Perfect mimicry between Heliconius butterflies is constrained by genetics and development

Mullerian mimicry strongly exemplifies the power of natural selection. However, the exact measure of such adaptive phenotypic convergence and the possible causes of its imperfection often remain unidentified. The butterfly species Heliconius erato and Heliconius melpomene have a large diversity of co-mimicking geographic races with remarkable resemblance in melanic patterning across the mid-forewing that has been linked to expression patterns of the gene WntA. Recent CRISPR/Cas9 experiments have informed us on the exact areas of the wings in which WntA affects color pattern formation in both H. erato and H. melpomene, thus providing a unique comparative dataset to explore the functioning of a gene and its potential effect on phenotypic evolution. We therefore quantified wing color pattern differences in the mid-forewing region of 14 co-mimetic races of H. erato and H. melpomene and measured the extent to which mimicking races are not perfectly identical. While the relative size of the mid-forewing pattern is generally nearly identical, our results highlight the areas of the wing that prevent these species from achieving perfect mimicry and demonstrate that this mismatch can be largely explained by constraints imposed by divergence in the gene regulatory network that define wing color patterning. Divergence in the developmental architecture of a trait can thus constrain morphological evolution even between relatively closely related species.