More than a century ago, a bluish butterfly flitted among the sand dunes of the Sunset District in San Francisco and laid its eggs on a plant known as deerweed. As the city’s development overtook the dunes and deerweed, the butterflies vanished, too. The last Xerces blue butterfly was collected in 1941 from Lobos Creek by an entomologist who would later lament that he had killed what was one of the last living members of the species.
But was this butterfly truly a unique species?
Scientists could all agree the grim fate of the Xerces blue — the first butterfly known to go extinct in North America because of human activities — was a loss for biodiversity. But they were divided over whether Xerces was its own distinct species, a subspecies of the widespread silvery blue butterfly Glaucopsyche lygdamus, or even just an isolated population of silvery blues.
This may seem a scientific quibble, but if Xerces blue was not in fact a genetically distinct lineage, it would not technically be truly extinct.
Now, researchers have sequenced a near-complete mitochondrial genome of a 93-year-old museum specimen, which suggests the Xerces blue was a distinct species, which they say could be properly named Glaucopsyche xerces, according to a paper published Wednesday in Biology Letters.
“It goes to show how critically important it is not only to collect specimens but to safeguard them,” said Corrie Moreau, the director and curator of the Cornell University insect collection and an author on the paper. “We can’t imagine the ways they will be used 100 years from now.”
Durrell Kapan, a senior research fellow at the California Academy of Sciences who was not involved with the research, said he found the new findings “suggestive and very exciting,” but added that there could be limits to this kind of research because “what makes two organisms different species isn’t always directly addressable with genetic information.”
Dr. Kapan is working on a separate genomic project on Xerces blue butterflies and close relatives with Revive & Restore, a nonprofit initiative to restore extinct and endangered species through genetic engineering and biotechnology.
The researchers started working on the project several years ago, when Dr. Moreau was at the Field Museum in Chicago. She and Felix Grewe, now the director of the phylogenomics initiative of the Grainger Bioinformatics Center at the museum, sifted through museum archives of Xerces blue butterflies to find the least damaged specimen, which would theoretically produce the best-preserved DNA.
“You’re grinding up a piece of an extinct butterfly,” Dr. Moreau said. “You only get one chance.”
Dr. Moreau removed a third of the butterfly’s abdomen, a body part loaded with muscle, fat and other tissues, and sequenced it. DNA this old degrades into short fragments. Historically, researchers would sequence long, uninterrupted stretches of DNA by chopping it up and puzzling it back together. But new sequencing technology allows researchers to work with already-chopped, fragmented DNA. “We just leave that step out,” Dr. Grewe said.
After recovering their sequences, the researchers examined publicly available data of other related butterfly specimens.
Their mitochondrial DNA sequences did not appear similar. They suggested that the Xerces blue was a distinct species and that two other butterflies traditionally believed to be subspecies of the silvery blue butterfly — the australis and pseudoxerces clades — may also be distinct species, and the closest living relatives of the Xerces blue.
These results are surprising, as those two butterflies are found in Southern California, a long way from the Xerces blue’s original home on the San Francisco Peninsula.
The new paper’s sequencing focused on the CO1 bar coding mitochondrial gene. Mitochondrial DNA is an excellent option for older museum specimens because a single cell contains many more copies of the mitochondrial genome than the nuclear genome, the researchers said. Mitochondrial DNA is inherited from the mother, while nuclear DNA is inherited from both parents.
But the CO1 gene represents a “very small sample of the genome,” Dr. Kapan said, adding that he did not think the new paper definitely settled the species debate.
At the California Academy of Sciences, Athena Lam, a genomics researcher, Dr. Kapan and others want to illuminate where Xerces falls on the evolutionary scale, Dr. Lam said.
These kinds of genomic studies, Dr. Kapan said, could reveal where to find populations of surviving species in the Glaucopsyche genus that may be well suited for potential reintroduction to San Francisco’s sand dunes. According to the new paper, good candidates to investigate would be australis or pseudoxerces, the latter of which has wings that recall Xerces’ brilliant blue hue.
Dr. Moreau said she hoped the new study shined a light on blue butterflies that are currently endangered, such as the El Segundo blue, which lives in coastal sand dunes in Southern California, and the Karner blue, which is found most commonly in Wisconsin where wild lupine grows.
And though the Xerces blue is long gone, the deerweed it once needed has recently been replanted in the sand dunes in the Presidio, awaiting a somewhat familiar future butterfly.