Chemical signals act as the main reproductive barrier between sister and mimetic Heliconius butterflies

Colour pattern has been long recognised as the trait that drives mate recognition between Heliconius species that are phylogenetically close. However, when this cue is compromised such as in cases of mimetic, sympatric and closely related species, alternative mating signals must evolve to ensure reproductive isolation and species integrity. The closely related species Heliconius melpomene malleti and H. timareta florencia, occur in the same geographic region and despite being co-mimics they display strong reproductive isolation. In order to test which cues differ between species, and therefore potentially contribute to reproductive isolation, we quantified differences in wing phenotype and male chemical profile. As expected, wing colour pattern was indistinguishable between the two species while the chemical profile of their male sex pheromones showed marked differences. We then conducted behavioural experiments to study the importance of these signals in mate recognition by females. In agreement with our previous results, we found that pheromones and not wing colour pattern drive the preference of females by conspecific males. In addition, experiments with hybrid males and females suggested an important genetic component for both pheromone production and preference. Altogether, these results suggest that pheromones are the major reproductive barrier opposing gene flow between these two sister and co-mimic species.