DNA solves riddle of extinct Xerces butterfly

Near-century-old butterfly DNA  has confirmed that the Xerces blue butterfly, famously wiped out by humans in the 1940s, was a distinct species, according to a study published in Biology Letters.

A team of researchers, led by Felix Grewe of the Chicago Field Museum’s Grainger Bioinformatics Center, US, analysed the DNA of a 93-year-old museum specimen of the butterfly, which was last seen in San Francisco in the 1940s. They found that it was a distinct species that became extinct because of humans, confirming a century-old hypothesis.

“There was a long-standing question as to whether the Xerces blue butterfly was truly a distinct species or just a population of a very widespread species called the silvery blue that’s found across the entire west coast of North America,” says Corrie Moreau, director of the Cornell University Insect Collections, US, who began work on the study as a researcher at the Field Museum.

“The widespread silvery blue species has a lot of the same traits. But we have multiple specimens in the Field Museum’s collections, and we have the Pritzker DNA Lab and the Grainger Bioinformatics Center that has the capacity to sequence and analyse lots of DNA, so we decided to see if we could finally solve this question.”

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To extract the tissue used for DNA extraction, the team used forceps to pinch off a tiny amount of abdomen from butterfly specimens that were collected in 1928.

“It was nerve-wracking, because you want to protect as much of it as you can,” recalls Moreau. “Taking the first steps and pulling off part of the abdomen was very stressful, but it was also kind of exhilarating to know that we might be able to address a question that has been unanswered for almost 100 years that can’t be answered any other way.”

The abdomen tissue was then sent to the Pritzker DNA Laboratory and DNA was isolated using traditional extraction methods.

“DNA is a very stable molecule; it can last a long time after the cells it’s stored in have died,” says Grewe.

“When this butterfly was collected 93 years ago, nobody was thinking about sequencing its DNA. That’s why we have to keep collecting, for researchers 100 years in the future.”

Regardless, DNA does degrade over time, so the team had to use multiple pieces from different cells, stringing them together like a puzzle to get the whole picture.

“It’s like if you made a bunch of identical structures out of Legos, and then dropped them,” says Moreau. “The individual structures would be broken, but if you looked at all of them together, you could figure out the shape of the original structure.”. The DNA evidence confirmed the Xerxes butterfly was a distinct species.

“It’s really terrible that we drove something to extinction, but at the same time what we’re saying is, ‘okay, everything we thought does in fact align with the DNA evidence,” says Moreau.

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