Editors' ChoiceSensory Perception

Understanding hygrosensation: How flies sense changes in humidity

Sci. Signal.  31 May 2016:
Vol. 9, Issue 430, pp. ec127
DOI: 10.1126/scisignal.aag2207

Finding areas of proper humidity is critically important for small-bodied animals to avoid desiccation. Fruit flies use transient receptor potential (TRP) channels for detecting the presence or absence of moisture in the air, a phenomenon called hygrosensation. Enjin et al. found that another family of ionotropic channels that are related to ionotropic glutamate channels (iGluRs) is important for differentiating between gradations in humidity. When presented with a choice between 20 and 70% relative humidity, Drosophila melanogaster, a species of fruit fly that naturally inhabits temperate and neotropical climates, moved to the area with the higher humidity. When given the choice between 70 and 85% relative humidity, the flies chose 70% relative humidity, indicating that they have the ability to differentiate between small differences in moisture. Not surprisingly, Drosophila species native to arid climates preferred 20 to 70% relative humidity, and species native to tropical climates preferred 85 to 70% relative humidity. Although the TRP channels water witch (Wrtw) and nanchung (Nan) enable flies to distinguish between 0 and 100% relative humidity, both wrtw and nan mutant flies nevertheless exhibited a preference for 70 over 20% relative humidity. A functional deficiency screen of various receptors present in sensory neurons of the antennae identified the ionotropic receptor IR25a and its co-receptors IR93a and IR40a as important for the ability of flies to distinguish between 20 and 70% relative humidity. These data indicated that whereas TRP channels are important for detecting absolute humidity, IR25a, IR93a, and IR40a are important for gauging relative humidity. IR25a and IR93a were also important for temperature preference; IR40a was not. Neurons in the antennae that expressed both IR40a and IR93a projected into two regions of the brain. In one of these regions, neuronal activity increased in response to dry air and decreased in response to moist air. The other brain region was responsive to changes in temperature, suggesting that the brain may integrate thermosensory and hygrosensory information to fine tune humidity sensing (see Kim and Wang).

A. Enjin, E. E. Zaharieva, D. D. Frank, S. Mansourian, G. S. B. Suh, M. Gallio, M. C. Stensmyr, Humidity sensing in Drosophila. Curr. Biol. 26, 1352–1358 (2016). [PubMed]

S. M. Kim, J. W. Wang, Hygrosensation: Feeling wet and cold. Curr. Biol. 26, R408–R410 (2016). [PubMed]