This proposal is to study the genetic basis of quantitative variation at the level of individual loci (quantitative trait loci, or QTLs), using the model system of chemotaxis in Caenorhabdidtis elegans. The inbred strains N2 and B0 differ in their behavioral response to benzaldehyde, and a reference set of 180 recombinant inbred (RI) lines between these strains that has been genotyped for molecular markers is available. the behavioral response to benzaldehyde has been measured on a subset of 80 RI lines, and QTLs for chemosensation have been mapped to the X chromosome by association with the molecular markers. The same analysis will be completed for the remaining set of 100 RI lines. The X chromosome QTLs will be mapped more precisely. A high density molecular marker map of the X chromosome will be constructed for these strains. The chromosome regions from B0 associated with the difference in chemotactic response between B0 and N2 will first be introgressed into the N2 background in 10 cM fragments, using markers to ensure the desired B0 region is being retained and to select for maximum replacement of the N2 background genotype each generation. The effects of the regions will be assessed in the common background, then further fine-scale mapping of the important region(s) will be accomplished by direct screening for informative recombination events in F2 progeny of crosses between the B0 introgression lines and N2. Over 80 loci affecting chemosensation in C. elegans have been identified by mutagenesis and are thus candidate QTLs. The map positions of two of these loci, odr-1 and odr-5, coincide with peaks of probability for the location of the X chromosome QTLs for response to benzaldehyde. The hypothesis that the QTLs interact with these loci, and other candidate loci that map to the same genomic region as the QTLs, will be tested by quantitative complementation. Most of the chemosensory mutations that have been studied are defective in the attraction response to low concentrations of benzaldehyde and not the repulsion response to high concentrations of this odorant. the correlation between the two responses is not unity; therefore, a mutagenesis screen for mutations defective in the repulsion response will be conducted. These mutations will be mapped, and will provide additional candidate loci. Finally, the extent to which allelic variation at the candidate loci (initially, odr-1 and odr-5) contributes to naturally occurring variation for chemotactic response will be determined. Natural isolates of C. elegans will be sampled, and their phenotypic variation in chemotactic response estimated. Molecular variation at the candidate loci will be assessed, and associations between phenotypic and molecular variation sought. The wild-derived lines will also be crossed to congenic strains containing null mutant or wild type alleles at the candidate loci, and the significant of the quantitative complementation effect (i.e., difference in mean chemotactic behavior between the two F1 hybrid classes) will be tested to determine whether naturally occurring alleles interact with the candidate locus mutations.
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