On Sept. 19, 2025, researchers at the Wellcome Sanger Institute and the Institute of Evolutionary Biology, Barcelona reported that the Atlas blue butterfly, also known as Polyommatus atlantica, has been genetically confirmed as having the highest number of chromosomes out of all multicellular animals in the world. 

This insect boasts 229 pairs of chromosomes, while many of its close relatives have only 23 or 24 pairs. Researchers have revealed that these chromosomes have been broken up over time, instead of duplicated.

The first genomic study of this butterfly, published (10 September) in Current Biology, allows experts to begin to explore the evolutionary reasons behind this extreme number of chromosomes. Chromosome changes are also seen in human cancer cells, and therefore, understanding this process in different species could help inform cancer research.

This is the first time that the Atlas blue butterfly has been sequenced. From this, experts have produced a gold-standard reference genome for this species, allowing researchers to compare this extreme genome to other butterflies and moths to understand more about how species form and change over time.

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.