A single gene is responsible for regulating the copy-cat pattern on swallowtail butterfly's wings. The described gene, dubbed "doublesex," is already known to play a role in insect sexual differentiation.
"Conventional wisdom says that it should be multiple genes working together to control the whole wing pattern of a butterfly," Marcus Kronforst, Neubauer Family Assistant Professor of Ecology & Evolution at the University of Chicago and senior author of the study, said in a University of Chicago Medical Center news release. "But in this case, it's just this one. This single gene that controls sexual differentiation has been co-opted to do a totally new job."
The butterflies mimic other toxic species in order to trick predators into thinking they are poisonous. Researchers pinpointed the region of the genome that was responsible for the mimicry process in swallowtail butterflies.
Researchers assumed the region contained a "supergene," which is "multiple tightly-linked genes, each controlling a subset of the wing pattern," the news release reported.
The researchers looked at the Papilio polytes, an Asian swallowtail that employs sex-limited mimicry. The researchers looked at the genomes of about 500 offspring of butterflies with various wing patterns; this allowed them to identify five genes involved in mimicry. They then sequenced the genomes of 30 butterflies, split between mimetic and non-mimetic, to scout out correlations between wing patterns.
The team was only able to find a correlation in one doublesex, which "functions through alternative splicing," the news release reported.
"When copied into messenger RNA, it is cut and rearranged into different isoforms, which then go on to instruct cells whether they should be male or female," the news release reported.
By tracking these butterflies throughout every stage of their lives the researchers were able to determine that the expression of doublesex overlapped with the wing pattern.
"When you look at the wing tissue in a chrysalis five days after it forms the pupa, it's just a floppy piece of white tissue," Kronforst said. "But when you look at where doublesex is being manufactured on the wing, it looks just like the future adult wing pattern."
The researchers are still unsure of how one gene controls so many functions. Kronforst believes regulatory DNA that controls where the gene is expressed could be a factor.
"We've illustrated the genetic basis of female-limited mimicry in these butterflies," Wei Zhang, PhD, postdoctoral fellow at the University of Chicago and a lead study author, said in the news release. "But this is just the first step. How doublesex became involved in this process is still uncertain, and requires further study."
