Interallelic cis-regulatory dominance promotes robustness and evolutionary innovation

Dominance is a central principle of genetics, yet the mechanistic basis and the evolutionary consequences of dominance arising from cis-regulatory variation remain poorly understood. We examined the evolutionary trajectories of a pleiotropic developmental enhancer in Drosophila. A genotype–phenotype map between D. melanogaster and D. simulans enhancer sequences reveals extensive epistasis, and many homozygous evolutionary paths reduce transcriptional output. In heterozygotes, however, regulatory dominance masks variants that reduce gene expression, potentially relaxing evolutionary constraints. Using allele-specific reporters and imaging, we show that this dominance arises from interallelic interactions (also known as transvection) reinforced by transcriptional hubs. Importantly, this enhancer dominance is cell-type specific, raising the possibility that it conceals deleterious effects in essential tissues while revealing novel, ectopic activity in others. Interallelic regulatory hubs may therefore expand the range of mutational paths available to diploid genomes while preserving essential transcriptional output.