Identification of ASYNAPTIC4, a Component of the Meiotic Chromosome Axis

International audienceDuring the leptotene stage of prophase I of meiosis, chromatids become organized into a linear looped array via a protein axis that forms along the loop bases. Establishment of the axis is essential for the subsequent synapsis of the homologous chromosome pairs and the progression of recombination to form genetic crossovers. Here, we describe ASYNAPTIC4 (ASY4), a meiotic axis protein in Arabidopsis (Arabidopsis thaliana). ASY4 is a small coiled-coil protein that exhibits limited sequence similarity with the carboxyl-terminal region of the axis protein ASY3. We used enhanced yellow fluorescent protein-tagged ASY4 to show that ASY4 localizes to the chromosome axis throughout prophase I. Bimolecular fluorescence complementation revealed that ASY4 interacts with ASY1 and ASY3, and yeast two-hybrid analysis confirmed a direct interaction between ASY4 and ASY3. Mutants lacking full-length ASY4 exhibited defective axis formation and were unable to complete synapsis. Although the initiation of recombination appeared to be unaffected in the asy4 mutant, the number of crossovers was reduced significantly, and crossovers tended to group in the distal parts of the chromosomes. We conclude that ASY4 is required for normal axis and crossover formation. Furthermore, our data suggest that ASY3/ASY4 are the functional homologs of the mammalian SYCP2/SYCP3 axial components

Crossover interference mechanism: New lessons from plants

Plants are the source of our understanding of several fundamental biological principles. It is well known that Gregor Mendel discovered the laws of Genetics in peas and that maize was used for the discovery of transposons by Barbara McClintock. Plant models are still useful for the understanding of general key biological concepts. In this article, we will focus on discussing the recent plant studies that have shed new light on the mysterious mechanisms of meiotic crossover (CO) interference, heterochiasmy, obligatory CO, and CO homeostasis. Obligatory CO is necessary for the equilibrated segregation of homologous chromosomes during meiosis. The tight control of the different male and female CO rates (heterochiasmy) enables both the maximization and minimization of genome shuffling. An integrative model can now predict these observed aspects of CO patterning in plants. The mechanism proposed considers the Synaptonemal Complex as a canalizing structure that allows the diffusion of a class I CO limiting factor linearly on synapsed bivalents. The coarsening of this limiting factor along the SC explains the interfering spacing between COs. The model explains the observed coordinated processes between synapsis, CO interference, CO insurance, and CO homeostasis. It also easily explains heterochiasmy just considering the different male and female SC lengths. This mechanism is expected to be conserved in other species

Caractérisation génétique et moléculaire de mutants d'embryogenèse précoce d'Arabidopsis thaliana

Chez les plantes à fleurs, le développement des graines est initié par une double fécondation qui conduit à la formation de l'embryon et l'albumen. Ce processus a été décrit il a plus d'un siècle mais les mécanismes moléculaires par lesquels la fécondation active le programme embryogène sont encore peu compris. La voie la plus étudiée concerne la répression du développement de la cellule centrale avant la fécondation. Nous avons utilisé une approche de génétique classique dans le but de caractériser des gènes essentiels impliqués dans l'activation de l'embryogenèse précoce chez la plante modèle Arabidopsis thaliana. Les analyses ont porté sur des mutants embryons défectueux (emb) bloqués dans les toutes premières étapes de l'embryogenèse et étiquetés par une insertion d'ADN-T. Ces mutations sont sporophytiques et récessives. Notre étude s'est concentrée sur la caractérisation phénotypique et moléculaire des mutants zeus (zeu) arrêtés après l'élongation du zygote et des mutants cyclops (cyl) bloqués après la première division asymétrique du zygote. Un gène ZEU et deux gènes CYL ont été identifiés et vérifiés par complémentation fonctionnelle. Les profils d'expression de chaque gène suggèrent une régulation transcriptionnelle au moment de la fécondation cohérente avec leur rôle dans l'activation du développement de la graine. Les protéines ZEU1 et CYL2 sont impliquées dans la réplication de l'ADN. La fonction potentielle de CYL1 suggère qu'une voie de signalisation similaire à celle nécessaire pour l'induction de l'embryogenèse somatique serait impliquée in vivo pour l'induction du développement de l'embryon zygotique.In flowering plants, the seed development is initiated by a double fertilization which induces the formation of the embryo and the endosperm. This process has been described more than one century ago but the molecular mechanisms by which the fertilization activates the embryogenic programm are poorly understood. The most studied pathway concerns the repression of the central cell development before fertilization. We have used a forward genetic approach in order to characterize essential genes involved in the activation of early embryogenesis in the model plant Arabidopsis thaliana. The analysis have been made on embryo-defective (emb) mutants arrested during the first steps of embryogenesis and tagged by a T-DNA insertion. The mutations are sporophytic and recessive. We focused on the phenotypic and molecular characterization of zeus (zeu) mutants arrested after the zygote elongation and cyclops (cyl) mutants stopped after the first assymetric division of the zygote. One ZEU and two CYL genes have been identified and confirmed by fonctional complementation. The expression profil of each gene suggest a transcriptional regulation at the moment of fertilization consistent with their role in the activation of the seed development. The ZEU1 and CYL2 proteins are involved in DNA replication. The putative function of CYL1 suggests that a signalling pathway similar to the one involved in the induction of somatic embryogenesis could be implicated in vivo to induce the zygotic embryo development.?PERPIGNAN-BU Sciences (661362101) / SudocSudocFranceF

Global transcriptome analysis of two ameiotic1 alleles in maize anthers: defining steps in meiotic entry and progression through prophase I

Abstract Background Developmental cues to start meiosis occur late in plants. Ameiotic1 (Am1) encodes a plant-specific nuclear protein (AM1) required for meiotic entry and progression through early prophase I. Pollen mother cells (PMCs) remain mitotic in most am1 mutants including am1-489, while am1-praI permits meiotic entry but PMCs arrest at the leptotene/zygotene (L/Z) transition, defining the roles of AM1 protein in two distinct steps of meiosis. To gain more insights into the roles of AM1 in the transcriptional pre-meiotic and meiotic programs, we report here an in depth analysis of gene expression alterations in carefully staged anthers at 1 mm (meiotic entry) and 1.5 mm (L/Z) caused by each of these am1 alleles. Results 1.0 mm and 1.5 mm anthers of am1-489 and am1-praI were profiled in comparison to fertile siblings on Agilent® 4 × 44 K microarrays. Both am1-489 and am1-praI anthers are cytologically normal at 1.0 mm and show moderate transcriptome alterations. At the 1.5-mm stage both mutants are aberrant cytologically, and show more drastic transcriptome changes. There are substantially more absolute On/Off and twice as many differentially expressed genes (sterile versus fertile) in am1-489 than in am1-praI. At 1.5 mm a total of 4,418 genes are up- or down-regulated in either am1-489 or am1-praI anthers. These are predominantly stage-specific transcripts. Many putative meiosis-related genes were found among them including a small subset of allele-specific, mis-regulated genes specific to the PMCs. Nearly 60% of transcriptome changes in the set of transcripts mis-regulated in both mutants (N = 530) are enriched in PMCs, and only 1% are enriched in the tapetal cell transcriptome. All array data reported herein will be deposited and accessible at MaizeGDB http://www.maizegdb.org/. Conclusions Our analysis of anther transcriptome modulations by two distinct am1 alleles, am1-489 and am1-praI, redefines the role of AM1 as a modulator of expression of a subset of meiotic genes, important for meiotic progression and provided stage-specific insights into the genetic networks associated with meiotic entry and early prophase I progression

Global transcriptome analysis of two <it>ameiotic1 </it>alleles in maize anthers: defining steps in meiotic entry and progression through prophase I

Abstract Background Developmental cues to start meiosis occur late in plants. Ameiotic1 (Am1) encodes a plant-specific nuclear protein (AM1) required for meiotic entry and progression through early prophase I. Pollen mother cells (PMCs) remain mitotic in most am1 mutants including am1-489, while am1-praI permits meiotic entry but PMCs arrest at the leptotene/zygotene (L/Z) transition, defining the roles of AM1 protein in two distinct steps of meiosis. To gain more insights into the roles of AM1 in the transcriptional pre-meiotic and meiotic programs, we report here an in depth analysis of gene expression alterations in carefully staged anthers at 1 mm (meiotic entry) and 1.5 mm (L/Z) caused by each of these am1 alleles. Results 1.0 mm and 1.5 mm anthers of am1-489 and am1-praI were profiled in comparison to fertile siblings on Agilent® 4 × 44 K microarrays. Both am1-489 and am1-praI anthers are cytologically normal at 1.0 mm and show moderate transcriptome alterations. At the 1.5-mm stage both mutants are aberrant cytologically, and show more drastic transcriptome changes. There are substantially more absolute On/Off and twice as many differentially expressed genes (sterile versus fertile) in am1-489 than in am1-praI. At 1.5 mm a total of 4,418 genes are up- or down-regulated in either am1-489 or am1-praI anthers. These are predominantly stage-specific transcripts. Many putative meiosis-related genes were found among them including a small subset of allele-specific, mis-regulated genes specific to the PMCs. Nearly 60% of transcriptome changes in the set of transcripts mis-regulated in both mutants (N = 530) are enriched in PMCs, and only 1% are enriched in the tapetal cell transcriptome. All array data reported herein will be deposited and accessible at MaizeGDB http://www.maizegdb.org/. Conclusions Our analysis of anther transcriptome modulations by two distinct am1 alleles, am1-489 and am1-praI, redefines the role of AM1 as a modulator of expression of a subset of meiotic genes, important for meiotic progression and provided stage-specific insights into the genetic networks associated with meiotic entry and early prophase I progression.</p