Geographically variable mate preferences shed light on the processes maintaining inversion polymorphism

An important question in evolution is to understand the mechanisms that maintain phenotypic diversity, despite selection that should drive homogeneity. For example, selection by predators may promote the convergence of colour patterns among defended prey, resulting in the formation of mimetic communities. However, certain aposematic species, such as the Neotropical butterfly Heliconius numata, are polymorphic. In H. numata, wing pattern polymorphism, which is associated with chromosomal inversions, may be maintained via disassortative mating preferences, thought to favour the co-occurrence of individuals with different mimicry phenotypes in Peru. To test whether environmental variation due to geography influence the mate choice, we investigate the occurrence of disassortative mating among the two co-existing forms of H. numata in French Guiana, and its potential role in the maintenance of this polymorphism. Our experimental approach demonstrates that the two forms display weak and slightly asymmetrical disassortative mate preferences. Modelling and simulations suggest that this pattern of mate preference alone is not sufficient to maintain polymorphism, and predict the loss of the choosiest form, unless this form enjoys a survival advantage. In this aposematic species, such an advantage could arise from mimicry, but further studies into the benefits of mimicry and predator generalization are needed to test this hypothesis. More importantly, our results suggest that the balance between selective forces influencing polymorphism may vary across geographic and ecological contexts, and this warrants further study.