In the study, the researchers collected and sequenced the genomes of 48 individual butterflies from locations across southeast Asia, from Thailand to the Spice Islands in Indonesia; some were from sexually dimorphic populations, while others were from sexually monomorphic populations.
To their surprise, genetic analysis revealed that rather than splitting neatly into two groups, sexual dimorphism arose repeatedly in different populations. “This is a really neat case of never-before-seen repeated traits; this sexual dimorphism has appeared repeatedly in these butterflies,” said first author Dee Ruttenberg, at the time an undergraduate student in the College, now a graduate student at Princeton University. “What’s more, when we look at these individual groups of sexually dimorphic butterflies, we found that one gene popped up repeatedly, and predicted if a butterfly would be monomorphic or dimorphic almost perfectly — that’s WntA.”
Important in a wide range of similar butterfly species and associated with melanin localization and regulation, the research team called WntA a “smoking gun” — the gene is well known for being involved in pigmentation in butterflies, and now appears to be regulating the transition between sexual dimorphism and monomorphism. The benefit is that individual populations of the butterfly can rapidly and easily “flip the switch” based on local pressures.
“We identified two key base pairs in a region of DNA that seems to be associated with regulating the activity of WntA,” said co-senior author David Lohman, an associate professor of biology at City Colleges of New York. “It seems like there are a fairly small number of genetic changes that contribute to this difference in female form, and it’s likely that this represents an example of gene reuse. The genetic machinery to make an orange wing or black wing didn’t evolve de novo but rather those sets of genes are turned on and off depending on the population. It really is like a mimicry switch.”
Because this butterfly is relatively understudied, the research is the first to clarify the distribution of this trait and the underlying genetics, but what is less clear is why. “We can go into the field and document these population patterns and then go in and find the genetic mechanism, but what we don’t know is, why is it that some populations have orange females and others don’t?” said Lohman. “Is it because the orange species that the females might mimic are rare or absent? Or that they’re not particularly toxic, so mimicking an orange species wouldn’t be very beneficial to the females?”
The other major remaining question is how the WntA gene is regulated in a sexually dimorphic way. “How is it that this gene that, up to this point, we have had no reason to think might be associated with sexual dimorphism, is now being regulated in that way?” said Ruttenberg. “Is there a pathway by which sexually dimorphic expression could be created in any gene?”
In future research, the investigators hope to dig more into how exactly WntA regulates the monomorphic and dimorphic color patterns.
Additional study authors include Nicholas W. VanKuren and Sumitha Nallu of UChicago, Shen-Horn Yen of National Sun Yat-Sen University, and Djunijanti Peggie of the Indonesian Institute of Sciences (LIPI).
Citation: “The evolution and genetics of sexually dimorphic ‘dual’ mimicry in the butterfly Elymnias hypermnestra.” Ruttenberg et al, Proceedings of the Royal Academy B, Jan. 13, 2021. DOI: 10.1098/rspb.2020.2192
Funding: National Geographic Society Fieldwork, National Science Foundation, National Institutes of Health, MOST.
—Adapted from an article first published by the University of Chicago Medicine.