The Chalk Hill Blue has some surprising claims to fame. Firstly, it is one of Britain's most beautiful butterflies, as evidenced by its flight over the fields of southern England in summer.
They also have a close and unusual relationship with ants: the larvae of the Lysandra corydon species, found throughout Europe, secrete a type of honeydew that the ants milk to refuel, and in return provide protection in underground nests that the ants build specifically for them. As a result, the Chalk Hill Blue is thriving, but its numbers are now under threat.
It's an extraordinary catalogue of characteristics, and scientists have just made a surprising addition to it, thanks to a pioneering new project, Psyche, which aims to sequence the genomes of all 11,000 European butterfly and moth species, revealing in detail how climate change and habitat loss are affecting them.
As part of psyche, scientists have discovered that Chalk Hill Blue's cells in different locations have different numbers of chromosomes – chunks of DNA that contain the genetic blueprint. In southern Europe, they have a total of 87 chromosomes, and as you move north, they gain one more chromosome, so that at their northern limit, the Chalk Hill Blue has 90 chromosomes.
“This goes completely against the conventional wisdom that certain species have certain numbers of chromosomes,” said Charlotte Wright, an evolutionary biologist at the Wellcome Sanger Institute near Cambridge.
“It's intriguing why this change is happening in the Chalk Hill Blue. It's clear that since the end of the last Ice Age, it has been migrating in Europe as the glaciers retreated, adding one chromosome at a time as it made its way north. It's a surprising observation.”
Evolutionary biologist Charlotte Wright holds a moth and butterfly trap at the Wellcome Sanger Institute. Photo: Robin McKee/Observer
This point is supported by Mark Blaxter, also at the Wellcome Sanger Institute: “If we go back a million years or so, we can see when two species diverged from a single origin, but how did that happen? And more importantly, how do we tell what was going on back then? Maybe that's what we're seeing here. We're seeing the process of two species arising from one. We're shedding light on the evolutionary process.”
Project Psyche research is being carried out at the Wellcome Sanger Institute in collaboration with six other leading European research centres, including the University of Oulu in Finland and the Institute of Evolutionary Biology in Barcelona. The project is named after the Greek goddess of the soul, who was said to have been given butterfly wings by Zeus and was often depicted by the ancient Greeks as enveloped in butterflies.
Before modern genomics, the relationship of moths to butterflies was a matter of great debate, “but DNA techniques have revealed that butterflies are essentially a subgroup of moths, although moths are generally more colorful,” Wright says.
Moths and butterflies make up one in every 10 named species on Earth, making them particularly sensitive to changes in habitat, temperature and the plants they live in, Blaxter added. “This means that the more we know about moths and butterflies, the better we can understand the changes affecting the natural world as a whole. The changes affecting Chalk Hill Blue are a perfect example of that knowledge.”
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With 87 to 90 chromosomes, the butterfly's number may seem extreme compared to humans' 23 pairs, but scientists say these large numbers of gene clusters are common among moths and butterflies, with the record holder being another blue butterfly species, Polyommatus atlanticus, which has a staggering 229 chromosomes.
Another interesting example from the Lepidoptera is the recently extinct Xexus blue butterfly. By studying specimens from museum collections and studying its genome, scientists were able to determine that the species was highly inbred, making it vulnerable.
“Although this study was carried out in the United States, Psyche's aim is to similarly identify other similarly vulnerable species in Europe and propose the best targets for interventions to save them,” Wright added. “The genome is the perfect starting point for understanding how well an organism is doing in its environment.”
Their survival is important because butterflies and moths are important pollinators for plants and a vital food source for birds, Wright added. “This is blue-sky research that could have very practical consequences.”
