Butterfly wing patterns just got CRISPR

When people say their house have an infestation this is usually associated with something bad, say, an infestation of ants, wasps or rats for example. However, currently attracted to the flowering ivy in my backyard is a confetti of red admiral butterflies, painted with splashes of white and stripes of orange over a velvety dark spread*. They are quite the family favourite and thus it is not surprising that much recent research still focuses on the mechanistic and genetic underpinnings that create their elaborate wing patterns. Now with CRISPR on-board too to aid experimental studies new discoveries can be made.

* Maybe they all migrated from their release Sunday from Conwy nature reserve (http://www.bbc.co.uk/news/uk-wales-north-west-wales-41233011)!

Figure 1: A red admiral posing on my wall


CRISPR contribution

Many studies have already been conducted to find genes associated with wing pattern formation – but that’s just it, association. These include genetic association and mapping studies which try to link phenotype (in this case wing pattern) to genotypes (the genes). Two genes, WntA and optix, were repeatedly found in these studies that they became known as ‘adaptive hotspots’, meaning the genes are linked to physical changes that make the butterflies adapted to their environment. However, their exact functions remained unclear.

With CRISPR, scientists are now able to knock out the genes and examine the effect of their absence.

Colour by number

WntA, a signalling ligand, was mutated in seven different butterfly species disrupting its function. This led to wing patterns with faded marks and patterns and colours bleeding from their wild type positions (2). This strongly supports WntA’s contribution to setting the borders and boundaries of the wing for subsequent pigmentation. One of the most significant findings was the observation of lineage-specific mutant phenotypes, demonstrating that repeated modification of WntA in these different lineages has allowed for the complex variety of wing patterns to evolve. This supports the ‘tinkering’ model of evolution.

When optix was knocked out in four species the colour pigments were replaced with melanins, turning part or all of the wings black or grey. Hmmm, I wonder what optix is responsible for? Interestingly, in the Common Buckeye (for those who know their butterflies), the wing developed bright iridescent spots. Since iridescence is the effect when a colour changes when looked at different directions it suggests optix is also responsible for regulating microscopic structural changes of wing scales (3).

An outlook

There are many other evolutionary questions that can now be fuelled by the aid of CRISPR such as how a turtle got its shell, how fins changed to feet and then the environmental pressures selecting them. A good summary of the aid of CRISPR to this evo-devo (evolutionary development) discipline can be read here (http://www.nature.com/news/crispr-s-hopeful-monsters-gene-editing-storms-evo-devo-labs-1.20449)

Further Reading

(1)  http://www.nature.com/news/crispr-reveals-genetic-master-switches-behind-butterfly-wing-patterns-1.22628

(2)  Mazo-Vargas, A. et al. Macroevolutionary shifts of WntA function potentiate butterfly wing-pattern diversity Proc. Natl Acad Sci USA (2017)

(3)  Zhang, L, Mazo-Vargas, A & Reed, R.D Single master regulatory gene coordinates the evolution and development of butterfly colour and iridescence Proc. Natl Acad Sci USA (2017)

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