Behavioral responses of Drosophila to biogenic levels of carbon dioxide depend on life-stage, sex and olfactory context

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Abstract
Drosophila melanogaster (Meigen) detects and uses many volatiles for its survival. Carbon dioxide (CO2) is detected in adults by a special class of olfactory receptor neurons, expressing the gustatory receptor Gr21a. The behavioral responses to CO2 were investigated in a four-field olfactometer bioassay that is new for Drosophila. We determined (1) whether the sensitivity of this response changes with odor context, and (2) if it depends on sex and life stage. When CO2 was added to ambient air in one field and tested against ambient air in the three other fields, individually observed adults avoided CO2 (0.1-1% above ambient), but did not respond to a low rise of 0.02%. We relate this behavior to measurements of CO2 production in bananas and flies. When 0.02% CO2 was combined with the odor of apple cider vinegar in one field of the olfactometer and tested against ambient air in the three other fields, the addition of CO2 did not affect the attractiveness of apple cider vinegar alone. However, this combination of CO2 and vinegar became repellent when it was tested against vinegar at ambient CO2 concentrations in the three other fields. This `odor background effect' was female-specific, revealing a sexually dimorphic behavior. The new assay allowed us to test larvae under similar conditions and compare their behavior to that of adults. Like adults, they avoided CO2, but with lower sensitivity. Larvae lacking neurons expressing Gr21a lost their avoidance behavior to CO2, but kept their positive response to vinegar odor. Hence, Gr21a-expressing neurons mediate similar behaviors in larvae and adults.