Postmating responses depend on genome background in Drosophila melanogaster strains homoallelic for sex peptide

The phenotypic response of gene alleles can be influenced by environmental conditions and genome background. Although progress has been made focusing on polymorphisms at major-effect genes, our understanding of how genome background modulates phenotypic expression remains limited. To address this, we focus on two female postmating responses-egg laying and remating rate, both primarily triggered by sex peptide (SP) transfer from males-across different genome backgrounds and yeast environments. Using 10 Drosophila melanogaster Drosophila Genetic Reference Panel lines carrying an identical SP allele, we found significant effects of both genome background and yeast on egg laying and remating behaviour. SP expression analysis revealed no significant differences among lines, suggesting that the observed phenotypic variation is not driven by transcriptional regulation of the focal gene. However, a genome-wide association analysis of remating under yeast supplementation identified genetic variants non-randomly distributed across chromosomes and implicated potential effectors to this complex trait. Together, our results highlight how genome background can contribute substantially to phenotypic variation that is commonly ascribed to allelic differences at major regulators of reproductive success.