The Atlas blue butterfly, with a record-breaking 229 pairs of chromosomes, is helping scientists unravel mysteries of evolution, adaptation, and even human cancer.
The Atlas blue butterfly, scientifically named Polyommatus atlantica, has been confirmed as the multicellular animal with the greatest number of chromosomes ever recorded.
This butterfly carries 229 pairs of chromosomes, a striking contrast to many of its close relatives that typically have only 23 or 24 pairs. Scientists at the Wellcome Sanger Institute and the Institute of Evolutionary Biology (IBE: CSIC-UPF), Barcelona, discovered that the increase resulted from chromosomes splitting into smaller sections over time, rather than being duplicated.
Details of the first genomic analysis of this species were published in Current Biology. The findings open the door to exploring why such an unusually high chromosome count evolved. Because chromosome alterations are also seen in human cancer cells, studying how this process occurs in other species may provide valuable insights for medical research.
This marks the first sequencing of the Atlas blue butterfly’s genome. The project has produced a high-quality reference genome that researchers can use to compare with other butterflies and moths, helping to uncover how species diversify and evolve over millions of years.
Evolution and the development of new species happen over millions of years, making it hard to study practically. Instead, experts can use the DNA of a species and compare this to others in the same family to understand which genes and traits have been kept and which have been lost and then make informed guesses as to why.
Insights Into Adaptation and Conservation
Having the genetic story of a species also allows us to understand how the next chapter might go. For example, we might be able to understand how a species could respond to the increasing global temperature and if they have any genes or mechanisms that might protect them. This could inform conservation efforts as well as research into producing more resilient crops.
The Atlas blue butterfly is found in the mountain ranges of Morocco and northeast Algeria. While it had been suspected to have the most chromosome pairs in the animal kingdom, this is the first time experts have sequenced the butterfly genome to confirm. For comparison, a close relative found widely in the UK, the Common blue butterfly, has 24 chromosomes.
Changes in chromosome numbers are thought to contribute to the process of new species forming and help species adapt to their environment. The group to which the Atlas blue butterfly belongs contains many closely related species that evolved over a short period of time.
In this new research, the team found that the chromosomes had been split up at points where the DNA is less tightly wound. This means there was roughly the same amount of genetic information, but it was packaged in smaller sections. All of the chromosomes, apart from the sex chromosomes, were cut up, and the researchers estimate that this caused the chromosome number to go from 24 to 229 over roughly three million years — a relatively short amount of time by evolutionary standards.
Usually, it is assumed that this kind of extreme chromosomal change is negative; however, the Atlas blue butterfly has evolved and survived for millions of years. It is only now, due to climate change and human impacts on the environment, such as the destruction of cedar forests and overgrazing, that its populations are under threat.
This research raises multiple questions that can now be addressed in the future. Splitting up the chromosomes could help give greater genetic diversity by allowing more frequent shuffling of genome parts or give other unknown benefits. While this may help butterflies to rapidly adapt, species with many chromosomes may also face challenges due to the extra complexity of this, potentially making them more vulnerable to extinction over time. Further investigations and comparisons with other butterflies could highlight whether any genes have been lost or preserved, giving us more information on the biology of the butterflies, but also a deeper understanding of evolution.
Links to Human Health
Chromosomal rearrangements also happen in human cancers, and therefore, studying these processes in the Atlas blue butterfly DNA could lead to new developments in human health and highlight possible ways to reduce or stop this phenomenon in cancer cells.
Dr. Roger Vila, senior author at the Institute of Evolutionary Biology, said: “Breaking down chromosomes has been seen in other species of butterflies, but not on this level, suggesting that there are important reasons for this process which we can now start to explore. Additionally, as chromosomes hold all the secrets of a species, investigating whether these changes impact a butterfly’s behavior could help form a full picture of how and why new species occur.”
Dr. Charlotte Wright, first author at the Wellcome Sanger Institute, said: “When we set out to start to understand evolution in butterflies, we knew we had to sequence the most extreme, and somewhat mysterious, Atlas blue butterfly. Thanks to Roger Vila, who had previously worked with his colleague to find and identify this elusive butterfly, we were able to sequence this species, highlighting the collaborative nature of science. Being able to see, in detail, how the Atlas blue butterfly chromosomes have been split over time in specific places, we can start to investigate what benefits this might have, how it impacts their ability to adapt to their environment, and whether there are any lessons we can learn from their DNA that might aid conservation in the future.”
Professor Mark Blaxter, senior author at the Wellcome Sanger Institute, said: “Genomes hold the key to how a creature came to be, but also, where it might go in the future. To be able to tell the story of our planet, we must have the story of each species and see where they overlap and interact with each other. It also allows us to apply learnings from one genome to another. For example, rearranging chromosomes is also seen in human cancer cells, and understanding this process in the Atlas blue butterfly could help find ways to limit or stop this in cancer cells in the future.”
Reference: “Constraints on chromosome evolution revealed by the 229 chromosome pairs of the Atlas blue butterfly” by Charlotte J. Wright, Dominic Absolon, Martin Gascoigne-Pees, Roger Vila, Mara K.N. Lawniczak and Mark Blaxter, 10 September 2025, Current Biology.
DOI: 10.1016/j.cub.2025.08.032
Funding: Wellcome Trust
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