Researchers from the Wellcome Sanger Institute and the Spanish Institute of Biology have mapped the female genome of the Atlas Blue Butterfly (Polyommatus atlantica), revealing 227 pairs of autosomes and four sex chromosomes, marking it as the organism with the highest chromosome count among all multicellular animals globally.

The Atlas Blue Butterfly is native to the mountainous regions of Morocco and Northeast Algeria.

Previously suspected to have the highest chromosome count in the Animal Kingdom, this is the first instance where scientists have successfully sequenced the butterfly’s genome to confirm this assumption.

In comparison, the more commonly observed Common Blue Butterfly (Polyommatus icarus) has only 24 chromosomes.

Variations in chromosome numbers are believed to facilitate the formation of new species and assist in adaptation to changing environments.

The Atlas Blue Butterfly belongs to a group of closely related species that have evolved rapidly over a short geological timeframe.

“The genome is crucial for understanding how organisms develop and what the future may hold,” stated Professor Mark Blaxter from the Wellcome Sanger Institute.

“To narrate the stories of our planet, we must explore various tales and observe their interactions.”

“Insights gained from one genome can also enrich our understanding of others.”

“For instance, chromosomal rearrangements are also present in human cancer cells, and investigating these patterns in the Atlas Blue Butterfly could lead to methods for mitigating cancer cell growth in the future.”

In their research, Professor Blaxter and his team discovered that chromosomal structure was altered due to less tightly packed DNA.

This indicates that while the amount of genetic information remained similar, it was organized into smaller segments.

Except for the sex chromosomes, all chromosomes were found to be fragmented, leading researchers to estimate a dynamic range of 24 to 229 chromosomes emerging over approximately 3 million years, a brief period in evolutionary terms.

Generally, such drastic chromosomal modifications are considered detrimental; however, the Atlas Blue Butterfly has thrived for millions of years.

Its population faces threats primarily from climate change and human environmental impact.

This study opens numerous avenues for future exploration.

Chromosomal division is thought to enhance genetic diversity by allowing for increased genomic mixing or possibly offering other unforeseen advantages.

While this may enable butterflies to adapt quickly, possessing numerous chromosomes can also introduce complications, potentially making them more susceptible to extinction in the long run.

Further studies comparing other butterfly species will clarify whether genes are lost or retained, offering greater insights into butterfly biology and evolution.

“Observing chromosomal degradation at this level is uncommon, yet evident in butterflies of other species, hinting at a significant need for exploration in this area,” noted Dr. Roger Villa, a researcher at the Evolutionary Biology Institute in Spain.

“Moreover, chromosomes hold the secrets of species, and examining how these changes influence butterfly behavior could help us form a comprehensive understanding of species emergence.”

“When we embarked on studying butterfly evolution, we realized that sequencing the extraordinary Atlas Blue Butterfly was essential,” remarked Dr. Charlotte Wright from the Wellcome Sanger Institute.

“This research emphasizes the collaborative spirit of scientific inquiry.”

“By examining how the chromosomes of the Atlas Blue Butterfly have split over time in specific environments, we can begin to uncover the potential benefits of this phenomenon, how it influences adaptability, and whether there are lessons in the DNA that could aid our future conservation efforts.”

The findings have been published in this week’s edition of Current Biology.

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Charlotte J. Wright et al. Chromosomal evolutionary constraints revealed by the 229 chromosome pairs of the Atlas Blue Butterfly. Current Biology, published online on September 10th, 2025. doi: 10.1016/j.cub.2025.08.032