Coloring outside species boundaries: A ‘super’-cool example of adaptive introgression in butterflies

Although the main topic on this blog is avian hybridization, I occasionally venture into the world of other creatures (see for example here and here). Surprisingly, I have not covered hybridization in butterflies yet. But a recent paper in Current Biology is just too “super” to ignore.

 

Wondrous Wing-patterns

The butterfly Heliconius numata flutters around Amazonian forests in a variety of color morphs. Each wing-pattern closely matches the colors and shapes of other toxic butterflies. A textbook example of Batesian mimicry. But what is the genetic basis of this color palette? Previous work showed that the wing-pattern morphs are associated with large chromosomal rearrangements. The reshuffling of the genome has brought together several genetic loci involved in the build-up of color patterns, culminating in an inversion that can considered a supergene.

 

Heliconius_numata.jpg

Just one of the many color morphs of Heliconius numata (from http://www.wikipedia.com/)

 

Helpful Hybridization

Paul Jay and his colleagues reconstructed the evolutionary history of this supergene. First, they looked for this particular inversion in several closely related species. They found it only in H. pardalinus. Interestingly, numata and pardalinus are not sister species. How did they come to share the same inversion? Could it be (wait for it…) introgressive hybridization?!

Further analyses confirmed this suspicion. The inversion probably introgressed from pardalinus into numata about 2.3 million years ago. Quite a long time after these species diverged around 3.5 million years ago. After the introgression event, additional rearrangements in adjacent positions contributed to the variety of color morphs we see today. This study highlights the potential evolutionary significance of introgression. The authors conclude:

Beyond suggesting a mechanism for supergene evolution, these findings demonstrate how introgression, when involving structural variants, can trigger the emergence of novel genetic architectures.

 

inversion.jpg

The inversion (P1 – the purple boxes) was found in H. pardalinus (blue) and H. numata (red). Further analyses revealed hybridization was involved (from Jay et al. 2018 Current Biology)

 

A Major Evolutionary Force

Ernst Mayr once wrote that ‘the available evidence contradicts the assumption
that hybridization plays a major evolutionary role.’ I guess it is safe to say he was wrong. Hybridization is an important player that evolutionary biologists need to take into account.

 

References

Jay, P., Whibley, A., Frezal, L., Rodriguez de Cara, M.A., Nowell, R.W., Mallet, J., Dasmahapatra, K.K. & Joron, M. (2018) Supergene Evolution Triggered by the Introgression of a Chromosomal Inversion. Current Biology, 28:1-7.

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