How mutational events in genomes of butterflies and moths cause ADAPTATION to environmental signals

These two fascinating articles [below] elucidate some mechanisms that help explain how an organism’s genome can be “forced” by strong selective environmental pressures into changing an organism’s outward appearance (heritable trait) to something that will benefit survival of the species in this (changing adverse) environment.

Discovering mutational events that fuel adaptation to environmental change––remains an important challenge for evolutionary biology. The classroom example of a visible evolutionary response is: industrial melanism in the peppered moth (Biston betularia): replacement, during the Industrial Revolution, of the common pale typica form by a previously unknown black (carbonaria) form, appeared to have been driven by interaction between bird predation and coal pollution. The carbonaria locus had been coarsely localized to a 200-kb region, but the specific identity and nature of the sequence difference that controls the carbonariatypica polymorphism, and any gene it influences, has remained unknown.

The first [attached] article, pp 102–105, shows that the mutational event giving rise to industrial melanism in Britain was insertion of a large tandemly repeated transposable element into the first intron of the gene cortex. Statistical inference––based on the distribution of recombined carbonaria haplotypes––indicates that this transposition event occurred around 1819 [!!!], consistent with the historical record. The authors then demonstrated that the mode-of-action of the transposable element is to increase abundance of a cortex transcript, the protein product of which plays an important role in cell-cycle regulation during early wing disc development.


These findings add to our knowledge about this iconic example of microevolutionary change––adding a further layer of insight into the mechanism of adaptation in response to natural selection. Discovery that the mutation itself is a transposable element will stimulate further debate about the importance of ‘jumping genes’ [the discovery of which by Barbara McClintock resulted in her winning the 1983 Nobel Prize] as a source of major phenotypic novelty.

Wing patterns of butterflies and moths (Order of insects called Lepidoptera) are striking examples of evolutionary diversification by natural selection. Lepidopteran wing color patterns are novel, consisting of arrays of colored scales. Yet, there still remains a lack a general understanding of how these patterns are controlled and whether this control shows any commonality––across the 160,000 moth, and 17,000 butterfly, species. In the second [attached] article, pp 106–110, authors use fine-scale mapping with population genomics and gene expression analyses to identify the gene cortex, that regulates pattern switches in multiple species across the mimetic radiation in Heliconius butterflies.

The gene cortex belongs to a fast-evolving subfamily of the otherwise highly conserved fizzy family of cell-cycle regulators, suggesting that it probably regulates pigmentation patterning by regulating scale cell development. In parallel with findings in the peppered moth (Biston betularia), these results suggest that this mechanism is common within Lepidoptera and that cortex has become a major target for natural selection––acting on color and pattern variation in this group of insects.

Nature   2 June 2o16;  534:  102-105 & 106–110   (two back-to-back articles) Plus one brief editorial (page 5)

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