Zygote | EPFL Graph Search

A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellular organisms, the zygote is the earliest developmental stage. In humans and most other anisogamous organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring. German zoologists Oscar and Richard Hertwig made some of the first discoveries on animal zygote formation in the late 19th century. Humans Development of the human body and Human fertilization In human fertilization, a released ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete) combine to form a single diploid cell called the zygote. Once the single sperm fuses with the ooc

Characterizing the localization and role of lin-5 and era-1 mRNAs in early C. elegans embryos

Zoltán Péter Spiró

Asymmetric cell division is essential for the generation of diversity during development and the function of stem cell lineages. The Caenorhabditis elegans zygote is an attractive model to investigate the mechanisms of spindle positioning during asymmetric cell division. In this polarized cell, the asymmetric distribution of cortical force generators along the antero-posterior axis and pulling on astral microtubules leads to the unequal cleavage of the one-cell embryo. The mechanisms underlying such cortical force generation are thought to act strictly at the protein level. In this thesis work we report that the mRNA encoding the cortical force generator component LIN-5 is enriched around centrosomes in early embryos, in a manner that depends on microtubules and dynein. We established that the lin-5 coding sequence is necessary and sufficient for mRNA enrichment around centrosomes in C. elegans. In addition, we found that lin-5 mRNA is mislocalized in lin-5(ev571) mutant embryos, which harbor a 9 nucleotide insertion in the coding sequence. Moreover, an intragenic revertant of lin-5(ev571), lin-5(ev571he63), also exhibits mislocalized lin-5 mRNA distribution. We demonstrated that this is accompanied by diminished pulling forces on the posterior spindle pole, suggesting that centrosomal localization of lin-5 mRNA is important for robust pulling forces. We found also that lin-5 mRNA centrosomal enrichment is slightly asymmetric during anaphase, with more transcripts present on the anterior side. We developed a novel FRAP-based assay, which revealed that lin-5 is translated/folded preferentially in the cytoplasm compared to centrosomes. Furthermore, we found that morpholino-mediated inhibition of lin-5 translation diminishes pulling forces on the posterior side during anaphase. Together, these findings lead us to propose that preferential translation/folding of lin-5 in the posterior cytoplasm following release of the mRNA from the posterior centrosome contributes to asymmetric cortical distribution of force generators, and thus to proper spindle positioning. Moreover, we found that the mRNA of an uncharacterized gene, era-1 is enriched on the anterior side of the zygote and is inherited by the anterior blastomeres. Similar to era-1 mRNA, a YFP fusion of ERA-1 protein is also asymmetrically distributed. Moreover, asymmetric distribution of both era-1 mRNA and YFP-ERA-1 protein requires the era-1 3'UTR. Furthermore, the RNA-binding protein MEX-5 is needed for both asymmetric era-1 mRNA localization and for its translational activation. Furthermore, we report that the clathrin heavy chain CHC-1 negatively regulates pulling forces acting on centrosomes during interphase and on spindle poles during mitosis in one-cell C. elegans embryos. We establish a similar role for the cytokinesis/apoptosis/RNA-binding protein CAR-1 and uncover that CAR-1 is needed to maintain normal levels of CHC-1. We demonstrate that CHC-1 is necessary for proper organization of the cortical acto-myosin network and for full cortical tension. Furthermore, we establish that the centrosome positioning phenotype of embryos depleted of CHC-1 is alleviated by stabilizing the acto-myosin network. Conversely, we demonstrate that slight perturbations of the acto-myosin network results in excess centrosome movements. Overall, our findings lead us to propose that clathrin plays a critical role in centrosome positioning by promoting acto-myosin cortical tension.

EPFL2014

Mechanisms of differential centriole inheritance in C. elegans

Lukas von Tobel

Centrosomes are the principal microtubule organizing center (MTOC) of animal cells and comprise a pair of centrioles surrounded by pericentriolar material (PCM). Centriole number must be carefully regulated to ensure bipolar spindle formation and thus faithful chromosome segregation. In the germ line of most metazoan species, centrioles are maintained during spermatogenesis, but eliminated during oogenesis. Such differential behavior ensures that the appropriate number of centrioles is present in the newly fertilized zygote. In this doctoral thesis work, I describe our investigation of centriole elimination during oogenesis in C. elegans and I characterize sas-1, a factor important for maintaining centriole integrity, especially during spermatogenesis, to ensure that two centrioles are present in the zygote. Despite being a fundamental feature of sexual reproduction in metazoans, themechanisms governing centriole elimination during oogenesis are poorly understood. We investigate this question using antibodies directed against centriolar components and serial-section electron microscopy and we establish that centrioles are eliminated during the diplotene stage of the meiotic cell cycle. Moreover, we show that centriole elimination is compromised upon depletion of the helicase CGH-1. We also find that somatic cellsmake a minor contribution to this process, and demonstrate that the germcell karyotype is important for timely centriole elimination. These findings set the stage for a mechanistic dissection of centriole elimination in ametazoan organism. How centriole integrity is ensured after organelle assembly is poorly understood, including in sperm cells, where such integrity is particularly critical. In a genetic screen in C. elegans, sas-1 was identified as a locus important for the maintenance of centrioles in the early embryo. Our analysis reveals that sperm-derived sas-1 mutant centrioles lose their integrity shortly after fertilization, and that a related defect occurs when maternal sas-1 function is absent. We establish that sas-1 encodes a C2 domain containing protein that localizes to centrioles once they are assembled. Futhermore, we demonstrate that SAS-1 can bind and stabilize microtubules when expressed in human cells. We also identify C2CD3, a protein required for proper centriole formation, as a putative mammalian SAS-1 homolog. Together, we uncover an important component required for maintaining centrioles in sperm cells to ensure that two centrioles are present after fertilization and thus guarantee the continuity of the species.

EPFL2014

Analysis of centriole elimination during C. elegans oogenesis

Pierre Gönczy, Graham Knott, Tamara Mikeladze-Dvali, Petr Strnad, Lukas von Tobel

Centrosomes are the principal microtubule organizing centers (MTOCs) of animal cells and comprise a pair of centrioles surrounded by pericentriolar material (PCM). Centriole number must be carefully regulated, notably to ensure bipolar spindle formation and thus faithful chromosome segregation. In the germ line of most metazoan species, centrioles are maintained during spermatogenesis, but eliminated during oogenesis. Such differential behavior ensures that the appropriate number of centrioles is present in the newly fertilized zygote. Despite being a fundamental feature of sexual reproduction in metazoans, the mechanisms governing centriole elimination during oogenesis are poorly understood. Here, we investigate this question in C. elegans. Using antibodies directed against centriolar components and serial-section electron microscopy, we establish that centrioles are eliminated during the diplotene stage of the meiotic cell cycle. Moreover, we show that centriole elimination is delayed upon depletion of the helicase CGH-1. We also find that somatic cells make a minor contribution to this process, and demonstrate that the germ cell karyotype is important for timely centriole elimination. These findings set the stage for a mechanistic dissection of centriole elimination in a metazoan organism.

2012

Mechanisms of differential centriole inheritance in C. elegans

Lukas von Tobel

EPFL2014

Characterizing the localization and role of lin-5 and era-1 mRNAs in early C. elegans embryos

Zoltán Péter Spiró

EPFL2014