Scientists at the Wellcome Sanger Institute and the Institut de Biologia Evolutiva in Spain have sequenced the genome of a female Atlas blue butterfly (Polyommatus atlantica) and found that it has 227 pairs of autosomes and four sex chromosomes, the highest number of chromosomes out of all multicellular animals in the world.
The Atlas blue butterfly (Polyommatus atlantica). Image credit: Roger Vila.
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 (Polyommatus icarus), 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.
“Genomes hold the key to how a creature came to be, but also, where it might go in the future,” said Professor Mark Blaxter, a researcher at the Wellcome Sanger Institute.
“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.”
In the study, Professor Blaxter and colleagues found that the chromosomes had been spilt 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.
“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,” said Dr. Roger Vila, a researcher at the Institut de Biologia Evolutiva in Spain.
“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.”
“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,” said Dr. Charlotte Wright, a researcher at the Wellcome Sanger Institute.
“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.”
The results were published this week in the journal Current Biology.
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Charlotte J. Wright et al. Constraints on chromosome evolution revealed by the 229 chromosome pairs of the Atlas blue butterfly. Current Biology, published online September 10, 2025; doi: 10.1016/j.cub.2025.08.032
