The genome of the Xerces Blue butterfly, icon of human-caused extinction, has been sequenced

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The wings of the Xerces Blue butterfly (Glaucopsyche xerces) were of an intense iridescent blue color, contrasting with the characteristic white spots on its ventral area (abdomen). The last specimens of this species, endemic to the coastal dunes around the city of San Francisco (California), were found in 1941 and its disappearance made it a global icon of human-caused extinctions. This week a team led by researchers from the Institute of Evolutionary Biology (IBE-CSIC-UPF) and the Museum of Natural Sciences of Barcelona details in the scientific journal eLife the genome sequencing of four specimens of this butterfly, a work that allows us to better understand the events that led to its extinction and that opens the door to the possible recovery of the species.

In addition to Xerces Blue, the authors have also sequenced the genomes of seven other Silvery Blue specimens (Glaucopsyche lygdamus), its closest relative in evolutionary terms, who lived between 80 and 100 years ago, from the Smithsonian Institution in Washington. So, the researchers have been able to recompose the evolutionary path of both species -which diverged between one and two million years ago- and confirm that they are two different evolutionary lineages.

Comparison of the genomes of both species has further revealed that the DNA of the xerces blue butterfly contained a high incidence of inbreeding, a sign of population decline, a finding that could be used to identify other insect species threatened by human activity. The extinction patterns of these animals, contrary to what happens with vertebrates, are not well understood today, despite the fact that the disappearance of insects, especially pollinators, is a serious ecological problem.

“There are many endangered insects whose situation goes unnoticed because it is extremely difficult to census their populations, which in general seem very abundant to us”, explains Roger Vila, researcher at the IBE (CSIC-UPF) and lead author of the research. “However, they can be very sensitive to climate variations and human action, for example, to pesticides. That’s why we believe that genomic traits signaling population decline of the Xerces blue butterfly can serve as an early warning and help detect vulnerable insects in future studies.”

The results of the research indicate that the Xerces Blue butterfly experienced a large population decline over tens of thousands of years, probably caused by climate changes (which did not affect Silvery Blue in the same way). The authors explain having detected the characteristics of a small population in the genome, traits that include low genetic diversity, long chromosomal fragments without genetic variation, and a high frequency of deleterious alleles (which impair the viability of individuals and make the species into vulnerable).

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