A Balanced Inversion Polymorphism Exhibits a Dominance Reversal at the Gene Expression Level that Depends on Developmental Context.

How genetic variance for fitness is maintained is incompletely understood. Mutation-selection balance and single-locus overdominance cannot account for the large variance observed. Recent work suggests that antagonistic balancing selection, favoring different alleles in different contexts and involving beneficial dominance reversals, might contribute to maintaining fitness variance. However, while this mechanism is plausible, evidence for dominance reversals remains scarce. Here, we study how In(3R)Payne, a balanced inversion polymorphism in Drosophila melanogaster, affects gene expression and chromatin accessibility by using RNA-seq and ATAC-seq (assay for transposase-accessible chromatin with sequencing). We find that, in embryos, the inverted (INV) arrangement tends to have dominant effects, while the standard (STD) arrangement behaves like a recessive Mendelian allele. Yet, in wing discs, this pattern is reversed: STD has mostly dominant effects, whereas INV behaves recessively. Since this shift in the dominance of the INV "allele" between developmental contexts affects the expression of suites of genes in a concerted manner, it might be mediated by a dominance modifier, for example, a transcription factor. In favor of this idea, 25% of the differentially expressed genes between INV and STD encode transcription factors. Interestingly, while only four differentially expressed genes are shared between embryos and wing discs, one of them is HP1c, a chromatin-binding protein and major transcriptional regulator, and thus a promising candidate for mediating the context-dependent change in dominance. Although the relationship between these patterns and fitness is presently unknown, our observations are consistent with a potential role of reversals (or, more generally, shifts) of dominance in maintaining inversion polymorphism.