Contact pheromones of 2 sympatric beetles are modified by the host plant and affect mate choice

Host-plant shifts have significantly contributed to the diversification of phytophagous insects. The contact sex pheromones of such insects may be modified by the plant they feed on, thereby contributing to the formation/maintenance of sister species on different plants. Here, we addressed this issue using 2 sister species of specialist phytophagous flea beetles Altica fragaria and Altica viridicyanea, and their oligophagous F-1 hybrids. Specifically, we tested 1) if males from these Altica species recognize conspecific females based on their cuticular hydrocarbon (CHC) profile, 2) if the host plant affects the CHC profile of hybrid females, and 3) whether hybrid males distinguish between hybrid females raised on different host plants. Mate choice tests revealed that males use CHCs to identify conspecific mates. We then identified different CHC profiles in females of the 2 species and showed that the profile of CHCs in hybrids is modified by the host plant in which the beetles develop. Finally, we found that hybrid males raised on one host plant choose females with a matching profile, but this is not the case for males raised on the other plant. Our results suggest that plasticity in the expression of CHCs may have contributed to the original speciation process between the parental species. This reinforces the key role of host plants in shaping the evolution of reproductive isolation among herbivore populations.