Two types of cis-trans compensation in the evolution of transcriptional regulation

Because distant species often share similar macromolecules, regulatory mutations are often considered responsible for much of their biological differences. Recently, a large portion of regulatory changes has been attributed to cis-regulatory mutations. Here, we examined an alternative possibility that the putative contribution of cis-regulatory changes was, in fact, caused by compensatory action of cis- and trans-regulatory elements. First, we show by stochastic simulations that compensatory cis-trans evolution maintains the binding affinity of a transcription factor at a constant level, thereby spuriously exaggerating the contribution of cis-regulatory mutations to gene expression divergence. This exaggeration was not observed when changes in the binding affinity were compensated by variable transcription factor concentration. Second, using reciprocal introgressions of Drosophila, we show that relative expression of heterozygous alleles from two distinct species often varied significantly between different species backgrounds, indicating the possible action of cis-trans compensation. Taken together, we propose that cis-trans hybrid incompatibilities are accumulating much faster than generally considered.