What is it about?

Butterflies often use colors to recognize potential mates. In the case of sulphur butterflies, the most important color for mating is ultraviolet and thus invisible to us, but it distinguishes the males of two species of sulphur butterflies that inhabit the same fields of alfalfa across North America. We found that while these two species hybridize extensively, their sex chromosome remain genetically differentiated, keeping them distinct. This include a difference at a gene called bric-a-brac that determines if the expression of bric-a-brac (bab) is on or off in the wing. Using CRISPR genome editing, we demonstrated that bab acts a repressor of ultraviolet coloration : when the gene is removed, butterflies that are normally not iridescent became brightly iridescent.

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Why is it important?

This work uncovers key aspects about the degree of hybridization between two butterfly species that were once geographically separated, but are now reunited. In this case, sex chromosome act as a barrier to genetic exchange and maintain some degree of isolation between the two species. Ultraviolet signals are a key aspect to it, and our work illustrate how repressor genes can be used in the genome to set-up important on and off switches that impact evolution of sexual traits in animals.


Anthropogenic change is often used as a way to catch evolution in action, as in peppered moths that adapted to sooty bark during the Industrial Revolution. Here we harnessed this principle to catch a time-frozen glimpse of speciation where previously diverged species reunited, due to a massive range expansion that trailed with alfalfa agriculture. Using this scheme, we identified a gene that is a key regulator of a color signal involved in sexual communication and acts as a switch off for the male trait – the butterflies have evolved ways to change where Bab is expressed, and where it is expressed, ultraviolet colors are removed. This means that ultraviolet evolved as a sort of color by default, and the butterfly genomes is wired with erasers that remove it from places where it is not wanted.

Arnaud Martin
The George Washington University

Read the Original

This page is a summary of: A genetic switch for male UV iridescence in an incipient species pair of sulphur butterflies, Proceedings of the National Academy of Sciences, January 2022, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2109255118.
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