Sequencing the genome of a butterfly whose extinction was caused by humans

O blue xerces butterfly (xerces glaucopsyche) was native to the coastal dunes of San Francisco in the United States. As the city grew, much of its habitat was destroyed and its population was relegated to Golden Gate National Park. Its wings were a deep iridescent blue with characteristic white spots on the underside.

The last specimens of its species were found in 1941 by the entomologist W. Harry Lange. It is considered the first extinct insect species in historic times. His disappearance made him a global icon of human-induced extinction, to the point of giving his name to a famous North American conservation association, the Xerces Society.

Now, a team led by researchers from the Institute of Evolutionary Biology (IBE-CSIC-UPF) and the Museum of Natural Sciences in Barcelona has managed to sequence the genome of four of these butterflies. Also the seven specimens of Silvery Blue (Glaucopsyche lygdamus) between 80 and 100 years old, from the Smithsonian Institution in Washington. The results are published in the journal eLife.

A previous 2022 study retrieved mitochondrial DNA from a Xerces Blue butterfly specimen and compared it to that of the Silvery Blue, its closest living relative, confirming that they were indeed different species and not just different populations. The researchers were able to infer that the two species diverged between one and two million years ago and that they constituted two distinct evolutionary lineages.

Genome analysis of both species reveals that the DNA of the Xerces Blue butterfly contained a high incidence of inbreedinga sign of population decline that could be used to identify other insect species threatened by human activity and whose extinction patterns, unlike vertebrates, are still little known.

Furthermore, it could also pave the way for extinction (set of techniques that make it possible to engender a specimen or even revive a complete extinct and already disappeared species) of this popular butterfly, a project that has aroused the interest of the scientific community for years.

Low genetic diversity

Research results indicate that the Xerces Blue butterfly experienced a large population decline over tens of thousands of years, likely caused by changes in climatic factors that did not affect the Silvery Blue. O Destruction of their habitat by humans, however, would be the trigger for their final extinction.

The study thus concludes after detecting in its genome the characteristics of a small population, among which are included a low genetic diversitylong chromosome fragments without genetic variation and a high frequency of deleterious allelesthat undermined the viability of individuals and turned it into what we would now call vulnerable species.

“Detecting species of endangered mammals is easier, because in many cases it is possible to count the individuals”, he explains. Roger Villaresearcher at IBE (CSIC-UPF) and co-leader of this research.

“There are many insects in danger of extinction, however, whose situation goes unnoticed because it is extremely difficult to carry out a census of their populations, which generally seem to us to be very abundant. However, they can be very sensitive to climate variations and human action, for example, pesticides“, To add.

“For this reason, we believe that the genomic characteristics that indicate the reduction in the population of the Xerces Blue butterfly can, currently, serve as an alert and help to detect vulnerable insects in future studies”, highlights Vila.

Image of a male and female Xerces Blue on a branch of the plant it was feeding on: Acmispon glaber or deer. / Martí Franch

First step towards de-extinction

The disappearance of insects, mainly pollinators, is a very serious ecological problem all over the world. Therefore, the extinction of species such as the Xerces Blue butterfly, for genetic engineering techniques based on CRISPR, is of great interest to the scientific community.

“The Xerces Blue butterfly is an excellent candidate for de-extinction because it is an insect that disappeared relatively recently, which would reduce the ecological impact of its reappearance, and does not imply risk of pests or overgrowth due to the limited time it appears in adults (between March and April) and for its ecological expertise. That’s why we hope that having its complete genome can help to extinguish it in future initiatives”, says the other expert of the study, Carles Lalueza-Foxresearcher at the IBE (CSIC-UPF) and director of the Museum of Natural Sciences in Barcelona.

The Xerces Blue butterfly is an icon of insect extinction around the world. According to the researchers, sequencing its genome could help prevent the extinction of other endangered insects, whose sharp demographic declines are not evident.