, 2007). Unlike olfactory CO2-sensing neurons, the gustatory neurons require high CO2 concentrations for detection, with aqueous CO2 activating at 0.2% and volatile CO2 activating at 10%. Behaviorally, flies show a weak preference for CO2 in solution, taste peg CO2 sensors mediate this preference, and artificially activating these neurons also triggers

acceptance behavior. The molecules responsible for detection have not been described. Why do flies taste CO2? One possibility is that it acts as a proxy for detecting growing microorganisms like yeast that emit CO2 and are consumed Selleckchem AZD6244 by flies to obtain essential nutrients. Taken together, these studies highlight the importance of CO2 detection for insects and demonstrate that CO2 acts as a repellent in air and a palatable ATM Kinase Inhibitor molecular weight taste in solution. Like mammals, flies detect CO2 with the gustatory system and the olfactory system. Long-range, short-range, volatile, and nonvolatile CO2 may be interpreted as different cues triggering different behaviors. The gustatory and olfactory systems compartmentalize the CO2 environment to allow animals to respond differently depending on the CO2 source. It is interesting to speculate that CO2 detection by both the olfactory and gustatory systems may co-operate to determine the value of a food source. Perhaps flies accept rotting fruit with high local concentrations of growing yeast

but avoid it once yeast produce enough CO2 for long-range detection. In this scenario, the taste and smell of CO2 would allow the fly to identify fruit with and the right amount of rottenness. Of course, studies of plasticity argue that there are multiple ways to modulate the CO2 response (see below). The finding that a single compound can act as either a taste or a

smell is not unique to CO2. Recent studies of water detection in Drosophila argue that there are olfactory neurons that respond to high or low humidity ( Liu et al., 2007) and gustatory neurons that detect water to elicit drinking behavior ( Cameron et al., 2010). A general strategy that animals may use to mine additional information about important yet common compounds like water and CO2 is to set up multiple methods of detection that are context-dependent. Although O2 and CO2 are associated with innate behaviors in C. elegans, Drosophila and mammals, these behaviors are also plastic allowing animals to adjust their responses depending on the environment. As both O2 and CO2 are generic signals emitted by numerous organisms, their ability to be interpreted in the context of other sensory cues is essential. Two examples illustrate this plasticity well: one is variation in O2 sensation in different C. elegans strains, the second is modulation of olfactory CO2 avoidance behavior in Drosophila. Two commons strains of C.