Globetrotting not in the genes

Painted lady butterflies are world travelers. The ones we encounter in Europe fly from Africa to Sweden, ultimately returning to areas north and south of the Sahara. But what determines whether some butterflies travel long distances while others travel short distances? A group of scientists, including from the Institute of Science and Technology Austria (ISTA), shows that the different migration strategies are shaped by environmental conditions rather than being encoded in the butterfly’s DNA.

It is a warm summer day in June. A group of scientists with sunhats and nets is hiking along a trail in the Catalan mountains. They meticulously search for painted ladies—vibrant orange butterflies with an intricate black-and-white pattern. Capturing painted ladies is not an easy task; they are strong, determined flyers, a fact that evolutionary biologist Daria Shipilina also had to acknowledge.

Previously working with plants and birds, the scientist tries to catch one of the beautiful butterflies in the air. Her net sways in all directions, but not where it needs to go. Finally, a few butterflies are taking a nectar break, giving Shipilina a long-awaited chance. A swift “swoosh”, and she nets one. A great moment for the biologist and an even greater testament to the resilience and endurance of these incredible butterflies.

Each year, painted ladies embark on a huge migratory journey from the northwest of Africa all the way up to Sweden and back to find the perfect environmental conditions that ensure their survival and reproduction.
A group of scientists, alongside citizen science projects, has been trying to decode the butterfly travel map. A recent interdisciplinary publication provides new insights. It features contributions from Shipilina—formerly at the University of Uppsala and currently a postdoc with Nicholas Barton's group at the Institute of Science and Technology Austria (ISTA)—along with collaborators from several institutions: the University of Ottawa, the CSIC-CMCNB in Barcelona, SOS Savane, the Polytechnic Higher School of Dakar, and the Technical University of Darmstadt. The results are now published in PNAS Nexus.

No distance too far, no burden too heavy

“The painted lady is a strikingly beautiful and colorful butterfly species,“ says Shipilina. “Watching them form large aggregations is a true spectacle. But what makes them particularly special is their incredible long-distance migrations.”

These butterflies go on a yearly 10,000 km journey between Africa and Europe. They do so through a succession of generations, looking for the best breeding conditions for their offspring. “Each individual travels in one section of the annual migratory cycle, with its offspring continuing their journey,” Shipilina continues.

The colorful insects begin their grand voyage in spring, starting from Northwest Africa and flying over the Mediterranean Sea to Europe. Subsequent generations then make their way to Great Britain, even reaching the Arctic tundra of Sweden to spend the summer.

Until recently, it was believed that once the butterflies reach Sweden, they perish due to the colder climates that arise there at the end of summer. However, studies have shown that painted ladies return to warmer regions in autumn, confirming a circular migratory pattern. While some end up staying in the Mediterranean area, others travel back to Africa, even crossing the Sahara. But how come? Do they have a different GPS systems?

Where have you been, where are you headed?

Shipilina and colleagues set out to understand this phenomenon. To achieve this, the scientists went on field trips and collected painted ladies from regions both north and south of the Sahara, including Benin, Senegal, Morocco, Spain, Portugal, and Malta.

They utilized isotope geolocation to estimate the geographic origin of each butterfly. “The key principle of this method is that the isotopic makeup—or the stable isotopes—of the adult butterfly’s wings mirrors the isotopic signature of the plants they ate as a caterpillar,” explains Shipilina. Isotopes are different forms of the same element, with identical chemical properties but slightly different atomic masses.

Co-first author Megan Reich and Clement Bataille from the University of Ottawa spent several years developing this technique, testing different isotopes, refining statistical approaches, and incorporating machine-learning techniques to enhance accuracy and resolution.

The analysis confirmed the diverse travel behavior among individuals: some took a long migration trip south from Scandinavia, crossing the Sahara, while others migrated a short distance, staying north of the desert in the Mediterranean region.

Is it in their genes?

The scientists then used whole genome sequencing to compare DNA sequences of each individual. Interestingly, there was no genetic difference between short-trip and long-trip butterflies.

“This finding fundamentally differs from what is observed in some birds, another well-studied migratory group,” Shipilina says. “For example, in willow warblers, a large chromosomal region has been associated with variable migratory direction, illustrating how different phenotypes arise from distinct genomic compositions.” Additionally, migration patterns in painted ladies could not be associated with factors such as sex, wing size, or wing shape.

Painted ladies adapt to the environment

According to the scientists, so-called phenotypic plasticity might explain the different migration styles. “Phenotypic plasticity is the ability of an organism to change its phenotype—in this case, its engagement in long- or short-distance migration—in response to environmental conditions without altering its genetic makeup,” explains Shipilina.

For instance, in summer, butterflies in Sweden might be prompted to migrate a long distance south across the Sahara due to the quick shift in day lengths or other seasonal cues. In contrast, butterflies in Southern France, where the days are longer, may not encounter those migratory cues and therefore only undertake short-distance journeys, staying in the Mediterranean area.

Compared to the other butterflies, like the well-studied monarch, much remains unknown about the migration of painted ladies. Does the observed pattern apply to the broad geographic distribution of the painted lady? Is this phenomenon unique to butterflies, or could it be observed in other insects as well? ISTA researcher Daria Shipilina and her colleagues are determined to close this knowledge gap—one paper at a time.

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Information on animal studies
In order to better understand fundamental processes, for example, in the fields of neuroscience, immunology, or genetics, the use of animals in research is indispensable. No other methods, such as in silico models, can serve as alternative. The animals are raised, kept, and treated according to the strict regulations of the respective countries, the research was conducted.