophyte (spore producing) generation and yields haploid [...]

As part of the life sciences series, this article describes the role of deoxyribonucleic acid and ribonucleic acid, genes and chromosomes. The processes of mitosis and meiosis are discussed and [...]

Introduction Detailed reports on variation in meiotic behaviour within individual organisms are rare. Sybenga et al. (2008) reported on the variation between tillers of single plants of rye (Secale cereale [...], Meiotic metaphase I configuration frequencies were determined in different tillers of genetically related plants of rye (Secale cereale L.) heterozygous for reciprocal translocation T248W (between chromosome arms 1RS and 6RS) and with an additional (telocentric) arm 1RS. Seventeen different configurations could be recognized, grouped into three categories. Very different configuration frequencies were found not only between sister plants from the same parents but also between tillers of the same plant grown under identical conditions (climate chambers at 15°C and 20°C). The heterogeneity reflects variation in chromosome pairing and crossing over, and is variable and unpredictable. Anthers within florets were homogeneous. Between tiller heterogeneity is insufficient to explain differences between sister plants. It is ascribed to random somatic variation in the conditions of the chromatin which, at meiosis, govern chromosome pairing. During sexual differentiation, these conditions are fixed and subsequent cell lineages have the same pairing and crossing over characteristics. As homology search is an activity of DNA, this control of pairing and crossing over, consistent over long cell lineages, may be considered to be epigenetic even when no realistic suggestions concerning its character can be given. Key words: translocation heterozygote, single-arm-trisomiy, Secale, meiosis, intraplant heterogeneity. Les frequences des configurations observees lors de la metaphase I de la meiose ont ete mesurees au sein de differents talles de chacun de plusieurs plants de seigle (Secale cereale L.) genetiquement apparentes et heterozygotes pour la translocation reciproque T248W (entre bras chromosomiques 1RS et 5RS) ainsi que porteurs d'un bras (telocentrique) 1RS additionnel. Dix-sept configurations differentes ont ete distinguees et formaient trois categories. Des frequences tres differentes de ces configurations ont ete observees non seulement entre plantes soeurs ayant les memes parents, mais aussi entre talles de la meme plante cultivee dans des conditions identiques (chambres de croissance a 15°C ou 20°C). Cette heterogeneite reflete de la variation au niveau de l' appariement chromosomique et des enjambements, est variable et imprevisible. Les antheres au sein d' une fleur etaient homogenes. L' heterogeneite observee entre talles etait insuffisante pour expliquer les differences entre plantes soeurs. Ces dernieres sont plutot attribuees a une variation somatique aleatoire dans l' etat de la chromatine qui, lors de la meiose, determinerait l' appariement. Au cours de la differenciation sexuelle, ces conditions seraient fixees et les lignees cellulaires derivees partageraient les memes caracteristiques en matiere d'appariement et de recombinaison. Comme la recherche d'homologie est un processus qui depend de l'ADN, on peut considerer ce controle de l' appariement et de l' enjambent, conserve tout au long de lignees cellulaires, comme etant un processus epigenetique bien qu'aucune hypothese realiste concernant sa nature ne puisse etre mise de l'avant. Mots-cles : heterozygote pour une translocation, trisomie pour un seul bras, Secale, meiose, heterogeneite intra- plante. [Traduit par la Redaction]

Introduction Polyploidy has played a major role in the evolution of higher plants, and polyploids have often been selected during the evolution of crop plants (Levin 2002). As now generally [...], The wild species Brassica fruticulosa Cyr. (FF, 2n = 16) is closely related to the cultivated Brassica species. Through interspecific reciprocal crosses between B. fruticulosa and three cultivated Brassica allotetraploids (AABB, AACC, and BBCC where A = 10, B = 8, and C = 9), four trigenomic hybrids (F.AC, 2n = 27; F.AB, 2n = 26; F.BC, 2n = 25; BC.F, 2n = 25) were produced. By chromosome doubling of respective hybrids, three allohexaploids (FF.AACC, 2n = 54; FF.AABB, 2n = 52; BBCC.FF, 2n = 50) were synthesized. In pollen mother cells (PMCs) of the trigenomic hybrids, 1-2 autosyndetic bivalents were detected within A, B, and C genomes but only one within F genome; 1-3 allosyndetic bivalents between any two genomes were observed, and a closer relationship of F and B genomes than F and A genomes or F and C genomes was revealed. The allohexaploids showed a generally low but different pollen fertilities. The chromosomes in PMCs were predominantly paired as bivalents but some univalents and multivalents at variable frequencies were observed. The bivalents of homologous pairing for each genome prevailed, but allosyndetic quadrivalents and hexavalents involving any two genomes were observed, together with autosyndetic quadrivalents for A, B, and C genomes but not the F genome. The nondiploidized cytological behaviour of these allohexaploids contributed to their low fertility. The relationships between the genome affinity and meiotic behavior in these allohexaploids were discussed. Key words: cultivated Brassica allotetraploids, Brassica fruticulosa, interspecific hybrids, autosyndesis, allosyndesis, diploidization. L'espece sauvage Brassica fruticulosa Cyr. (FF, 2n = 16) est tres apparentee aux especes cultivees du genre Brassica. Suite a des croisements interspecifiques reciproques entre le B. fruticulosa et les trois Brassica allotetraploides (AABB, AACC et BBCC oU A = 10, B = 8 et C = 9), quatre hybrides trigenomiques (F.AC, 2n = 27; F.AB, 2n = 26; F.BC, 2n = 25; BC.F, 2n = 25) ont ete produits. Suite a un doublement chromosomique chez ces hybrides, trois allohexaploides (FF.AACC, 2n = 54; FF.AABB, 2n = 52; BBCC.FF, 2n = 50) ont ete obtenus. Au sein des microsporocytes (PMC) des hybrides trigenomiques, 1-2 bivalents autosyndetiques ont ete detectes au sein des genomes A, B et C, mais un seul a ete observe pour le genome F. Un a trois bivalents allosyndetiques entre n'importe lesquels des genomes ont ete observes, et une plus grande proximite entre les genomes F et B qu'entre les genomes F et A ou F et C a ete constatee. Les allohexaploides affichaient une fertilite du pollen differente mais generalement faible. Les chromosomes dans les microsporocytes etaient principalement apparies sous forme de bivalents, bien que des univalents et des multivalents aient ete observes a des frequences variables. Les bivalents impliquant des chromosomes homologues de chaque genome etaient predominants, bien que des quadri-/hexavalents allosyndetiques impliquant deux genomes aient ete observes, de meme que des quadrivalents autosyndetiques pour les genomes A,B et C, mais pas pour le genome F. Le comportement cytologique non-diploide chez ces allohexaploides a contribue a leur faible fecondite. Les auteurs discutent des relations entre l'affinite des genomes et le comportement meiotique au sein de ces allohexaploides. Mots-cles : Brassica allotetraploides cultives, Brassica fruticulosa, hybrides interspecifiques, autosyndese, allosyndese, diploidisation. [Traduit par la Redaction]

Introduction Meiosis is a highly specialized cell division that requires a significant retooling of the normal cell cycle machinery. In spite of the need for many meiosis-specific factors, meiosis still [...], The spindle assembly checkpoint (SAC) plays an important role in mitotic cells to sense improper chromosome attachment to spindle microtubules and to inhibit [APC.sup.Fzy]-dependent destruction of cyclin B and Securin; consequent initiation of anaphase until correct attachments are made. In Drosophila, SAC genes have been found to play a role in ensuring proper chromosome segregation in meiosis, possibly reflecting a similar role for the SAC in [APC.sup.Fzy] inhibition during meiosis. We found that loss of function mutations in SAC genes, Mad2, zwilch, and mps1, do not lead to the predicted rise in [APC.sup.Fzy]-dependent degradation of cyclin B either globally throughout the egg or locally on the meiotic spindle. Further, the SAC is not responsible for the inability of [APC.sup.Fzy] to target cyclin B and promote anaphase in metaphase II arrested eggs from cort mutant females. Our findings support the argument that SAC proteins play checkpoint independent roles in Drosophila female meiosis and that other mechanisms must function to control APC activity. Key words: Drosophila, meiosis, anaphase promoting complex, spindle assembly checkpoint. Le point de controle de l'assemblage du fuseau (ou SAC pour << spindle assembly checkpoint >>) joue un role important au sein des cellules mitotiques pour detecter des connexions chromosomiques incorrectes avec les microtubules du fuseau et pour inhiber la destruction de la cycline B et de la securine qui depend de [l'APC.sup.Fzy], ainsi que l'initiation de l'anaphase qui s'en suit, jusqu'a ce que de bonnes connexions aient ete etablies. Chez le Drosophila, il a ete observe que les genes SAC jouaient un role semblable pour assurer une bonne segregation des chromosomes lors de la meiose, ce qui suggere un role semblable du SAC dans l'inhibition de [l'APC.sup.Fzy] au cours de la meiose. Les auteurs rapportent que des mutations entrainant une perte de fonction au sein des genes SAC Mad2, zwilch et mpsl ne causent pas la hausse attendue de la degradation de la cycline B dependante de [l'APC.sup.Fzy] a un niveau global dans l'ensemble de l'oeuf ou localement sur le fuseau meiotique. De plus, le SAC n'est pas responsable de l'incapacite de [l'APC.sup.Fzy] a cibler la cycline B et a promouvoir l'anaphase chez des oeufs arretes en metaphase II chez des femelles mutantes cort. Ces observations supportent l'hypothese voulant que les proteines SAC jouent des roles autres que la surveillance du cycle cellulaire lors de la meiose chez les Drosophila femelles et que d'autres mecanismes doivent intervenir pour controler l'activite d'APC. Mots-cles : Drosophila, meiose, complexe promoteur de l'anaphase, point de controle de l'assemblage du fuseau. [Traduit par la Redaction]

In contrast to somatic cells, formation of acentriolar meiotic spindles relies on the organization of microtubules (MTs) and MT-organizing centers (MTOCs) into a stable bipolar structure. The underlying mechanisms are still unknown. We show that this process is impaired in hepatoma up-regulated protein (Hurp) knockout mice, which are viable but female sterile, showing defective oocyte divisions. HURP accumulates on interpolar MTs in the vicinity of chromosomes via Kinesin-5 activity. By promoting MT stability in the spindle central domain, HURP allows efficient MTOC sorting into distinct poles, providing bipolarity establishment and maintenance. Our results support a new model for meiotic spindle assembly in which HURP ensures assembly of a central MT array, which serves as a scaffold for the genesis of a robust bipolar structure supporting efficient chromosome congression. Furthermore, HURP is also required for the clustering of extra centrosomes before division, arguing for a shared molecular requirement of MTOC sorting in mammalian meiosis and cancer cell division. doi/ 10.1083/jcb.201005065

The egg's competency to activate at fertilization and transition to embryogenesis is dependent on its ability to generate a fertilization-specific [Ca.sup.2+] transient. To endow the egg with this capacity, [Ca.sup.2+] signals remodel during oocyte maturation, including inactivation of the primary [Ca.sup.2+] influx pathway store-operated [Ca.sup.2+] entry (SOCE). SOCE inactivation is coupled to internalization of the SOCE channel, Orail. In this study, we show that Orail internalizes during meiosis through a caveolin (Cav)--and dynamin-dependent endocytic pathway. Cav binds to Orail, and we map a Cav consensus-binding site in the Orail N terminus, which is required for Orail internalization. Furthermore, at rest, Orail actively recycles between an endosomal compartment and the cell membrane through a Rho-dependent endocytic pathway. A significant percentage of total Orail is intracellular at steady state. Store depletion completely shifts endosomal Orail to the cell membrane. These results define vesicular trafficking mechanisms in the oocyte that control Orail subcellular localization at steady state, during meiosis, and after store depletion. doi/ 10.1083/jcb.201006022

Recent evidence indicates that meiosis arose very early in eukaryotic evolution, which suggests that essential features of meiosis were already present in the prokaryotic ancestors of eukaryotes. Furthermore, in extant [...]

The evolutionary appearance of p53 protein probably preceded its role in tumor suppression, suggesting that there may be unappreciated functions for this protein. Using genetic reporters as proxies to follow in vivo activation of the p53 network in Drosophila, we discovered that the process of meiotic recombination instigates programmed activation of p53 in the germ line. Specifically, double-stranded breaks in DNA generated by the topoisomerase Spo11 provoked functional p53 activity, which was prolonged in cells defective for meiotic DNA repair. This intrinsic stimulus for the p53 regulatory network is highly conserved because Spoil-dependent activation of p53 also occurs in mice. Our findings establish a physiological role for p53 in meiosis and suggest that tumor-suppressive functions may have been co-opted from primordial activities linked to recombination. 10.1126/science.1185640

Bloom's syndrome (BS) is an autosomal recessive disorder characterized by growth retardation, cancer predisposition, and sterility. BS mutated (BIm), the gene mutated in BS patients, is one of five mammalian RecQ helicases. Although BLM has been shown to promote genome stability by assisting in the repair of DNA structures that arise during homologous recombination in somatic cells, less is known about its role in meiotic recombination primarily because of the embryonic lethality associated with Blm deletion. However, the localization of BLM protein on meiotic chromosomes together with evidence from yeast and other organisms implicates a role for BLM helicase in meiotic recombination events, prompting us to explore the meiotic phenotype of mice bearing a conditional mutant allele of Blm. In this study, we show that BLM deficiency does not affect entry into prophase I but causes severe defects in meiotic progression. This is exemplified by improper pairing and synapsis of homologous chromosomes and altered processing of recombination intermediates, resulting in increased chiasmata. Our data provide the first analysis of BLM function in mammalian meiosis and strongly argue that BLM is involved in proper pairing, synapsis, and segregation of homologous chromosomes; however, it is dispensable for the accumulation of recombination intermediates. doi/10.1083/jcb.200909048

Meiotic crossovers (COs) are tightly regulated to ensure that COs on the same chromosome are distributed far apart (crossover interference, COl) and that at least one CO is formed per homolog pair (CO homeostasis). CO formation is controlled in part during meiotic double-strand break (DSB) creation in Caenorhabditis elegans, but a second level of control must also exist because meiotic DSBs outnumber COs. We show that the anti-recombinase RTEL-1 is required to prevent excess meiotic COs, probably by promoting meiotic synthesis-dependent strand annealing. Two distinct classes of meiotic COs are increased in rtel-1 mutants, and COl and homeostasis are compromised. We propose that RTEL-1 implements the second level of CO control by promoting noncrossovers. 10.1126/science.1183112

In many organisms, female and male meiosis display extensive sexual dimorphism in the temporal meiotic program, the number and location of recombination events, sex chromosome segregation, and checkpoint function. We show here that both meiotic prophase timing and germ-line apoptosis, one output of checkpoint signaling, are dictated by the sex of the germ line (oogenesis vs. spermatogenesis) in Caenorhabditis elegans. During oogenesis in feminized animals (fem-3), a single pair of asynapsed autosomes elicits a checkpoint response, yet an unpaired X chromosome fails to induce checkpoint activation. The single X in males and fem-3 worms is a substrate for the meiotic recombination machinery and repair of the resulting double strand breaks appears to be delayed compared with worms carrying paired X chromosomes. Synaptonemal complex axial HORMA domain proteins, implicated in repair of meiotic double strand breaks (DSBs) and checkpoint function, are assembled and disassembled on the single X similarly to paired chromosomes, but the central region component, SYP-1, is not loaded on the X chromosome in males. In fem-3 worms some X chromosomes achieve nonhomologous self-synapsis; however, germ cells with SYP-1-positive X chromosomes are not preferentially protected from apoptosis. Analyses of chromatin and X-linked gene expression indicate that a single X, unlike asynapsed X chromosomes or autosomes, maintains repressive chromatin marks and remains transcriptionally silenced and suggests that this state locally precludes checkpoint signaling.

Nanos is one of the evolutionarily conserved proteins implicated in germ cell development. We have previously shown that NANOS2 plays an important role in both the maintenance and sexual development of germ cells. However, the molecular mechanisms underlying these events have remained elusive. In our present study, we found that NANOS2 localizes to the P-bodies, known centers of RNA degradation that are abundantly accumulated in male gonocytes. We further identified by immunoprecipitation that the components of the CCR4-NOT deadenylation complex are NANOS2-interacting proteins and found that NANOS2 promotes the localization of CNOT proteins to P-bodies in vivo. We also elucidated that the NANOS2/CCR4-NOT complex has deadenylase activity in vitro, and that some of the RNAs implicated in meiosis interact with NANOS2 and are accumulated in its absence. Our current data thus indicate that the expression of these RNA molecules is normally suppressed via a NANOS2-mediated mechanism. We propose from our current findings that NANOS2-interacting RNAs may be recruited to P-bodies and degraded by the enzymes contained therein through NANOS2-mediated deadenylation. germ cells | P-body | meiosis doi/10.1073/pnas.0908664107

Sister chromatid cohesion is essential to maintain stable connections between homologues and sister chromatids during meiosis and to establish correct centromere orientation patterns on the meiosis I and II spindles. However, the meiotic cohesion apparatus in Drosophila melanogaster remains largely uncharacterized. We describe a novel protein, sisters on the loose (SOLO), which is essential for meiotic cohesion in Drosophila. In solo mutants, sister centromeres separate before prometaphase I, disrupting meiosis I centromere orientation and causing nondisjunction of both homologous and sister chromatids. Centromeric foci of the cohesin protein SMC1 are absent in solo mutants at all meiotic stages. SOLO and SMC1 colocalize to meiotic centromeres from early prophase I until anaphase II in wild-type males, but both proteins disappear prematurely at anaphase I in mutants for mei-S332, which encodes the Drosophila homologue of the cohesin protector protein shugoshin. The solo mutant phenotypes and the localization patterns of SOLO and SMC1 indicate that they function together to maintain sister chromatid cohesion in Drosophila meiosis. doi/10.1083/jcb.200904040

Crossing over establishes connections between homologous chromosomes that promote their proper segregation at the first meiotic division. However, there exists a backup system to ensure the correct segregation of those chromosome pairs that fail to cross over. We have found that, in budding yeast, a mutation eliminating the synaptonemal complex protein, Zip1, increases the meiosis I nondisjunction rate of nonexchange chromosomes (NECs). The centromeres of NECs become tethered during meiotic prophase, and this tethering is disrupted by the zip1 mutation. Furthermore, the Zip1 protein often colocalizes to the centromeres of the tethered chromosomes, suggesting that Zip1 plays a direct role in holding NECs together. Zip3, a protein involved in the initiation of synaptonemal complex formation, is also important for NEC segregation. In the absence of Zip3, both the tethering of NECs and the localization of Zip1 to centromeres are impaired. A mutation in the MAD3 gene, which encodes a component of the spindle checkpoint, also increases the nondisjunction of NECs. Together, the zip1 and mad3 mutations have an additive effect, suggesting that these proteins act in parallel pathways to promote NEC segregation. We propose that Mad3 promotes the segregation of NECs that are not tethered by Zip1 at their centromeres. spindle checkpoint | Mad3 | nondisjunction | Zip3 | centromere www.pnas.org/cgi/doi/10.1073/pnas.0913435107

Two critical stages of mammalian oocyte regulation are prophase I arrest, which is important for sustaining the oocyte pool, and the progression through meiosis I (MI) to produce fertilizable eggs. We have found that the spindle assembly checkpoint protein BubR1 regulates both stages in mouse oocytes. We show that oocytes depleted of BubR1 cannot sustain prophase I arrest and readily undergo germinal vesicle breakdown, a marker for reentry into MI. BubR1-depleted oocytes then arrest before completing MI, marked by failure of polar body extrusion. Both meiotic defects in BubR1-depleted oocytes are due to reduced activity of the master regulator known as the anaphase-promoting complex (APC), brought about through diminished levels of the APC coactivator Cdh1. 22 April 2009; accepted 24 September 2009 10.1126/science.1175326

During meiosis, homologues become juxtaposed and synapsed along their entire length. Mutations in the cohesin complex disrupt not only sister chromatid cohesion but also homologue pairing and synaptonemal complex formation. In this study, we report that Pds5, a cohesin-associated protein known to regulate sister chromatid cohesion, is required for homologue pairing and synapsis in budding yeast. Pds5 colocalizes with cohesin along the length of meiotic chromosomes. In the absence of Pds5, the meiotic cohesin subunit Rec8 remains bound to chromosomes with only minor defects in sister chromatid cohesion, but sister chromatids synapse instead of homologues. Double-strand breaks (DSBs) are formed but are not repaired efficiently. In addition, meiotic chromosomes undergo hypercondensation. When the mitotic cohesin subunit Mcd1 is substituted for Rec8 in Pds5-depleted cells, chromosomes still hypercondense, but synapsis of sister chromatids is abolished. These data suggest that Pds5 modulates the Rec8 activity to facilitate chromosome morphological changes required for homologue synapsis, DSB repair, and meiotic chromosome segregation.

Double-strand breaks (DSBs) in DNA are lethal unless repaired. Faithful repair requires processing of the DSB ends and interaction with intact homologous DNA, which can produce genetic recombinants. To determine the role of nucleases in DSB end-processing and joint molecule resolution, we studied recombination at the site of a single DSB, generated by induction of the I-Scel endonuclease, during meiosis of fission yeast lacking Rec12 (Spo11 homolog) and, hence, other DSBs. We find that in the presence of the MRN (Rad32-Rad50-Nbs1) complex efficient recombination requires Ctp1, the ortholog of the nuclease Sae2, but not the nuclease activity of MRN. In the absence of MRN, exonuclease 1 (Exo1) becomes the major nuclease required for efficient recombination. Our data indicate that MRN enables access of Ctp1 to the DSB but blocks access of Exo1. In our assay, the Rad16-Swi10 nuclease, required for nucleotide excision-repair, is required for efficient recombination, presumably to remove heterologous DNA at the end of the I-Scel cut site. Another nuclease, the Mus81-Eme1 Holliday junction resolvase, is required to generate crossovers accompanying gene conversion at the I-Scel cut site. Additional, previously published evidence indicates that these 5 nucleases play similar roles in wild-type fission yeast meiotic recombination and in the repair of spontaneous and damage-induced mitotic DSBs. We propose that in wild-type meiosis MRN, in conjunction with Ctp1, removes the covalently attached Rec12 protein from the DNA end, which is then resected by Ctp1 and other activities to produce the single-stranded DNA necessary for further steps of DSB repair. DNA resection | DSB repair | S. pombe | Sae2

Normally, meiotic crossovers in conjunction with sister-chromatid cohesion establish a physical connection between homologs that is required for their accurate segregation during the first meiotic division. However, in some organisms an alternative mechanism ensures the proper segregation of bivalents that fail to recombine. In Drosophila oocytes, accurate segregation of achiasmate homologs depends on pairing that is mediated by their centromere-proximal heterochromatin. Our previous work uncovered an unexpected link between sister-chromatid cohesion and the fidelity of achiasmate segregation when Drosophila oocytes are experimentally aged. Here we show that a weak mutation in the meiotic cohesion protein ORD coupled with a reduction in centromere-proximal heterochromatin causes achiasmate chromosomes to missegregate with increased frequency when oocytes undergo aging. If ORD activity is more severely disrupted, achiasmate chromosomes with the normal amount of pericentric heterochromatin exhibit increased nondisjunction when oocytes age. Significantly, even in the absence of aging, a weak ord allele reduces heterochromatin-mediated pairing of achiasmate chromosomes. Our data suggest that sister-chromatid cohesion proteins not only maintain the association of chiasmate homologs but also play a role in promoting the physical association of achiasmate homologs in Drosophila oocytes. In addition, our data support the model that deterioration of meiotic cohesion during the aging process compromises the segregation of achiasmate as well as chiasmate bivalents.

The bipolar spindle forms without centrosomes naturally in female meiosis and by experimental manipulation in mitosis. Augmin is a recently discovered protein complex required for centrosome-independent microtubule generation within the spindle in Drosophila melanogaster cultured cells. Five subunits of Augmin have been identified so far, but neither their organization within the complex nor their role in developing organisms is known. In this study, we report a new Augmin subunit, wee Augmin component (Wac). Wac directly interacts with another Augmin subunit, Dgt2, via its coiled-coil domain. Wac depletion in cultured cells, especially without functional centrosomes, causes severe defects in spindle assembly. We found that a wac deletion mutant is viable but female sterile and shows only a mild impact on somatic mitosis. Unexpectedly, mutant female meiosis showed robust microtubule assembly of the acentrosomal spindle but frequent chromosome misalignment. For the first time, this study establishes the role of an Augmin subunit in developing organisms and provides an insight into the architecture of the complex.

Saccharomyces cerevisiae can be infected with four amyloid-based prions: [URE3], [[PSI.sup.+]], [[PIN.sup.+]], and [[SWI.sup.+]], due to self-propagating aggregation of Ure2p, Sup35p, Rnq1p and Swi1p, respectively. We searched for new prions of yeast by fusing random segments of yeast DNA to SUP35MC, encoding the Sup35 protein lacking its own prion domain, selecting clones in which Sup35MC function was impaired. Three different clones contained parts of the Q/N-rich amino-terminal domain of Mca1p/Yca1p with the Sup35 part of the fusion protein partially inactive. This inactivity was dominant, segregated 4:0 in meiosis, and was efficiently transferred by cytoplasmic mixing. The inactivity was cured by overexpression of Hsp104, but the prion could arise again in the cured strain (reversible curing). Overproduction of the Mca1 N-terminal domain induced the de novo appearance of the prion form of the fusion. The prion state, which we name [MCA], was transmitted to the chromosomally encoded Mca1p based on genetic, cytological and biochemical tests. cytoduction | metacaspase | prion

In contrast to the extensive analysis of the regulation of Cyclin B protein levels during developmental progression through meiosis in oogenesis, little is known about Cyclin A. Repression of cyclin A translation early in prophase I in Drosophila is important to maintain the oocyte in meiosis, and this has been shown to be mediated by deadenylation of the mRNA and inhibition by the Bruno repressor. We find that at oocyte maturation as meiosis resumes, Cyclin A protein reappears, coincident with polyadenylation of the mRNA and loss of Bruno repressor. Cyclin A is multiphosphorylated in a pattern consistent with autophosphorylation, and this form accumulates aberrantly in metaphase I if the Cortex form of the Anaphase Promoting Complex/Cyclosome is inactive. The PAN GU (PNG) kinase positively promotes translation of Cyclin A, beginning in oogenesis, an earlier onset than previously recognized. After egg activation and the completion of meiosis, PNG promotes further polyadenylation of cyclin A mRNA and appears to antagonize repression of translation by the PUMILIO inhibitor. Epistasis studies with png; apc mutants indicate that PNG acts solely to promote translation, rather than having a parallel function to inhibit degradation. These studies reveal multiple levels of posttranscriptional regulation of Cyclin A protein by translational and proteolytic control during oocyte maturation and the onset of embryogenesis. APC/C | Drosophila | oocyte maturation | PNG kinase | translation

Altered spindle microtubule dynamics at anaphase onset are the basis for chromosome segregation. In Xenopus laevis egg extracts, increasing free calcium levels and subsequently rising calcium-calmodulin-dependent kinase II (CaMKII) activity promote a release from meiosis II arrest and reentry into anaphase. CaMKII induces the activation of the anaphase-promoting complex/cyclosome (APC/C), which destines securin and cyclin B for degradation to allow chromosome separation and mitotic exit. In this study, we investigated the calcium-dependent signal responsible for microtubule depolymerization at anaphase onset after release from meiotic arrest in Xenopus egg extracts. Using Ran-guanosine triphosphate-mediated microtubule assemblies and quantitative analysis of complete spindles, we demonstrate that CaMKII triggers anaphase microtubule depolymerization. A CaMKII-induced twofold increase in microtubule catastrophe rates can explain reduced microtubule stability. However, calcium or constitutively active CaMKII promotes microtubule destabilization even upon APC/C inhibition and in the presence of high cyclin-dependent kinase 1 activity. Therefore, our data demonstrate that CaMKII turns on parallel pathways to activate the APC/C and to induce microtubule depolymerization at meiotic anaphase onset.

The mating-type bias (mat-bias) of gene conversion was previously described as a phenomenon in which the number of prototrophic recombinants in an ura4A heteroallelic two-factor cross relates to the mating types of the parents. We show now that the mat-bias is restricted neither to ura4A nor to recombination hotspots, but occurs at other genomic loci, too. It is specific for gene conversion and absent in azygotic meiosis. Thus, the mat-bias must originate from mating-type-specific 'imprinting' events before karyogamy takes place. Structural variations of the mating-type locus, such as [h.sup.+N], [h.sup.+s], [h.sup.-s], [h.sup.+smt[DELTA]], or [h.sup.-smt[DELTA]], showed mat-bias manifestation. Mutations in genes coding for histone acetylase (gcn5, ada2) and histone deacetylase (hos2, clr6) activities smooth or abolish the mat-bias. In addition, the mat-bias depends on the presence of Swi5. We propose a new role for Swi5 and the histone acetylation status in mat-bias establishment through directionality of repair from the intact chromatid to the broken chromatid.

Reproductive fitness in many animals relies upon a tight balance between the number of cells that proliferate in the germ line and the number of cells that enter meiosis and differentiate as gametes. In the Caenorhabditis elegans germ line, the GLP-1/Notch signaling pathway controls this balance between proliferation and meiotic entry. Here we describe the identification of the proteasome as an additional regulator of this balance. We show that a decrease in proteasome activity, through either genetic mutation or RNAi to core components of the proteasome, shifts this balance toward excess germ-line proliferation. We further demonstrate that there are likely two or more proteasome targets that contribute to excess germ-line proliferation when proteasome activity is reduced. One of these targets is likely a component or regulator of the Notch-signaling pathway, while the other functions on one of the two major redundant genetic pathways downstream of GLP-1/Notch signaling. We propose a model in which the proteasome degrades proteins that are necessary for proliferation as cells switch from proliferation to meiotic entry.

The subspecies of honeybee indigenous to the Cape region of South Africa, Apis mellifera capensis, is unique because a high proportion of unmated workers can lay eggs that develop into females via thelytokous parthenogenesis involving central fusion of meiotic products. This ability allows pseudoclonal lineages of workers to establish, which are presently widespread as reproductive parasites within the honeybee populations of South Africa. Successful long-term propagation of a parthenogen requires the maintenance of heterozygosity at the sex locus, which in honeybees must be heterozygous for the expression of female traits. Thus, in successful lineages of parasitic workers, recombination events are reduced by an order of magnitude relative to meiosis in queens of other honeybee subspecies. Here we show that in unmated A. m. capensis queens treated to induce oviposition, no such reduction in recombination occurs, indicating that thelytoky and reduced recombination are not controlled by the same gene. Our virgin queens were able to lay both arrhenotokous male-producing haploid eggs and thelytokous female-producing diploid eggs at the same time, with evidence that they have some voluntary control over which kind of egg was laid. If so, they are able to influence the kind of second-division meiosis that occurs in their eggs post partum.

Although ~1 in 10,000 animal species is capable of parthenogenetic reproduction, the evolutionary causes and consequences of such transitions remain uncertain. The microcrustacean Daphnia pulex provides a potentially powerful tool for investigating these issues because lineages that are obligately asexual in terms of female function can nevertheless transmit meiosis-suppressing genes to sexual populations via haploid sperm produced by environmentally induced males. The application of association mapping to a wide geographic collection of D. pulex clones suggests that sex-limited meiosis suppression in D. pulex has spread westward from a northeastern glacial refugium, conveyed by a dominant epistatic interaction among the products of at least four unlinked loci, with one entire chromosome being inherited through males in a nearly nonrecombining fashion. With the enormous set of genomic tools now available for D. pulex, these results set the stage for the determination of the functional underpinnings of the conversion of meiosis to a mitotic-like mode of inheritance.