Systems genetics of single nucleotide substitutions at the Drosophila Obp56h locus

Individual variation in complex traits results from allelic variants of multiple segregating genes, which are expressed as coregulated ensembles that are modulated by the environment. Coregulated transcriptional networks around focal genes, defined as their 'transcriptional niches', are sensitive to genetic and environmental perturbations. Understanding how single base pair substitutions affect this complex genotype-phenotype relationship by perturbing transcriptional niches is possible in Drosophila, which allows precise control of both the genetic background and the environment. We used a two-step CRISPR-Cas9 mediated gene deletion and reinsertion strategy to generate in a common genetic background five single nucleotide substitutions in the D. melanogaster Obp56h gene that correspond to naturally occurring allelic variants. Changes in single base pairs give rise to differential, sexually dimorphic effects on a plethora of fitness traits, including viability, sex ratio, feeding behavior, starvation resistance, recovery from a chill-induced coma, response to heat shock, activity, and sleep traits. These pleiotropic effects are accompanied by sexually dimorphic shifts in the transcriptional niche of Obp56h. Pairwise comparisons between the lines show common coregulated genes along with varying numbers of transcripts unique to one or few Obp56h alleles. Gene ontology enrichment analyses indicate that fundamental cellular processes for each sex underlie the phenotypic pleiotropy revealed by the Obp56h allelic series. Furthermore, different Obp56h alleles in a common genetic background give rise to allele-specific, sexually dimorphic microenvironmental variation. The reverse genetic engineering strategy, illustrated here, can be generally applied to other genes to dissect variation in the genotype-phenotype relationship at single base pair resolution. Overall design: RNA seq profiles (post rRNA depletion) of heads of CSB, Obp56h knockout and 5 allelic variants in both sexes.