Olfactory variation among closely related cactophilic Drosophila species.

Chemosensation plays an important role in a wide range of behaviors including host identification and localization, oviposition site selection, and mate recognition. Variation in the ability to detect chemical signals may influence behavior in animals like insects that use volatile cues emitted from plants when discriminating between potential hosts. Differences in odor detection has been demonstrated to play a crucial role in driving changes in host use within and between insect species, leading to reproductive isolation between populations and eventual speciation through specialized host adaptation. We examined between-species variation in odor tuning and asked whether it is linked to shifts in host plant use in the Drosophila repleta species group, a taxonomic radiation of flies specializing on cacti that exhibits multiple shifts in host plant use across their phylogeny resulting in three current states: (1) Opuntia cactus specialists, (2) columnar cactus specialists, and (3) cactus "generalists" which use both hosts. We measured odor response profiles from select olfactory sensillar subtypes across multiple species within the group as well as for the outgroup D. melanogaster. Variation in both sensitivity and specificity to odors was observed, with some olfactory sensory neurons exhibiting differences associated with host cactus use. This study is the first in-depth analysis of the olfactory system across the repleta species group and provides the opportunity to test for conserved mechanisms in the olfactory system underlying divergence and host shift.