Co-Occurring Sister Taxa of Mountain Butterflies Exhibit Distinct Cuticular Hydrocarbon Profiles

Invisible to human perception, differentiation in chemical traits such as insects cuticular hydrocarbons (CHCs) might contribute to speciation. The species-rich mountain butterfly genus Erebia represents a well-established model for studying speciation because closely related taxa form stable secondary contact zones. However, to which degree these taxa would also differ in their chemical composition of the cuticle has remained unexplored. We compared CHCs of males and females from four locally sympatric or parapatric sister taxa pairs with varying levels of gene flow. Rarely hybridizing taxa pairs (E. cassioides-E. tyndarus, E. euryale-E. ligea) exhibited significant CHC differentiation at both interspecific and intersexual levels. Conversely, taxa pairs with no prior contact (E. melampus-E. sudetica) or frequent ongoing hybridization in their contact zones (E. euryale adyte-E. e. isarica) showed limited CHC differentiation. Our findings suggest that differentiation in CHC profiles scales with among-species gene flow. Although it remains unclear whether CHCs are involved in mate recognition in Erebia, the observed differentiation could play a role in reproductive isolation, particularly under environmental changes that promote novel interspecific interactions. Future research should explore the role of CHC divergence across hybrid zone gradients and pinpoint the genomic regions underlying CHC synthesis and perception.