Near the early 1940s, the Xerces blue butterfly was last seen fluttering its iridescent periwinkle wings in San Francisco. It's widely assumed to be extinct, the first American insect species to be wiped off by urbanization. However, there are some doubts over whether it was ever a separate species or perhaps a sub-population of another popular butterfly.
Studying 93-Year-Old DNA
died the DNA of a 93-year-old Xerces blue specimen in museum collections for a new study published in Biology Letters and discovered that its DNA is distinct enough to be declared a species. Furthermore, the study indicates that the Xerces blue did indeed become extinct and that insect conservation is an issue that must be addressed.
"It's fascinating to confirm what people have suspected for over a century: that this was a species pushed to extinction by human activities," says Felix Grewe, co-director of the Field's Grainger Bioinformatics Center and lead author of the project's Biology Letters publication.
"There had been a long-standing question about whether the Xerces blue butterfly was truly a distinct species or just a population of a prevalent 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, who started working on the study as a researcher at Chicago's Field Museum.
"Many of the same characteristics may be seen in the widely distributed silvery-blue species. We chose to investigate whether we could ultimately address this riddle since we have many specimens in the Field Museum's collections, as well as the Pritzker DNA lab and the Grainger Bioinformatics Center, which have the potential to sequence and analyze large amounts of DNA."
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Butterfly Specimens and Data
Moreau and her colleagues examined pinned butterfly specimens housed in drawers in the Field's insect collections to verify whether the Xerces blue was a distinct species. First, she pinched a small portion of the abdomen of a butterfly captured in 1928 using forceps. "It was nerve-wracking because you want to protect as much of it as you can."
"Taking the first steps and removing a portion of the abdomen was extremely stressful, but it was also thrilling to think that we might be able to answer a question that has been unanswered for almost a century and cannot be addressed any other way."
After retrieving a section of the butterfly's body, the sample was sent to the Pritzker DNA Laboratory at the Field Museum, where the tissues were processed with chemicals to extract the residual DNA. "DNA is a highly durable molecule; it may survive for a long period after the cells in which it is stored have died," Grewe explains.
Integrity and Stability of DNA
Even though DNA is a stable molecule, it degrades with time. Every cell, however, has DNA, and scientists may reconstruct the original form by comparing several strands of DNA code. "It's as if you built a lot of similar Lego towers and then dumped them. Individual buildings would be shattered, but if you looked at them all together, you could see the original structure's shape, "Moreau agrees.
Grewe, Moreau, and their colleagues compared the genetic sequence of the Xerces blue butterfly to that of the more common silvery blue butterfly and discovered that the Xerces blue's DNA was distinct, indicating that it was a specific species.
The conclusions of the study have far-reaching consequences. "Because it is the first insect we know of in North America that people drove to extinction, the Xerces blue butterfly is the most emblematic bug for conservation. It is the name of an insect conservation society."
Moreau agrees. "It's awful that we pushed something to extinction, but at the same time, what we're saying is that what we assumed is supported by DNA evidence. If we had discovered that the Xerces blue was not truly extinct, it might have jeopardized conservation efforts."
Extinct Species
Extinct species DNA sequencing occasionally raises the possibility of resurrecting the species, à la Jurassic Park, but Grewe and Moreau argue in their study that those efforts would be better spent conserving species that currently exist. "Before we start putting a lot of work towards resurrection, let's put that energy into conserving what we already have and learning from our mistakes," Grewe adds.
Grewe claims that the initiative highlights the value of museum collections, in addition to the study's conservation implications. "Nobody thought of sequencing the DNA of this butterfly when it was collected 93 years ago. That is why we must continue to collect data for scholars in the next 100 years."
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