An Equatorial Hemispheric Barrier Shapes the Diversification of Migratory Belenois Butterflies.

Biogeographic barriers are typically considered prominent geographic features that block or severely restrict dispersal and gene flow. However, mating barriers can also emerge within continuous suitable habitats, driven by ecological or behavioural constraints. Migratory insects show an extraordinary capacity to traverse vast geographic ranges, as well as notable landscape features like mountains, deserts and oceans. Yet, their movements are not unrestricted: they are shaped by seasonal dynamics that dictate the feasibility of migration across these landscapes. Hemisphericity, the existence of inverted seasonal regimes and orientation cues in the two latitudinal hemispheres, has been proposed as a potential abiotic barrier involved in the diversification of migratory insects. Here, we use population genomic data to investigate patterns of diversification in migratory caper butterflies (Belenois spp.) across Africa. We identify a striking phylogeographic break around the equator in Belenois aurota, and emerging population structure between northern and southern African populations in Belenois creona, consistent with migratory divides aligned with hemispheric barriers. These divergences largely predate the Last Glacial Maximum, when major environmental changes such as contractions-expansions of equatorial rainforests and savannahs occurred. This reinforces the hypothesis that long-term abiotic factors, such as hemisphericity, had a role in limiting north-south dispersal. Given the absence of detectable gene flow detected even in sympatric populations of B. aurota in their contact zone in Kenya, Uganda, and Tanzania, we argue that populations from the Northern and Southern Hemispheres represent different species, and reinstate the taxon Belenois syrinx (Wallengren 1860) reinst. stat. for the Southern African lineage. Our findings provide genomic evidence of migratory divides in insects, which surprisingly emerge in the absence of physical barriers in the landscape, highlighting a role of hemisphere-specific adaptations in driving reproductive isolation and diversification in migratory insects.