A north-south hemispheric migratory divide in the butterfly Vanessa cardui

Reversed seasonality and distinct navigation cues in the Earth's two hemispheres may shape the evolution of migratory behaviour in animals. Migratory divides-contact zones where populations have evolved alternative migratory strategies-are well-documented in birds and typically occur longitudinally. We hypothesise that insect migratory divides are less likely to emerge longitudinally, but may exist latitudinally, driven by hemisphere-specific sensory adaptations that lead to spatial and temporal isolation. Here, we examine this hypothesis in the cosmopolitan painted lady butterfly (Vanessa cardui), whose Southern Hemisphere dynamics remain unexplored. Investigating the genomes of 300 individuals across Africa and Europe, we identify a 9 Mb chromosomal inversion on chromosome 8, which exhibits strong haplotype structure aligned with hemispheric origin, with a few potential heterozygotes near the equator. The inversion harbours 336 genes, including several directly relevant to migration. Notably, one inversion breakpoint intersects the gene encoding the GABA-B receptor, which responds to the neuropeptide γ-aminobutyric acid (GABA), crucial for insect navigation. Our findings provide genomic evidence of a migratory divide in insects and highlight the role of inverted seasonality in the two hemispheres and genomic rearrangements as isolating barriers for highly mobile species.