Biodiversity is a wonderful interplay between genetics and evolution, and butterflies are a fascinating example with their variety of patterns and colors. Understanding how the same gene networks engender visual effects so diverse in thousands of Lepidoptera species is a longtime ambition for many entomologists and evolutionary biologists. The good news is that scientists nowadays have a straightforward technique working with organisms that were difficult to manipulate with conventional biotech tools. Obviously, we are talking about CRISPR. Two papers published in PNAS last week describe how genome editing was used to alter the genetic palette of colors in butterflies and how their wings changed as a result. We’ve asked the entomologist Alessio Vovlas, from the Polyxena association, to comment these stunning experiments.
Robert Reed of Cornell University and his colleagues silenced two genes, proving that they play a key role in designing the insects outfits. WntA defines the outline of the sketch while optix fills in the colors. Is it the beginning of a new season of research in entomology?
The specimens CRISPR-edited are from 7 species belonging to a family called Nymphalidae because they inhabit woods. Besides including further Lepidoptera, it would be interesting to extend the experiment to Odonata. Indeed the pigment acted upon by the optix gene (ommochrome) gives male and female dragonflies different colors. Coccinellidae would also be worth investigating, to unveil the genetic mechanism governing the number of spots on the back of ladybugs. CRISPR may contribute to these discoveries which are fascinating not only for aesthetic reasons but also for the relevance of the biological functions involved. Applications in entomology may be numerous.
Why are patterned and multicolored wings useful?
Shape and colors have two core functions. They allow recognition of individuals of the same species for mating purpose, as well as the use of mimicry to hide from predators. Some species mimic features of the nourishing plant leaves (Gonepteryx cleopatra), or bark. Polyommatus icarus males are blue while females are brown to camouflage themselves on the soil during egg laying. Aposematism is a kind of warning coloration displayed by Machaon and other yellow and black butterflies. It is no coincidence that humans use the same colors in warning signals. At the caterpillar stage these species seize toxic alkaloids from the plants they feed on. Then predators learn to recognize bad-tasting specimens from their colors.
Turn the WntA gene off and the wing pattern changes. Turn optix off and most butterflies darken, getting gray and black. However, the common buckeye (Junonia coenia) develops spots of iridescent blue. What is the effect that amazes you the most?
Butterfly wings are covered with scales, which can be pigmented or have microscopic structural features producing light refraction phenomena. I’m not so surprised that iridescence may be affected by altering pigments. More broadly, however, the specimens becoming melanic after genome editing remind me of the cult of aberrations that was once widespread among collectors. I wonder if the same mutations intentionally induced by CRISPR occurred spontaneously in specimens appreciated for their rarity at butterfly exhibitions and trade fairs.
(Photo: Heliconius sara, the right half of the picture is from an edited specimen; credit George Washington University)