An unassuming butterfly called the Atlas blue has been confirmed to have the greatest known number of chromosome pairs of any animal.
The elusive species (Polyommatus atlantica), which is native to the mountains of North Africa, possesses as many as 229 pairs per cell.
To put that in perspective, most other butterflies have 31 or 32 pairs. We humans have 23 pairs in the nucleus of each of our cells.
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Some plants have more chromosomes than the Atlas blue, like the Adders-tongue fern (Ophioglossum reticulatum) with 720 chromosomes per cell. But that’s because they can have up to ten sets of DNA – the Atlas blue has but two.
“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,” says first author Charlotte Wright, evolutionary biologist at the Wellcome Sanger Institute in the UK.
The elusive Atlas blue is an extreme case of chromosome evolution that scientists are keen to understand better. The carriers of its genes in each cell are exceptionally small, even for a butterfly.
Wright and her colleagues suspect that in the relatively ‘rapid’ space of three million years, the butterfly’s autosomes (non-sex chromosomes) became deeply fragmented.
Over hundreds of ‘splitting’ events, the original 24 or so ancestral chromosomes multiplied to become 229. The fractures occurred at points where DNA was less tightly wound and thus easier to disentangle.
Chromosome changes like these are generally considered detrimental to life, but the Atlas blue butterfly has survived for millions of years.
That said, its sex chromosomes largely resisted fragmentation, which suggests there is an evolutionary limitation.
“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,” says evolutionary biologist Roger Vila from Spain’s Institute of Evolutionary Biology. Vila led the team that collected wild specimens for the gene study.
“Rearranging chromosomes is also seen in human cancer cells,” adds genomicist Mark Blaxter from Wellcome Sanger, “and understanding this process in the Atlas blue butterfly could help find ways to limit or stop this in cancer cells in the future.”
The study was published in Current Biology.
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