Olfactory differences among cactophilic Drosophila species

Divergence of behavioral traits with adaptation to changes in the environment has been well documented in many animals and is often accompanied by divergence in sensory systems. Shifts in insect host plant preference behavior, in particular, are often associated with changes to the olfactory system, including differences in the sensitivity, specificity, or abundance of olfactory sensory neurons (OSNs). Here, we examine 12 species from the cactophilic Drosophila repleta group that represent three broad host-use categories: Opuntia specialists, columnar cactus specialists, and polymorphic species that use both host types. We recorded odor-evoked responses from all six maxillary palp OSN classes using single sensillum recordings and used electropalpograms to further evaluate species in which a pb2B-like class was not detected by single sensillum recordings (SSRs). The B-neuron classes clustered largely by OSN identity across species, retaining characteristic class-specific response profiles. The A-neuron classes were less clearly separated because of weak or overlapping response profiles. Despite these shared class-specific responses, species differed substantially in response magnitude and sensitivity to odorants. A pb2B-like class was not detected by SSR in D. mulleri, D. aldrichi, and D. straubae. Electropalpograms supported the absence or strong reduction of a phenol-responsive palpal signal in D. mulleri and D. straubae, while D. aldrichi retained a strong response. Principal component and phylogenetic principal component analyses did not reveal consistent clustering by host-use category. These results suggest that maxillary palp olfaction has evolved in a species- or lineage-specific manner rather than through a repeated host-category-specific shift in peripheral sensitivity.