Ir8a is the sole co-receptor required for the function of acid-sensing receptors. (A) Left, diagram of an ac1 sensillum and receptor expression. The acid-sensing ORN is in black. Middle, representative traces of responses to 2-oxovaleric acid in ac1 sensilla in WT, Ir8a1, Ir25a2, and Ir76b1 flies. Right, box plots overlaid with individual data points (n = 7 WT, 6 Ir8a1, 7 Ir25a2, and 12 Ir76b1 sensilla). (B) Similar to (A), but for ac2 sensilla tested with propionic acid and 2-oxovaleric acid (propionic acid: n = 10 WT, 13 Ir8a1, 7 Ir25a2, and 11 Ir76b1 sensilla; 2-oxovaleric acid: n = 9 WT, 13 Ir8a1, 7 Ir25a2, and 9 Ir76b1 sensilla). (C) Similar to (A), but for ac3 sensilla tested with propionic acid and 2-oxovaleric acid (propionic acid: n = 16 WT, 18 Ir8a1, 24 Ir25a2, and 20 Ir76b1 sensilla; 2-oxovaleric acid: n = 7 WT, 8 Ir8a1, 8 Ir25a2, and 7 Ir76b1 sensilla). (D) Similar to (A), but for ac4 sensilla tested with phenylacetic acid and phenylacetaldehyde (n = 7 WT, 6 Ir8a1, 7 Ir25a2, and 7 Ir76b1 sensilla for each odorant). Genotypes that are significantly different are indicated with different letters.