First, the incomparable figure of genetic interactions in the forewing of Heliconius melpomene by John R. G. Turner in 1972 (Zoologica NY vol. 56: 125-155).

Here were my rather feebler efforts for the genetic interactions in the Tarapoto, Peru Heliconius hybrid zones (I remember pasting the lettering onto the drawings! — those were the days!): –

(from Mallet et al. 1990 Genetics  124:921-936)

Here’s some photos of the actual colour patterns in the Peru hybrid zones showing linkage and interactions:

And here’s Chris Jiggins et al. with the molecular loci involved in H. melpomene/cydno mimicry switches:

(From Jiggins et al. 2017 Phil Trans Roy Soc B 372:20150485).

Heliconius numata seems to be able to switch its entire colour pattern (including orange/brown optix patterns) by means of a series of polymorphic inversions in the cortex region:

Illustration of Heliconius numata dominance hierarchy at inversion forms near cortex gene. From Le Poul et al. 2014. Evolution of dominance mechanisms at a butterfly mimicry supergene. Nature Communications 5:5644).

You’ll notice that Chris Jiggins et al. show the red pattern markings shown as being due to action of optix, and the yellow/white pattern markings as being due to action of cortex. But this is highly simplified to make the points he wished to make about developmental gene co-option in that paper.

In Peru Heliconius erato seems to be able to switch its forewing yellow band on or off via action of the optix region (i.e. the DRy colour pattern locus in my own colour pattern diagram, see above). Chris Jiggins and Owen McMillan in the 1990s discovered that the yellow forewing band in himera/erato crosses was switched at the cortex locus, prompting me to look again at my Peru broods from the 1980s, and it is now clear to me that the cortex locus (Cr) also influenced the expression of yellow, explaining some fuzzy intermediate phenotypes in the forewing band in H. erato in those broods.

We also know that in H. melpomene there’s another colour pattern locus “M” that appears to be able to switch on yellow forewing bands recessively (Mallet 1989 Proc Roy Soc).  M appears to be linked to the B/D chromosome and therefore to optix (Simon Baxter pers. comm.)!  In contrast, the N locus at cortex switches on yellow dominantly (see the Turner 1972 diagram above). And in Turner’s crosses, B and D seem an awfully long way apart on the optix chromosome — they’re linked in repulsion with around 30% recombination rate between them.  So are they both really regulators of optix?

I think it’s true to say that we don’t fully understand all of these gene interactions yet, and perhaps we won’t until the regulatory pathways leading to colour pattern expression have been better worked out.