Meiosis is a specialized division of germ cells in sexually reproducing organisms, which is a fundamental process with key implications for evolution and biodiversity. In two consecutive rounds of cell division, meiosis I and meiosis II, a normal, diploid set of chromosome is halved. From diploid mother cells haploid gametes are generated to create genetic individual cells. This genetic uniqueness is obtained during prophase of meiosis I by essential meiotic processes in meiotic recombination, as double strand break (DSB) formation and repair, formation of crossovers (CO) and holiday junctions (HJs). Checkpoint mechanisms ensure a smooth progress of these events. Despite extensive research key mechanisms are still not understood. Based on an analysis of Massive Parallel Sequencing (MPS) data I could identify 2 genes, Mcmdc2 and Prr19, with high implication in meiotic recombination. In the absence of Mcmdc2 both sexes are infertile and meiocytes arrest at a stage equivalent to mid-­‐pachytene in wt. Investigations of the synaptonemal complex (SC) formation revealed severe defects suggesting a role for MCMDC2 in homology search. Moreover, MCMDC2 does not seem to be essential for DSB repair, as DSB markers of early and mid recombination nodules, like DMC1 and RPA, are decreased in oocytes. Nevertheless, late recombination nodules, which are positive for MutL homolog 1 (MLH1), do not form in both sexes. The absence of the asynapsis surveillance checkpoint mechanism in Hormad2 deficient ovaries with Mcmdc2 mutant background allowed survival of oocytes. This points into the direction that Mcmdc2 knock­out oocytes get eliminated after prophase I due to failed homologous synapsis. Interestingly, MCMDC2 contains a conserved helicase domain, like the MCM protein family members MCM8 and MCM9. I therefore hyphothesize that Mcmdc2 promotes homolgy search.

The cohesin complex is essential for mitosis and meiosis. The specific meiotic roles of individual cohesin proteins are incompletely understood. We report in vivo functions of the only meiosis-specific STAG component of cohesin, STAG3. Newly generated STAG3-deficient mice of both sexes are sterile with meiotic arrest. In these mice, meiotic chromosome architecture is severely disrupted as no bona fide axial elements (AE) form and homologous chromosomes do not synapse. Axial element protein SYCP3 forms dot-like structures, many partially overlapping with centromeres. Asynapsis marker HORMAD1 is diffusely distributed throughout the chromatin, and SYCP1, which normally marks synapsed axes, is largely absent. Centromeric and telomeric sister chromatid cohesion are impaired. Centromere and telomere clustering occurs in the absence of STAG3, and telomere structure is not severely affected. Other cohesin proteins are present, localize throughout the STAG3-devoid chromatin, and form complexes with cohesin SMC1β. No other deficiency in a single meiosis-specific cohesin causes a phenotype as drastic as STAG3 deficiency. STAG3 emerges as the key STAG cohesin involved in major functions of meiotic cohesin.

Orientador: Lilian Madi-Ravazzi / Coorientador: Maria Tercília Vilela de Azeredo-Oliveira / Banca: Blanche Christine Pires de Bitner-Mathé Leal / Banca: Maria Izabel Camargo Mathias / Banca: Hermione Elly Melara de Campos Bicudo / Banca: Patrícia Vilamaior / Resumo: Zaprionus indianus é um drosofilídeo nativo da região Afrotropical que colonizou o continente Sul Americano, apresentando uma ampla distribuição geográfica enquanto Z. sepsoides é restrita a algumas regiões africanas. As duas espécies diferem em relação ao tamanho dos testículos e dos espermatozoides que é maior em Z. indianus do que em Z. sepsoides. Com o intuito de conhecer aspectos da biologia e o grau de diferenciação destas espécies, o presente estudo avaliou a espermatogênese de machos de diferentes idades (1, 3, 5 e 8 dias) de ambas as espécies por meio de técnicas de coloração convencional e de ultraestrutura. A espermatogênese e ultraestrutura dos espermatozoides foram semelhantes nas espécies em que foi confirmado o número diploide de cromossomos com 2n = 12. Entretanto, foi observada uma quantidade maior de espermatozoides em machos jovens (1 a 3 dias de idade) em Z. indianus do que em Z. sepsoides, o qual apresentou maior frequência de estágios iniciais da espermatogênese nestas idades. A porção da cabeça dos espermatozoides foi fortemente marcada nas duas espécies pela coloração por prata (AgNOR), orceína lacto-acética e pela reação de Feulgen. Quando submetidos à reação de P.A.S., os testículos de Z. sepsoides e Z. indianus apresentaram grânulos de glicogênio. As espécies possuem a mesma ultraestrutura flagelar, em que o axonema mostra um arranjo de 9+9+2 microtúbulos, com a presença de dois derivados mitocondriais de diferentes tamanhos e o número de 64 espermatozoides por feixe, em ambas as espécies. A grande semelhança observada no padrão do arranjo de microtúbulos do axonema e nos derivados mitocondriais com diferentes tamanhos nas espécies de Zaprionus, comparadas com outras espécies de Drosophila, é indicativa da conservação destas estruturas na família Drosophilidae... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Zaprionus indianus is a drosophilid native to the Afrotropical region that has colonized South America. Z. indianus exhibits a wide geographical distribution, whereas Z. sepsoides is restricted to certain African regions. The two species differ in the size of their testes, which are larger in Z. indianus than in Z. sepsoides. To better understand the biology and the degree of differentiation of these species, the current study evaluated spermatogenesis in males of different ages (1, 3, 5 and 8 days old) from both species by conventional staining techniques and ultrastructural analysis. Spermatogenesis and the ultrastructure of spermatozoa were similar in the two species, for which the diploid number was confirmed to be 2n = 12 chromosomes. However, a greater number of spermatozoa were observed in young Z. indianus males (1-3 days old) than in young Z. sepsoides males, which showed a higher frequency of cells at the early stages of spermatogenesis at this age. A portion of the head of the sperm was strongly marked in both species by silver staining (AgNOR), lacto-acetic orcein and the Feulgen reaction. Additionally, when submitted to P.A.S. reaction, the testes of both Z. sepsoides and Z. indianus exhibited glycogen granules. The two species also presented the same flagellar ultrastructure, in which the axoneme includes a 9+9+2 arrangement of microtubules, two mitochondrial derivatives of different sizes are present and the number of spermatozoa per bundle is 64. The great similarity in the pattern of microtubule arrangement in the axoneme and in the mitochondrial derivatives of the species Zaprionus, as compared with other species of Drosophila, indicates that these structures are preserved in the family Drosophilidae. The differences observed between the young males of Z. indianus and Z. sepsoides, including the number and frequency of sperm... (Complete abstract click electronic access below) / Doutor

Orientador: Maria Tercília Vilela Azeredo Oliveira / Coorientador: Patrícia Simone Leite Vilamaior / Banca: Alba Regina de Abreu Lima / Banca: Silvana Gisele Pegorin de Campos / Resumo: Os triatomíneos são insetos hematófagos de grande importância para a saúde pública, uma vez que são considerados como a principal forma de transmissão do protozoário Trypanosoma cruzi, agente etiológico da doença de Chagas. Além da importância epidemiológica, esses insetos se destacam em estudos celulares, pois apresentam algumas peculiaridades, como cromossomos holocêntricos, meiose invertida para os cromossomos sexuais e persistência nucleolar durante a meiose. O fenômeno de persistência nucleolar é definido pela presença do nucléolo ou corpúsculos nucleolares durante todas as fases da meiose. Esse fenômeno foi descrito, até o momento, para 24 espécies de triatomíneos, dos gêneros Triatoma, Rhodnius e Panstrongylus. No entanto, somente a presença dessa estrutura nuclear em todas as fases da meiose não garante a sua atividade transcricional. Diante disso, o presente estudo teve como objetivo descrever a nucleologênese em novas espécies de, pelo menos, sete novos gêneros (Cavernicola, Psammolestes, Dipetalogaster, Eratyrus, Meccus, Mepraia, Nesotriatoma), com o intuito de analisar se a persistência nucleolar é uma sinapomorfia da subfamília Triatominae. Além disso, esse trabalho também teve como finalidade avaliar a atividade nucleolar durante a meiose das espécies Triatoma infestans, Rhodnius montenegrensis e Panstrongylus megistus, por meio da técnica citoquímica Acridine Orange, com o objetivo de caracterizar, com base na presença de RNA, se existe atividade transcricional... / Abstract: Triatomines are hematophagous insects of great importance for public health, since they are considered as the main form of transmission of the protozoan Trypanosoma cruzi, etiologic agent of Chagas disease. Besides the epidemiological importance, these insects stand out in cellular studies, as they present some peculiarities, such as holocentric chromosomes, inverted meiosis for sex chromosomes and nucleolar persistence during meiosis. The phenomenon of nucleolar persistence is defined by the presence of the nucleolus or nucleolar corpuscles during all phases of meiosis. So far, this phenomenon has been described for 24 triatomine species of the genus Triatoma, Rhodnius and Panstrongylus. However, only the presence of this nuclear structure in all phases of meiosis does not guarantee its transcriptional activity. So, the objective of the present study was to describe the nucleologenesis in new species of at least seven new genera (Cavernicola, Psammolestes, Dipetalogaster, Eratyrus, Meccus, Mepraia, Nesotriatoma), in order to analyze if nucleolar persistence is a synapomorphy of the subfamily Triatominae. Besides that, this work had the purpose to evaluate the nucleolar activity during the meiosis of the species Triatoma infestans, Rhodnius montenegrensis and Panstrongylus megistus, by means of the cytochemical technique Acridine Orange, with the objective of characterizing, based on the presence of RNA, if there is transcriptional activity in the persistent nucleolar material. All 59 species analyzed, distributed in seven genera, presented nucleolar persistence during meiosis, thus corroborating that this phenomenon is a synapomorphy of the subfamily Triatominae. By means of the fluorochrome Acridine Orange, the persistent nucleolus has been found to be transcriptionally active. Thus, it was evident, for the first time, that the nucleolus is present in the spermatogenesis of ... / Mestre

Sexual reproduction requires meiosis, a specialized cell division program that halves the chromosome number of germ cells in order to generate gametes. Chromosome number reduction is achieved by two successive rounds of cell divisions after a single round of DNA replication. In meiosis I, homologous chromosomes (homologs) recombine to produce at least one reciprocal DNA exchange, called crossover (CO), in each chromosome. COs ensure proper segregation, as the resulting tetrad chromosomes are bisected during the first division to form dyads. In meiosis II, dyads split allowing segregation of single chromatids, resembling mitosis. During prophase I, the earliest stage of meiosis I, each pair of sister chromatids is arranged in series of loops tethered longitudinally by a structure called the chromosome axis, which serves as a scaffold for the machinery that promotes the formation of programmed DNA double-strand breaks (DSBs). Recombination is initiated when the DSBs are resected to produce single-stranded DNA (ssDNA) overhangs that seek out their homologs, promoting pairing. Then the synaptonemal complex (SC) forms, physically linking homolog axes through transverse filaments. In the context of the SC, meiotic recombination repairs the DSBs and turns a small fraction of them into COs. DSB formation will continue on unsynapsed axes and it is only terminated by the complete synapsis of all homolog pairs in pachytene stage. Meiocytes that fail to complete recombination and/or synapsis are eliminated (spermatocytes by mid-pachytene; oocytes from late prophase I, but at or before follicle formation), as these defects can cause chromosomal abnormalities that will be passed down to the offspring, causing severe diseases or death. Our group and others suggest that there are two simultaneous pachytene checkpoints, one that is activated by persistent DSBs and the other by asynapsis. This issue has proven very difficult to approach, since defective DSB formation/repair will inevitably affect synapsis by impairing homology search and strand invasion. The meiosis-specific HORMAD2 protein binds preferentially to unsynapsed axes, where HORMAD2 promotes the recruitment of ATR kinase. The ATR-dependent accumulation of γH2AX (histone H2AX phosphorylated on Ser139) is thought to generate a checkpoint signal from unsynapsed chromosomal regions as part of the synapsis checkpoint. In the case of spermatocytes ATR activity is concentrated to the non-homologous regions of sex chromosomes, which remain unsynapsed. This leads to γH2AX accumulation and subsequent transcriptional silencing of the sex chromosomes. This mechanism is called meiotic sex chromosome inactivation (MSCI) and is essential for the survival of spermatocytes, since the expression of sex chromosome-linked genes is toxic to spermatocytes beyond mid pachytene. Despite the importance of HORMAD2, the mechanism for its interaction with the unsynapsed chromosomes was unknown. Furthermore, it remained untested if HORMAD2 binding to the axis is actually required for the synapsis checkpoint. To address whether HORMAD2 binding to the axis is required for its function, I generated a mouse strain that expresses an altered version of the constitutive chromosome axis component SYCP2. The mutant SYCP2Δex16 lacks a short peptide sequence, called closure motif, which is predicted to bind HORMAD2. Immunofluorescence (IF) in spermatocytes showed that the localization of HORMAD2 to the chromosome axis is lost in Sycp2Δex16/Δex16, while axis formation is not impaired. Consistently, immunoblotting showed that HORMAD2 was depleted from insoluble fractions (chromatin-rich) of Sycp2Δex16/Δex16 testis extracts, while present in the soluble and total fractions. These results confirmed that the closure motif of SYCP2 serves as anchor for HORMAD2 on meiotic chromosome axis. Sycp2Δex16/Δex16 males are infertile. IF staining of cryosections of testes showed a complete loss of spermatocytes beyond pachytene. The number of cells undergoing apoptosis increased relative to the wild type, most of them in stage IV of the epithelium cycle, which corresponds to mid-pachytene. Thus, the onset of arrest coincides with the activation of the checkpoint that prevents asynaptic/DSB repair defective meiocytes from progressing beyond mid-pachytene. Nonetheless, global SC formation, and patterns of DSB formation and recombination foci in Sycp2Δex16/Δex16 are similar to the wild type. By early pachytene, the last stage reached by Sycp2Δex16/Δex16 spermatocytes, most cells have completed synapsis and DSBs are repaired. Only a small percentage of pachytene cells show a degree of incomplete synapsis. Although this suggests a synapsis-enhancing role of HORMAD2, it is thought to be concomitant to the MSCI failure observed in this mutant. In Sycp2Δex16/Δex16, 83.7% of pachytene spermatocytes have an abnormal distribution of γH2AX on sex chromosomes, accompanied by impaired accumulation of ATR and BRCA1 (another MSCI-promoting protein). The phenotype of Sycp2Δex16/Δex16 suggests a misregulation of pachytene checkpoint functions due to loss of HORMAD2-dependent ATR signaling from unsynapsed axes, and I propose that this misregulation accounts for the meiotic arrest phenotype. Females do not require MSCI, as their XX chromosomes can synapse. Accordingly, Sycp2Δex16/Δex16 females are fertile. Nonetheless, previous reports indicate that HORMAD2 has a role in sensing asynapsis, but not persistent DSB, as part of a checkpoint for quality control in oocytes. As expected, Sycp2Δex16/Δex16 cannot rescue oocytes in a DSB repair and synapsis defective background. Sycp2Δex16/Δex16 females have higher oocyte numbers than wild type. This could suggest that oocytes with defective synapsis that are usually culled in wild type ovaries, are not eliminated in Sycp2Δex16/Δex16, supporting HORMAD2 role in signaling asynapsis. Further testing in a synapsis defective background is underway. Sycp2Δex16/Δex16 seems to phenocopy Hormad2-/-, supporting a model where HORMAD2 binding to the unsynapsed axis is required for enabling its checkpoint-activating functions.:List of figures I List of tables II List of abbreviations III Acknowledgments V 1. Introduction 1 1.1. Meiosis and gametogenesis in mouse 1 1.2. Prophase of the first meiotic division 3 1.2.1. Meiotic recombination 4 1.2.2. The chromosome axis and the synaptonemal complex 5 1.2.3. The interplay between DSBs and synaptonemal complex formation 8 1.3. Surveillance mechanisms in prophase I 9 1.3.1. HORMAD1 and HORMAD2 in the male pachytene checkpoint 11 1.3.2. HORMAD1 and HORMAD2 in the female prophase checkpoint 12 1.4. HORMAD1/2 interaction with the chromosome axis 14 1.5. Aim of the project 15 2. Materials and methods 16 2.1. Generation of a mouse model lacking the closure motif 16 2.1.1. Generation of a mutant using CRISPR-Cas9 system 16 2.1.2. Identification of mutant candidates by PCR and HMA 17 2.1.3. Genomic DNA sequencing for the selection of founders 19 2.1.4. cDNA sequencing 20 2.2. Mice 21 2.2.1 Mice from a homogenized genetic background 21 2.3. Methods for the fixation of tissues 22 2.3.1. Nuclear surface spread spermatocytes 22 2.3.2. No-spin sucrose spreads 23 2.3.3. Testis fixation and cryosectioning 23 2.3.4. Ovary fixation and cryosectioning 23 2.3.4. Fixed Wild Type Cells (FWTC) 24 2.4 Staging 24 2.4.1. Staging of spermatocytes from nuclear surface spreads 24 2.4.2. Staging of epithelial cycles in cross-sections of seminiferous tubules 25 2.4.3. Staging of oocytes 26 2.5. Protein extraction 27 2.5.1. Total protein extraction and western blotting 27 2.5.2. Fractionation assay and western blotting 27 2.6. Immunofluorescence microscopy 29 2.6.1. Staining conditions 29 2.6.2. Imaging 30 3. Results 31 3.1. HORMAD2 recruitment to the axis requires the closure motif of SYCP2 31 3.1.1. Generation of a mouse mutant lacking the closure motif 31 3.1.2. Axial HORMAD2 localization is lost in Sycp2Δex16/Δex16 spermatocytes 34 3.1.3. HORMAD2 is present in Sycp2Δex16/Δex16 spermatocytes but greatly decreased from the chromatin-rich fraction 36 3.2. HORMAD1 localization to the axis seems to be independent of the closure motif of SYCP2 37 3.3. Loss of HORMAD2 localization to the axis causes infertility in males 38 3.3.1. Low levels of H1t signal indicate that Sycp2Δex16/Δex16 spermatocytes do not progress beyond mid-pachytene 39 3.3.2. Defect in spermatogenesis of Sycp2Δex16/Δex16 is caused by mid-pachytene arrest 41 3.4. Axis formation is not impaired in Sycp2Δex16/Δex16 spermatocytes 44 3.5. HORMAD2 localization to the axis plays a minor role in SC formation 45 3.5.1. In Sycp2Δex16/Δex16 spermatocytes, autosomal SC formation does not seem impaired, but abnormal XY synapsis is observed 45 3.5.2. IHO1 is properly removed from axes in Sycp2Δex16/Δex16 and Hormad2-/- spermatocytes 48 3.5.3. Synapsis at early pachytene is mildly defective in Sycp2Δex16/Δex16 and Hormad2-/- spermatocytes 50 3.6. HORMAD2 localization to the axis is not essential for DSB repair 52 3.7. Loss of HORMAD2-dependent ATR signaling from unsynapsed axes causes a misregulation of pachytene checkpoint 55 3.7.1. Abnormal sex body formation in Sycp2Δex16/Δex16 is an indicator of defective ATR signaling on unsynapsed sex chromosomes 55 3.7.2. ATR localization is affected by the loss of HORMAD2 localization to the axes 58 3.8. HORMAD2 seems to be required for a checkpoint mechanism in females that is activated by persistent asynapsis 60 3.8.1. HORMAD2 is not essential for fertility in females 60 3.8.3. HORMAD2 localization to the axis is not required for the elimination of oocytes with persistent DSB 60 4. Discussion 63 4.1. The closure motif in SYCP2 is the binding site of HORMAD2 to the unsynapsed axes 63 4.1.1 The role of HORMAD1 in the recruitment of HORMAD2 to the chromosome axis 64 4.2. Loss of HORMAD2 localization to the axes causes mid-pachytene arrest 65 4.2.1 Persistent DSBs and/or asynapsis are unlikely to trigger the mid-pachytene arrest in Sycp2Δex16/Δex16 spermatocytes 66 4.3. Loss of HORMAD2 from the unsynapsed axes impairs the recruitment of ATR activity on sex chromosomes, triggering the elimination by the mid-pachytene checkpoint 67 4.3.1 Increased asynapsis of sex chromosomes is most likely an effect of defective ATR signaling 68 4.3.2 ATR-dependent silencing of sex chromosomes is required for survival of spermatocytes beyond pachytene 69 4.4. HORMAD2 localization to the axis has a limited role in the prophase checkpoint in females 69 4.4.1 HORMAD2 seems to be required for a checkpoint mechanism in females that is activated by persistent asynapsis 70 4.4.2 HORMAD2 localization to the axis is not required for the elimination of oocytes with persistent DSB 70 4.4.3 HORMAD2 might play a role in the checkpoint that monitors unrepaired DSBs during female prophase 71 4.5. Final remarks 72 5. Summary 73 5. Zusammenfassung 75 References 78

In the intricate realm of biology, meiosis stands as the remarkable process responsible for generating genetically diverse haploid gametes from diploid cells. In 2000, Pezzi et al., identified STAG3 as a novel meiotic-specific synaptonemal-complex associated protein belonging to the highly conserved family of stromalin nuclear proteins. Later, over the years, research groups characterised the depletion phenotype of STAG3 in mice, where deficiency of STAG3 causes severe chromosomal defects and early meiotic arrest. These studies together collectively highlighted STAG3 as the most important meiotic cohesin. Traditionally, the role of the cohesin complex was understood as maintaining cohesion between chromatids during cell division. However, over the years, this perception has evolved significantly, expanding to include the regulation of dynamic chromosomal configurations during meiosis. With the realisation of STAG3's importance in meiotic progression, the next pressing question becomes: how does STAG3 coordinate this intricate process? This study sought to address that question by examining the STAG3 interactome in male germ cells, aiming to uncover novel pathways through which STAG3 contributes to maintaining meiotic progression. Through successful purification of the STAG3-REC8 complex, its ability to form functional complexes in-vitro was demonstrated. During my PhD thesis, I discovered links between STAG3 and DNA repair mechanisms beyond the well-known homologous recombination /non-homologous end joining pathway in meiotic recombination. By looking at the meiotic-specific protein interactome bound to the STAG3-REC8 complex through Mass Spectroscopic analysis, we identified STAG3 involvement in PARP-1-mediated repair of DNA double-strand breaks occurring outside of the programmed DSB repair during the zygotene stage of prophase I. PARP-1 is an ADP-ribose polymerase which acts as a first responder that detects DNA damage and facilitates the activation of the DNA repair pathway. STAG3 shows a preferential interaction with PARP-1 when spermatocytes are challenged with extensive DNA damage. Furthermore, the interaction of SMC3, another component of the cohesin complex, with PARP-1 during DNA damage suggests that STAG3, as part of the cohesin complex, contributes to DNA damage repair in spermatocytes. To gain deeper insights into the distinctive characteristics of STAG3, an extensive analysis of spermatogenesis in mice expressing a C-terminus truncated form of STAG3 was performed. The C-terminal region of STAG3 is not conserved among the stromalin family members, and hence it was speculated that this region might have unique functions to meiosis. Removal of the C-terminal end comprising 47 amino acids led to an early meiotic arrest, mirroring the phenotype in most cohesin subunit deletion mutants. The phenotype observed mimics the complete STAG3 depletion phenotype to some extent. The truncated STAG3 resulted in an arrest at a late zygotene/early pachytene-like stage during meiotic prophase I. One of the most notable observations was the significant reduction in the length of the axial elements (AE) in this mutant. Despite stable expression of and localisation of STAG3 to the axis, the axis length decreased by over 60%. This mutation compromised synaptonemal complex formation, leading to the early meiotic arrest. Although SYCP1 loads onto the axis and initiate synapsis, the shortened axial elements could not synapse, marked by HORMAD-1, a well-known asynapsis marker. The average number of SYCP3-marked stretches was 35 in this mutant. The increased number of AE and shortened axis length did not result from chromosome fragmentation because most chromosomes/axes had intact telomere and centromeric signals, validated by RAP1 and ACA foci, respectively. Centromeric and telomeric cohesion may be partially affected as some chromosome showed aberrant telomeric and centromeric defects. C terminal truncated STAG3 impairs synapsis between homologous chromosomes, but the sister chromatid cohesion remains largely unaffected. Also, this deletion did not affect the loading of the cohesin complex subunits onto the chromosome axis. The early meiotic arrest resulted in underdeveloped gonads, leading to infertility in otherwise healthy mice. Taken together, these results suggest novel roles for STAG3 in meiosis, and the meiotic-specific C terminal region of STAG3 is critical for proper meiotic progression in mice.

During cell division, chromosome segregation takes place on bipolar, microtubulebased spindles. Here, C. elegans is used to analyze spindle organization under both mitotic and meiotic conditions. First, the role of SAS-4 in organizing centrosome structure was analyzed. Partial depletion of SAS-4 in early embryos results in structurally defective centrioles. The study of this protein sheds light on the poorly understood role of the centrioles in dictating centrosome size. Second, the ultrastructure of wild-type mitotic spindle components was analyzed by electron tomography. This 3-D analysis reveals morphologically distinct microtubule end morphologies in the mitotic spindle pole. These results have structural implications for models of microtubule interactions with centrosomes Third, spindle assembly was studied in female meiosis. Specifically, the role of the microtubule severing complex katanin in spindle organization was analyzed. Electron tomography reveals fragmentation of spindle microtubules and suggests a novel katanin-dependent mechanism of meiotic spindle assembly. In this model, relatively long microtubules seen near the meiotic chromatin are converted into numerous short fragments, thus increasing the total number of polymers in an acentrosomal environment. Taken together, these results provide novel insights into the three-dimensional organization of microtubules during spindle assembly. / Die Segregation der Chromosomen während der Zellteilung wird duch bipolare, von Microtubuli-aufgebauten Spindlen gewährleistet. In der vorliegenden Arbeit wird C. elegans zur Analyse der Spindelorganisation unter mitotischen und meiotischen Bedingungen herangezogen. Erstens wird die Rolle von SAS-4 in der Organisation von Zentrosomen untersucht. Die partielle Depletierung von SAS-4 in frühen Embryonen führt zu strukturell defekten Zentriolen und wirft somit Licht auf die wenig verstandene Rolle der Zentriolen in der Bestimmung der Zentrosomengröße. Zweitens wird die Ultrastruktur der mitotischen Spindelkomponenten im Wildtyp durch Elektronentomographie untersucht. Diese 3-D-Analyse zeigt, dass im mitotischen Spindlepol unterschiedliche Morphologien der Mikrotubulienden zu finden sind. Diese Ergebnisse haben strukturelle Implikationen für Modelle der Mikrotubuli-Zentrosomen-Interaktionen. Drittens wird der Aufbau der Spindel in der weiblichen Meiose, speziell die Rolle des Mikrotubuli-schneidenden Kataninkomplexes in der Spindelorganisation, untersucht. Die Elektronentomographie zeigt hier eine Fragmentierung der Spindelmikrotubuli. Basierend auf diesem Ergebnis wird ein neues Katanin-abhängiges Modell der Formierung der Meiosespindel entwickelt, in dem relativ lange Microtubuli in Nähe des meiotischen Chromatins in zahlreiche kurze Mikrotubuli “zerschnitten” werden. Dies erhöht die Anzahl der verfügbaren Polymere in dieser azentrosomalen Situation. Zusammenfassend bringen diese Ergebnisse neue Einsichten in die räumliche Organisation der Mikrotubuli während des Spindelaufbaus.

During cell division, chromosome segregation takes place on bipolar, microtubulebased spindles. Here, C. elegans is used to analyze spindle organization under both mitotic and meiotic conditions. First, the role of SAS-4 in organizing centrosome structure was analyzed. Partial depletion of SAS-4 in early embryos results in structurally defective centrioles. The study of this protein sheds light on the poorly understood role of the centrioles in dictating centrosome size. Second, the ultrastructure of wild-type mitotic spindle components was analyzed by electron tomography. This 3-D analysis reveals morphologically distinct microtubule end morphologies in the mitotic spindle pole. These results have structural implications for models of microtubule interactions with centrosomes Third, spindle assembly was studied in female meiosis. Specifically, the role of the microtubule severing complex katanin in spindle organization was analyzed. Electron tomography reveals fragmentation of spindle microtubules and suggests a novel katanin-dependent mechanism of meiotic spindle assembly. In this model, relatively long microtubules seen near the meiotic chromatin are converted into numerous short fragments, thus increasing the total number of polymers in an acentrosomal environment. Taken together, these results provide novel insights into the three-dimensional organization of microtubules during spindle assembly. / Die Segregation der Chromosomen während der Zellteilung wird duch bipolare, von Microtubuli-aufgebauten Spindlen gewährleistet. In der vorliegenden Arbeit wird C. elegans zur Analyse der Spindelorganisation unter mitotischen und meiotischen Bedingungen herangezogen. Erstens wird die Rolle von SAS-4 in der Organisation von Zentrosomen untersucht. Die partielle Depletierung von SAS-4 in frühen Embryonen führt zu strukturell defekten Zentriolen und wirft somit Licht auf die wenig verstandene Rolle der Zentriolen in der Bestimmung der Zentrosomengröße. Zweitens wird die Ultrastruktur der mitotischen Spindelkomponenten im Wildtyp durch Elektronentomographie untersucht. Diese 3-D-Analyse zeigt, dass im mitotischen Spindlepol unterschiedliche Morphologien der Mikrotubulienden zu finden sind. Diese Ergebnisse haben strukturelle Implikationen für Modelle der Mikrotubuli-Zentrosomen-Interaktionen. Drittens wird der Aufbau der Spindel in der weiblichen Meiose, speziell die Rolle des Mikrotubuli-schneidenden Kataninkomplexes in der Spindelorganisation, untersucht. Die Elektronentomographie zeigt hier eine Fragmentierung der Spindelmikrotubuli. Basierend auf diesem Ergebnis wird ein neues Katanin-abhängiges Modell der Formierung der Meiosespindel entwickelt, in dem relativ lange Microtubuli in Nähe des meiotischen Chromatins in zahlreiche kurze Mikrotubuli “zerschnitten” werden. Dies erhöht die Anzahl der verfügbaren Polymere in dieser azentrosomalen Situation. Zusammenfassend bringen diese Ergebnisse neue Einsichten in die räumliche Organisation der Mikrotubuli während des Spindelaufbaus.

La grand majorité des espèces forment très peu de crossovers (CO) par chromosome en méiose, et ce quelle que soit la taille des chromosomes et en dépit de l'excès de précurseurs de recombinaison disponibles. Les mécanismes qui sous-tendent cette limitation restent, pour une grande part, inconnus. Pour identifier des facteurs limitant la formation des CO en méiose, nous avons mis en place un crible permettant d'isoler des mutants formant plus de CO que le sauvage. Chez Arabidopsis thaliana, les mutants présentant un défaut de CO (ex: les mutants zmm) présentent une réduction de fertilité directement observable sur la longueur des fruits. Notre crible repose sur l'idée qu'une mutation augmentant la formation des CO devrait restaurer la fertilité des mutants zmm.Le premier suppresseur identifié nous a permis de montrer que FANCM est un régulateur majeur de la formation des CO méiotiques, en limitant la voie dépendante de MUS81, normalement minoritaire chez Arabidopsis (Crismani et al., 2012). Nous avons ensuite montré que deux des cofacteurs de FANCM au sein de la voie de l'anémie de Fanconi (FA), MHF1 et MHF2, agissent dans la même voie pour limiter les CO. En dehors de ces trois protéines, les autres protéines FA ne sont pas des protéines anti-CO (Girard et al., 2014).Nous avons ensuite identifié FIDGETIN-Like-1 comme une protéine anti-CO agissant en parallèle de FANCM. En effet, si les deux mutations figl1 et fancm alimentent la voie dépendante de MUS81 de formation des CO, leurs effets sont cumulatifs et mènent à une augmentation d'un facteur 6 du nombre CO par méiose, et ce sans affecter la ségrégation des chromosomes. Ce résultat montre que FIGL1 et FANCM agissent à deux étapes indépendantes de la recombinaison, et nos données suggèrent que FIGL1 pourrait agir en amont de FANCM.Ce travail a révélé l'existence d'au moins deux mécanismes de limitation des CO méiotiques, et montre que la fréquence de CO peut être largement augmentée sans que la ségrégation des chromosomes ne soit affectée. / Most species only few meiotic crossovers (COs) per chromosome irrespective of their physical size and despite an excess of recombination precursors. However, the underlying mechanisms constraining CO frequency remain largely unknown. In order to find factors limiting meiotic COs, we performed a genetic screen to find mutants with increased CO frequency. CO-deficient mutants (e.g. zmm) of Arabidopsis thaliana display reduced fertility, easily noticeable by their obvious reduction in fruit length. We designed a screen based on the idea that mutations that increase CO frequency will restore the fertility of the zmm mutants.We showed first identified FANCM as a major anti-CO protein limiting MUS81-dependent COs, a normally minor pathway in Arabidopsis (Crismani et al., 2012). We then showed that two of FANCM's cofactors from the Fanconi anemia pathway of DNA repair, namely MHF1 and MHF2, act along FANCM to limit meiotic crossovers, whereas the other Fanconi proteins do not (Girard:2014).Another mutant revealed FIDGETIN-Like-1 (FIGL1) as an anti-CO factor that acts in parallel to FANCM. While both figl1 and fancm mutations fuels the MUS81-dependent CO pathway, the effect of both mutations is cumulative, leading to a six-fold increase in CO formation, without impairing chromosome segregation. This shows that FIGL1 and FANCM act independently, and our data suggest that FIGL1 could act at an earlier step in the recombination pathway than FANCM.This work reveals that at least two different mechanisms limit meiotic CO number and shows that CO frequency can be largely increased without affecting chromosome distribution at meiosis.

O óxido nítrico (NO) é um mensageiro químico detectado em vários tipos celulares como células endoteliais, neurônios e macrófagos, desempenhando também funções variadas como vasodilatação, neurotransmissão e indução de morte celular. O NO é gerado pela atividade da enzima óxido nítrico sintase (NOS), a qual foi detectada em vários órgãos incluindo o sistema reprodutor. O sistema NOS/NO parece desempenhar papel importante na maturação oocitária entre outras funções. No entanto, apesar das evidências, há poucos estudos sobre o possível papel desse sistema em oócitos da espécie bovina. O presente estudo teve por objetivo investigar a importância do sistema NOS/NO na maturação in vitro (MIV) de oócitos bovinos. Para isso, foram avaliados os efeitos da inibição da NOS durante a MIV na presença de concentrações crescentes do inibidor de NOS (L-NAME, 0-1mM) e do aumento do NO durante a MIV na presença de concentrações crescentes do doador de NO (SNAP, 0-1mM) sobre a taxa de maturação (metáfase II) e formação de blastocistos após a fecundação in vitro. Também foi avaliado o efeito da pré-maturação com butirolactona (10µM BLI), seguida de MIV, na presença ou não de doador ou inibidor de NO em cada fase de cultivo, sobre o desenvolvimento de blastocistos. Após 22h de MIV os grupos tratados com L-NAME apresentaram uma taxa de metáfase II (MII) inferior (~80%, P<0,05) ao controle (95,5%). A taxa de blastocisto foi similar entre os grupos (34 a 41,5%, P>0,05). Apenas 7,2% (P<0,05) dos oócitos maturados com a maior concentração de SNAP (10-3M), atingiram o estádio de MII. Os demais tratamentos (71,6; 72,4 e 54,8% para controle, 10-7 e 10-5M, respectivamente) não diferiram entre si (P>0,05). Altos níveis de SNAP (10-3M) bloquearam o desenvolvimento embrionário, enquanto os demais tratamentos apresentaram cerca de 38% de blastocistos (P>0,05). As taxas de blastocistos (26,3 a 34,1%) e de eclosão (14,8 a 22,0%) adicionando ou não L-NAME em baixa concentração (10-7M) durante a pré-maturação, a maturação ou ambas as fases foram similares (P>0,05). Também não houve diferença (P>0,05) na taxa de blastocistos (25,5 a 39,7%) com a adição ou não de baixa concentração de SNAP (10-7M) durante a pré-maturação, a maturação ou ambas as fases. O grupo com SNAP (10-7M) na pré-maturação e MIV superou a taxa de eclosão (27,5%, P<0,05) dos demais grupos que tiveram SNAP adicionado somente no bloqueio, na MIV ou sem adição de SNAP (14,2 a 18,6 %, P>0,05). Esses resultados exibiram o duplo efeito do NO sobre oócitos bovinos, que tiveram a maturação reduzida com a inibição da síntese de NO e a maturação nuclear e citoplasmática bloqueadas pelo excesso de NO. / Nitric oxide (NO) is a chemical messenger detected in several cell types such as endothelial cells, neurons and macrophages, performing also varied functions as vasodilatation, neurotransmission and induction of cell death. NO is generated by the activity of the enzyme nitric oxide synthase (NOS), which has been detected in several organs including the reproductive system. The NOS/NO system seems to play an important role in oocyte maturation besides other functions. However, despite the evidence, there are few studies on the possible role of this system in bovine oocytes. The present study aimed to investigate the importance of the NOS/NO system on in vitro maturation (IVM) of bovine oocytes. The effects of NOS inhibition during IVM in the presence of increasing concentrations of NOS inhibitor (L-NAME, 0-1mM) and of the increase in NO during IVM with the NO donor SNAP (0-1mM) on maturation rates (metaphase II) and on blastocyst development after in vitro fertilization were assessed. The effect of prematuration with butyrolactone I (10µM BLI) followed by IVM, in the presence or not of NO inhibitor or donor in each culture period on blastocyst development was also investigated. After 22h IVM, L-NAME treated groups showed a lower (~80%, P<0.05) metaphase II (MII) rate when compared with controls (95.5%). Blastocyst rates were similar among all groups (34 to 41.5%, P>0.05). Only 7.2% (P<0.05) of oocytes matured with the highest SNAP concentration (10-3M) reached MII. The other treatments (71.6; 72.4 and 54.8% for control, 10-7 and 10-5M, respectively) were similar among them (P>0.05). High SNAP concentration (10-3M) blocked blastocyst development, while the other treatments presented about 38% blastocyst rates (P<0.05). Blastocyst (26.3 to 34.1%) and hatching rates (14.8 and 22.0%) were similar (P>0.05) when low L-NAME concentration (10-7M) was added or not during prematuration, maturation or both. Blastocyst rates (25.5 to 39.7%) were also similar (P>0.05) whether SNAP (10-7M) was added or not during prematuration, IVM or both. When low concentration of SNAP (10-7M) was added during both prematuration and IVM, hatching rates were increased (27.5%, P<0.05) when compared with oocytes cultured in the presence of SNAP only during prematuration or IVM or without it (14.2 to 18.6 %, P>0.05). These results shoe the dual effect of NO on bovine oocytes, which had maturation rates decreased when NO synthesis was inhibited and nuclear maturation and blastocyst development were blocked by excess NO.