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Essential genes in the hDp16/hDp19 region of chromosome I in Caenorhabditis elegans McDowall, Jennifer Susan


This thesis describes the genetic analysis of a small region of the Caenorhabditis elegans genome. The region analyzed was defined by the 0.5 map unit interval between the breakpoints of the duplications hDp16 and hDp19, which lies within the dpy-5 unc-13 region on chromosome I. The analysis consisted of the identification and characterization of essential genes in this region. All the lethal mutations analyzed came from a set of 495 EMS-induced, sDp2-rescued lethals described in Howell (1989). The lethal mutations have been maintained as homozygotes, made viable by the presence of a wild-type allele on the sDp2 duplication. I used a combination of mapping techniques to analyze over 200 EMS-induced lethal mutations. A total of 189 new lethal mutations were positioned within the dpy-5 unc-13 interval. Three methods were used for mapping: recombination analysis, lethal rescue using duplications, and deficiency mapping. Duplication mapping was found to be the fastest and most precise method used. 178 of the new lethal mutations were mapped relative to the duplication breakpoints of hDp13, hDp16, and hDp19. These three duplications divided the dpy-5 unc-13 region into three approximately equal-sized zones. This study completes the mapping of the 495 lethals in the sDp2 set. In addition, thirty-three previously identified essential genes lying between dpy-5 unc-13 were positioned with respect to the breakpoints of the duplications hDp12, hDp13, hDp15, hDp16, hDp17, and hDp19. The dpy-5 unc-13 region carries a relatively large number of loci, therefore, I decided to concentrate on the smaller hDp16/hDp19 interval within this region. Complementation analysis was used to define the number of essential genes in the hDP16/hDp19 region. A total of eight new genes were described, six lying in the hDp16/hDP19 region, two lying just outside this region. This brings the total number of essential genes in the hDP16/hDp19 region to sixteen. In addition, as a result of my mapping data, the hDp16/hDp19 region has been subdivided into six intervals with respect to duplication and deficiency breakpoints. The stage of developmental arrest was determined for both the essential genes, and the new lethal mutations (ie. not yet defined by complementation tests), in the dpy-5 unc-13 interval. Although the number of genes studied was not great, the data suggests a relationship between map position and time of developmental arrest. The average forward mutation rate for C. elegans genes was determined to be 5.8 X 10⁻⁵mutations per gene. I have made a comparison of the forward mutation rates of the essential genes in the hDp16/hDp19 region, bli-4 was found to be the most mutable target in the region with nine mutant alleles, giving a forward mutation rate six times higher than average. In the hDp16/hDp19 region, ten of the sixteen essential genes were represented by more than one allele. The minimum estimate of the number of essential genes in the region using a truncated Poisson calculation was twenty. Therefore, the sixteen genes identified represent 80% of the essential genes in this region. This data was extrapolated to give a minimum estimate of approximately 225 essential genes in the 15 m.u. sDp2 region, and 4,500 essential genes in the C. elegans genome. This research has established the hDp16/hDp19 region as a genetically well-defined system for studying the genetic organization of essential genes, as well as the developmental regulation of gene expression, and the functional relationship between adjacent genes.

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