Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 198-220). Also available in electronic version.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2006. / Includes bibliographical references (leaves 222-232). Also available in electronic version.

Hamburg, Universiẗat, Diss., 2004.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 179-189). Also available in electronic version.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 193-218). Also available in electronic version.

Universiẗat, Diss., 2005--Heidelberg.

Metazoan development is a complex and tightly controlled process that not only requires precise cell fate differentiation, but also demands accurate timing of cell division and precise cell migration. Genetic regulation of cell cycle length throughout metazoans embryogenesis is largely unknown, mainly due to the technical hurdle in quantifying cell division timing during development. Caenorhabditis elegans embryogenesis provides an excellent opportunity to study the genetic regulation of division timing because of its invariant cell lineage and widespread division asynchronies between sister cells. A combination of in toto imaging and automated cell lineaging coupled with high throughput RNAi allows genetic screening of genes involved in regulation of Asynchrony of Division between Sister cells (ADS) or cousin cells. One of the most pronounced asynchronies between cousin cells during C. elegans embryogenesis is a significant elongation of division timing in two endoderm progenitor cells, Ea and Ep (E2), versus their cousins MSa and MSp (MS2) that mainly develop into mesoderm organs. Out of a total of 822 essential and conserved genes that were perturbed by RNAi in our previous genetic screening, 53 genes are found to produce significantly reduction specifically in the E2 cell cycle length (p<0.01). Surprisingly, nearly 70% of the 53 genes are involved in mRNA production or its regulation, indicating a differential requirement of transcription for division timing between E2 and MS2. Reduction in E2 cell cycle length is frequently associated with cell migration defect and gastrulation failure. Furthermore, our systematic data on cell division timings upon perturbation of a large cohort of essential genes provide a valuable source for inferring the function of uncharacterized gene. For example, phenotypical clustering based on cell division timings suggested that an essential gene, gad-1, is likely to be involved in general transcription, which is in agreement with its further functional assays. In summary, a combining of the published data with our own demonstrates that E2 specific cell cycle elongation requires robust and earlier zygotic genome activation (ZGA) during C. elegans embryogenesis. The cell-specific elongation might be important for coordinate fate differentiation, division timing and cell migration of E2 to ensure proper intestine development.

Hybrid incompatibility (HI) refers to lethality, sterility and other reduction in fitness of hybrid progeny between related species which has been frequently observed in different taxa. Its mechanism has been generalized as the consequence of conflicts between genomes of the related species that have been subject to independent evolution. HI plays a critical role in speciation by reducing or preventing exchange of the genetic materials between related species, gradually leading to reproductive isolation. According to the widely accepted Dobzhansky-Muller model, HI is produced by incompatible epistatic interactions between multiple genes that are independently diverged between the parental species. HI has been intensively studied especially in Drosophila species. A number of HI loci have been mapped in various species, several of which have been molecularly cloned. However, HI remains poorly understood in other taxa. My thesis focuses on systematic characterization of HI between two sequenced nematode species, C. briggsae and C. nigoni (C. sp. 9). The former has long been established as a companion species of C. elegans for comparative study while the latter is a recently identified species that can mate with the former and produce viable and fertile hybrids in spite of their different reproduction modes, allowing one to genetically and molecularly characterize HI between nematode species for the first time. Such a study is impossible with model organism C. elegans as it cannot produce viable hybrids with any other nematode species. To facilitate genome-wide mapping of HI loci between the two nematode species, over 90 dominant visible GFP markers have been randomly integrated into C. briggsae chromosomes, permitting repeated backcrossing the marker-associated C. briggsae genomic fragments into an otherwise C. nigoni background and definitive mapping of HI loci. Genotyping of the introgressions has been achieved by C. briggsae specific PCR, with primers as close as about half a million base pairs away from each other on average. Using the cost-effective protocol for introgression and genotyping, a genome-wide HI map between the two species has been obtained based on characterization of approximately 100 independent introgression lines. A remarkable proportion of C. nigoni genome was found to be replaceable by that from C. briggsae as judged by the fact that these introgressions are viable as a homozygote. A few HI loci critical to male inviability and sterility have been narrowed down to small intervals on the X chromosome by contrasting genotypes and HI phenotypes of independent introgression strains. Strains containing these introgressions have been examined for their gonadal structures using fluorescence microscopy. Our study provides genome-wide landscape of HI between nematodes for the first time, allowing comparative studies of HI between nematode and other species.

No description available.

Ubiquitin is a multifunctional 76 amino acid protein which plays critical roles in many aspects of cellular metabolism. Ubiquitin protein structure and gene structure are highly conserved among eukaryotes. In C. elegans the major source of ubiquitin RNA was shown to be the polyubiquitin locus, UbiA. UbiA was shown to be transcribed as a polycistronic mRNA which contained eleven tandem repeats of ubiquitin sequence and possessed a two amino acid carboxy terminal extension on the final repeat. Mature UbiA mRNA was demonstrated to acquire a 22 nucleotide leader sequence via a trans splicing reaction involving a 100 nucleotide splice leader RNA derived from a different chromosome. UbiA was also shown to be unique among known polyubiquitin genes in containing four cis spliced introns within its coding sequence. Thus UbiA was shown to be one of a small class of genes found in higher eukaryotes whose hnRNA undergoes both cis and trans splicing. The expression of the UbiA gene was studied under various heat shock conditions, and was monitored during larval molting and throughout the major stages of development. These studies indicated that the expression of the UbiA gene was not inducible by acute or chronic heat shock, and did not appear to be under nutritional or developmental regulation. A second ubiquitin gene, UbiB, was cloned from C. elegans and a related nematode species C. briggsae. This gene was comprised of (at least) one ubiquitin unit followed by a basic 52 amino acid tail sequence. / Medicine, Faculty of / Biochemistry and Molecular Biology, Department of / Graduate