Butterfly wing patterns emerge from ancient ‘junk’ DNA

Butterfly wing patterns have a basic blueprint, which is manipulated by non-coding regulatory DNA to create the diversity of wings seen in different species, according to new research.

The study, “Deep Cis-Regulatory Homologation of the Basis Plane of Butterfly Wing Pattern,” published as a cover story in the October 21 issue of Sciencesexplains how the DNA between genes, called “junk” DNA or noncoding regulatory DNA, conforms to a basic blueprint preserved for tens to hundreds of millions of years while allowing wing patterns to evolve extremely fast.

The research supports the idea that an ancient color pattern is already encoded in the genome and that non-coding regulatory DNA works like switches to turn on some patterns and turn off others.

“We’re interested in how the same gene can generate these very different-looking butterflies,” said Anyi Mazo-Vargas, Ph.D. ’20, the study’s first author and former graduate student in the lab of lead author Robert Reed, a professor of ecology and evolutionary biology in the College of Agriculture and Life Sciences. Mazo-Vargas is currently a postdoctoral researcher at George Washington University.

“We see that there is a highly conserved group of switches [non-coding DNA] that are working in different positions and are activated and driving the gene,” Mazo-Vargas said.

Previous work in Reed’s lab has uncovered key color pattern genes: one (WntA) that controls striping and another (Optix) that controls color and iridescence in butterfly wings. When the researchers turned off the Optix gene, the wings appeared black, and when the WntA gene was removed, the striped patterns disappeared.

This study focused on the effect of non-coding DNA on the WntA gene. Specifically, the researchers conducted experiments with 46 of these noncoding elements in five species of nymphalid butterflies, which is the largest family of butterflies.

For these noncoding regulatory elements to control genes, tightly coiled DNA coils unwind, a signal that a regulatory element is interacting with a gene to turn it on or, in some cases, turn it off.

In the study, the researchers used a technology called ATAC-seq to identify the regions of the genome where this unraveling occurs. Mazo-Vargas compared the ATAC-seq profiles of the wings of five butterfly species to identify the genetic regions involved in wing pattern development. They were surprised to find that a large number of regulatory regions were shared between very different butterfly species.

Mazo-Vargas and colleagues then employed CRISPR-Cas gene-editing technology to disable 46 regulatory elements one at a time, to see the effects on wing patterns when each of these non-coding DNA sequences was broken. When removed, each noncoding element changed one aspect of the butterflies’ wing patterns.

The researchers found that in four of the species, Junonia coenia (chestnut chestnut), Vanessa cardui (painted lady), Heliconius himera, and Agraulis vanillae (Gulf fritillary), each of these non-coding elements had similar functions with respect to the WntA. gene, showing that they were ancient and conserved, probably originating from a distant common ancestor.

They also found that D. plexippus (monarch) used different regulatory elements from the other four species to control its WntA gene, perhaps because it lost some of its genetic information throughout its history and had to reinvent its own regulatory system to develop its color. unique. patterns.

“We have progressively come to understand that most evolution occurs because of mutations in these non-coding regions,” Reed said. “What I hope is that this paper is a case study showing how people can use this combination of ATAC-seq and CRISPR to start interrogating these interesting regions in their own study systems, whether they’re working with birds, flies or worms.”

The study was funded by the National Science Foundation (NSF).

“This research is a major advance in our understanding of the genetic control of complex traits, and not just in butterflies,” said Theodore Morgan, NSF program manager. “The study not only showed how instructions for butterfly color patterns are deeply conserved throughout evolutionary history, but also revealed new evidence for how regulatory DNA segments positively and negatively influence traits such as color. and the shape.”

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Materials provided by Cornell University. Original written by Krishna Ramanujan, courtesy of Cornell Chronicle. Note: content can be edited for style and length.

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