The chalkhill blue has some surprising claims to fame. For a start, it is one of the UK’s most beautiful butterflies, as can be seen as they flutter above the grasslands of southern England in summer.
Then there is their close and unusual relationship with ants. Caterpillars of Lysandra coridon – found across Europe – exude a type of honeydew that is milked by ants and provides them with energy. In return, they are given protection in cells below ground especially created for them by the ants. Chalkhill blues thrive as a result, though their numbers are now coming under threat.
It is an extraordinary catalogue of features, to which scientists have now made a striking addition thanks to a pioneering new project, known as Psyche, which aims to sequence the genomes of all 11,000 species of butterflies and moths in Europe and reveal in fine detail how climate change and habitat loss are affecting them.
As part of Psyche, scientists have found that, depending on location, the cells of the chalkhill blue have different numbers of chromosomes – the packets of DNA that contain their genetic blueprint. In southern Europe, they have a total of 87 chromosomes, adding them one at a time as they head north until their northern limit is reached, where chalkhill blues have 90 chromosomes.
“That very much goes against the dogma which states that a given species has a given number of chromosomes,” said evolutionary biologist Charlotte Wright of the Wellcome Sanger Institute near Cambridge.
“Why this is changing in the chalkhill blue is intriguing. It is clear that, as it has moved in Europe as glaciers retreated since the end of the last ice age, it has added a chromosome one by one while progressing northwards. It is a surprising observation.”
This point was backed by Mark Blaxter, who is also based at the Wellcome Sanger Institute. “If we look back a million years or so, we can tell when two species have split from a single originator. But how does that happen? More to the point, how would we spot what was going on at the time? That is what we are probably seeing here. We are seeing two species in the act of being created from one. We are shining a light on evolution in action.”
Project Psyche research is being carried out at the Wellcome Sanger Institute in collaboration with six other leading European research centres including Oulu University, Finland, and the Institute of Evolutionary Biology in Barcelona. It is named after the Greek goddess of the soul, who was said to have been given butterfly wings by Zeus and was usually depicted by the ancient Greeks wreathed in butterflies.
Before the development of modern genomics, the relationship between moths and butterflies generated considerable debate. “However, DNA technology has made it clear that butterflies are essentially a sub-group of moths, albeit ones that are generally more colourful,” said Wright.
A tenth of all named species on Earth are moths or butterflies and they are uniquely sensitive to changes in habitats, temperature and plants on which they thrive, added Blaxter. “That means the more we know about them the better informed we will be about the changes affecting the natural world in general. The alterations affecting the chalkhill blue are a perfect example of that knowledge.”
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Its 87-to-90 complement of chromosomes may seem extreme compared with the 23 pairs possessed by humans. However, large numbers of these genetic packages are common among moths and butterflies, say scientists, with the record being held by another blue butterfly, the species Polyommatus atlanticus. It possesses a staggering set of 229 chromosomes.
Another intriguing example of lepidopteran life is provided by the xerces blue butterfly which was recently rendered extinct. By studying samples from museum collections, scientists have determined – by studying its genomes – that the species had become highly inbred and vulnerable.
“That research took place in the US but the aim of Psyche is to pinpoint other similarly vulnerable species in Europe in the same way and suggest which are the best targets for interventions to save them,” added Wright. “A genome is a perfect starting point for understanding how well an organism is doing in its environment.”
Butterflies and moths are crucial pollinators of plants and are also a key source of foods for birds, so their survival is important, added Wright. “This is blue-skies research that could have very practical outcomes.”
