RUI: X-Chromosome Meiosis in Caenorhabditis Elegans

Haverford College, Haverford PA

Investigators

Abstract

Early in meiosis, homologous chromosomes pair, exchange, and synapse. Both the number and the location of exchange events are highly regulated in a poorly understood process called crossover control. Crossover control is postulated to be closely related to chromosome compaction, although direct proof of this postulate has been lacking. Mutations that alter the number or the distribution of crossovers have been identified in many organisms, including C. elegans. Recessive mutations in the autosomal gene him-5 reduce the number of crossovers on the X chromosome and alter the distribution of the residual crossovers. Preliminary molecular evidence indicates that the predicted HIM-5 protein has a region of homology to RCC1, a well-known protein involved in chromosome compaction in vertebrates. Direct proof that this gene corresponds to him-5 will be obtained by determining the structure of transcripts from the locus and sequencing these in him-5 mutants. The expression pattern of the gene will be determined in both sexes, and in various mutant strains affecting meiosis and germ-line development. Using immunofluorescence, the location of the HIM-5 protein in the gonad will be determined in order to infer the meiotic processes affected. A closer homologue of RCC1 has also been shown to affect meiosis in C. elegans, but the gene involved has not been studied. Mutations of this gene will be identified and characterized genetically and cytologically. The expression of this gene will be studied as well. Epistatic relationships between this gene and him-5, as well as other meiotic mutants, will be used to clarify the functional relationships among the genes. As RCC1 is well-known in other organisms, the characterization of its role in meiosis in C. elegans could make a direct connection between chromosome structure and crossover control. One familiar genetic manifestation of crossover control is the phenomenon of interference, whereby one crossover prevents the occurrence of a second crossover nearby. Previous work has shown there is extremely high interference in C. elegans, and that most chromosomes have exactly one exchange. Interestingly, all of the known double crossover exceptions had one of the crossovers in a region of the X chromosome known to have other unusual meiotic properties. Genetic mapping experiments, combined with analysis from the genomic sequence, will be used to examine the unusual meiotic properties in this region and to ask if these have a common basis. Undergraduate students under the direction of the principal investigator will conduct all of these experiments. Because meiosis is a fundamental process whose main characteristics have been known for decades, most students are familiar with the basic properties. However, the molecular basis for crossover control is quite obscure. Therefore, the students will be carrying out significant research on a fundamental and familiar process.