Heliconius butterflies, native to the Neotropics and known to feed on passion flower vines for which they are eponymous, show a huge diversity of morphology, specifically in relation to the beautiful and complex colour patterns displayed on their wings. All the species within this genus use conspicuous wing colourations as warning signals to potential predators – an adaptation known as aposematism. Some of these butterflies, particularly those endemic to similar and overlapping geographical ranges, have evolved similar wing pigmentation patterns to signal to predators that they are inedible. This phenomenon, whereby different species share a common predator and so develop a common protective advantage, is known as Müllerian mimicry and is a hallmark of the Heliconius genus. The strongly adaptive function of wing patterns has generated an explosive diversity of morphologies in these butterflies, and as yet little is known about the genetic innovation that may have triggered such a rapid burst of evolution. Arnaud Martin from Cornell University, USA, and colleagues sought to investigate this by analysing the evolution of pigment patterns in these butterflies, as published in a recent study in EvoDevo.
The gene optix has previously been identified as identified as an important factor in the evolution of wing pattern diversification in two pairs of co-mimetic Heliconius butterflies. By looking at the developmental expression of optix in a broader sample of butterflies, Martin and colleagues identified the genetic conditions within Heliconius butterflies that gave rise to the spectacular red pattern adaptive radiation (a rapid speciation event resulting in increased diversity of derived lineages). The researchers used an antibody directed against the Optix protein, and compared the immunoreactivity in the developing wings of Heliconius and other butterflies at the pupal stage. At this time, the wings have not started to synthesise colour pigments, but expression of Optix acts as a genetic switch that specifies exactly which cells will make red pigments, thus specifying a molecular canvas for the presence and shape of red patterns involved in mimicry.
Their findings demonstrate that multiple recent co-options (the shift in function of a gene during evolution) of the Optix transcription factor, may have facilitated the extraordinary diversification of wing morphology that we see in nature today. The resulting images from their study highlight the tight association of Optix with a variety of wing features. Not only was Optix associated with the specification of red colour cells, but the study also reveals that it was recruited in the development male-specific vein structures seen in some of the butterflies (see below).
Many of the functions of Optix inferred by its developmental expression relate to evolutionary novelties – new phenotypic traits previously not observed in the Heliconiini clade. This supports the idea that these functions were key prerequisites to the phenotypic radiation of Heliconius butterflies. By identifying a prototypical expression of Optix in Dryas iulia, a species basal to the Heliconiini clade, the researchers showed that the colour patterning role of Optix preceded the evolutionary radiation of this taxa. Optix may therefore have helped to facilitate this radiation, resulting in a major diversification event that ultimately led to the wide array of patterns that characterise the wings of Heliconius butterflies.