Figure 6.

Asymmetric expression of parental orthologs in the F₁ hybrid resulting from cis-by-trans regulatory divergence, when the relative amount of divergence in cis compared to trans differs between species. In the above example, divergence occurred under selection for reduced expression. More spurious matches resulted between the binding site of species 1 and the transcription factor of species 2 (top right interaction in the hybrid) than in the inverse interaction (bottom right). Depending on allelic dominance, the overall expression level in the F₁ generation may be nearly the same in hybrids as in the parent species.