A groundbreaking genetic analysis by scientists at the Wellcome Sanger Institute and the Institut de Biologia Evolutiva in Spain has revealed that the Atlas blue butterfly (Polyommatus atlantica) possesses an astonishing 227 pairs of autosomes and four sex chromosomes. This extraordinary chromosomal count represents the highest number observed in any multicellular animal species globally.
The Atlas blue butterfly inhabits the mountainous terrains of Morocco and northeastern Algeria.
Although prior suspicions suggested this species might hold the record for the most chromosome pairs within the animal kingdom, this recent genome sequencing effort provides the definitive confirmation by experts.
For comparative context, a closely related species, the common blue butterfly (Polyommatus icarus), which is widespread in the UK, possesses only 24 chromosomes.
Alterations in chromosome numbers are widely believed to play a significant role in speciation — the formation of new species — and to facilitate species’ adaptation to their respective environments.
The taxonomic group to which the Atlas blue butterfly belongs is characterized by numerous closely related species that underwent rapid evolutionary divergence over a compressed timeframe.
Professor Mark Blaxter, a researcher affiliated with the Wellcome Sanger Institute, commented, “Genomes are instrumental in elucidating a creature’s origins and anticipating its future evolutionary trajectory.”
He further elaborated, “To fully comprehend the narrative of our planet, it is imperative to document the story of each species and identify their points of intersection and mutual influence.”
“Moreover, this endeavor enables us to transfer insights gleaned from one genome to another.”
“For instance, chromosomal rearrangements are also observed in human cancer cells. Consequently, a deeper understanding of this phenomenon in the Atlas blue butterfly could potentially lead to novel strategies for the mitigation or cessation of such processes in cancerous cells in the future.”
During the investigation, Professor Blaxter and his team observed that the chromosomes had undergone fragmentation at regions where the DNA exhibits reduced coiling.
This structural change implies a comparable quantity of genetic material, albeit organized into more numerous, smaller segments.
All chromosomes, with the exception of the sex chromosomes, were found to be fractured. The researchers estimate that this fragmentation process led to an increase in chromosome number from 24 to 229 over approximately three million years — a remarkably brief period by evolutionary metrics.
Typically, it is presumed that such profound chromosomal alterations have detrimental consequences; however, the Atlas blue butterfly has demonstrably adapted and persisted for millions of years.
It is only in the present era, primarily due to anthropogenic factors such as climate change and environmental degradation including habitat destruction and excessive grazing, that its populations are facing significant peril.
This research has instigated a multitude of research questions that are now poised for future inquiry.
The splitting of chromosomes might enhance genetic variability by permitting more frequent recombination of genomic components, or it could confer other presently unrecognized advantages.
While this mechanism could potentially facilitate rapid adaptation in butterflies, species characterized by an exceptionally high chromosome count might also encounter challenges stemming from the increased complexity, potentially rendering them more susceptible to extinction over extended periods.
Subsequent investigations and comparative analyses with other butterfly species could illuminate whether any genes have been either lost or retained, thereby providing enhanced insights into butterfly biology and a more profound comprehension of evolutionary processes.
Dr. Roger Vila, a researcher at the Institut de Biologia Evolutiva in Spain, stated, “Chromosomal fragmentation has been documented in other butterfly species, but not to this extent. This suggests the existence of significant underlying reasons for this process, which we can now commence exploring.”
He added, “Furthermore, given that chromosomes encapsulate all the genetic secrets of a species, examining whether these chromosomal modifications influence butterfly behavior could contribute to a comprehensive understanding of how and why new species emerge.”
Dr. Charlotte Wright, a researcher at the Wellcome Sanger Institute, remarked, “When we embarked on our quest to comprehend butterfly evolution, we recognized the necessity of sequencing the most genetically distinct, and somewhat enigmatic, species — the Atlas blue butterfly.”
“Our successful sequencing of this species underscores the inherently collaborative nature of scientific endeavors.”
“By meticulously observing the historical fragmentation of the Atlas blue butterfly’s chromosomes at specific loci, we can initiate investigations into its potential benefits, its impact on their adaptive capabilities, and importantly, whether their DNA holds any lessons that could inform future conservation strategies.”
The findings of this study were disseminated this week in the esteemed 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
