A study published in Nature Communications revealed that Vanessa cardui butterflies develop opposite migratory routes in each hemisphere, a phenomenon never before documented in insects.
While northern hemisphere populations fly south during the boreal autumn, those in the southern hemisphere move north in the austral autumn. Both avoid crossing the Equatorial Line, which functions as an invisible border and potential evolutionary barrier.
The genetic mechanism behind orientation
The finding is linked to the identification of a chromosomal inversion of nine million bases in chromosome 8 of the painted lady butterfly. This alteration contains genes such as the GABA-B neurotransmitter receptor, which could determine the orientation capability of the lepidopterans.
According to the researchers, this genetic mechanism influences how butterflies interpret key environmental signals, such as the magnetic field and the position of the sun, when defining their migratory paths.
International research
The work was led by the Botanical Institute of Barcelona (IBB, CSIC-CMCNB) in collaboration with the Institute of Evolutionary Biology (IBE, CSIC-UPF) and experts from Africa, Europe, and the United States. The team analyzed more than 300 specimens from 38 African and European countries, using genomic studies and advanced monitoring techniques.
Aurora García-Berro, a researcher at the IBB and lead author, highlighted that the detected chromosomal inversion is directly related to migration and orientation genes. Meanwhile, Daria Shipilina, from Uppsala University, emphasized that specific adaptations prevent crossing the Equatorial Line, keeping populations restricted to each hemisphere.
Evolutionary implications
Gerard Talavera, a CSIC scientist and principal investigator of the study, explained that this migratory division could become an evolutionary barrier, limiting genetic flow between populations and favoring species diversification. Unlike birds, whose migratory division is usually longitudinal, in these butterflies, the limit is latitudinal, with the Equatorial Line as a natural separation.
This phenomenon, typical in birds but unprecedented in insects, opens the possibility that migratory division acts as a little-recognized evolutionary mechanism, capable of explaining the existence of related but separated species in the northern and southern hemispheres.
Long-range migrations
In previous studies, the same group had documented routes of up to 15,000 kilometers between equatorial Africa and Europe, placing Vanessa cardui among the butterflies with the longest known migrations. The new finding adds an independent circuit in the southern hemisphere of Africa, expanding the understanding of these lepidopterans’ migratory patterns.
Ecological relevance
The discovery highlights the importance of studying biodiversity on a global level, especially in the southern hemisphere, which is less attended to in similar research.
Understanding how migratory insects interpret the environment is key to assessing their ecological role: from plant pollination to parasite transmission.
The research suggests that the Equatorial Line is not just a geographical demarcation but a true biological barrier for the migration of Vanessa cardui and possibly other flying insects or migratory animals. This natural limit contributes to the diversification of species and offers new perspectives on the evolutionary processes that shape life on Earth.
