Epistatic regulation of optix in hybrid patterns. (a-b) A synthetic hybrid zone was created by interbreeding Heliconius melpomene plesseni and H. melpomene madeira butterflies in a large greenhouse. Expression of optix in founders is reproduced from in situ hybridization and the plesseni white scales expresses optix. The N locus that modulates the color composition of the median forewing: non-Amazonian (non-rayed) races such as plesseni bear NB alleles, while Amazonian (rayed) races such as madeira bear NN alleles that result in a distal localization of the forewing red band[14, 48]. Panel b shows a sample of the phenotypic diversity observed on forewings of this hybrid population. Only butterflies that were homozygous for the plesseni optix allele are shown here (dominant rayed alleles excluded). Hindwings show no pattern variation in this population and are not shown. (c) Artificial selection of butterflies showing a band of distal red (yellow box) generated a stable population called ‘Narrow red’. The distal localization of red is explained by NN alleles that repress proximal expression of optix. (d) Crossing a ‘narrow red’ individual with a NB/NB butterfly does not recover the full expression of the forewing band red in the F1 progeny, in which Optix expression is limited to the distal portion of the white/red boundary. The absence of fully red forewing bands similar to the parental rosina pattern in the F1 shows the ‘narrow red’ parent is NN-homozygous. (e) Artificial selection of butterflies showing a mixture of red and white scales in the forewing band (blue box) generated a stable population called ‘Mixed red’. In the presence of NB alleles derived from plesseni, proximal optix expression (for instance overlapping with the discal cell) and only delimited by the total shape of the forewing bar.