(Center) Wild-type mosquitos prefer to lay eggs in fresh water (bottom) rather than salt water (top). When a female mosquito is deciding where to lay its eggs, it first 'dips' its legs and mouthparts into the water, exposing the neurons inside these body parts to the environment. (Left) Neurons (green) in the taste organs of the legs express ppk301 and other unknown salt sensing molecules. The ppk301 gene encodes a putative ion-channel subunit (teal) of the Pickpocket/Degenerin/ENaC (epithelial sodium channel) family. Ion channels in this family form homo-trimeric or hetero-trimeric complexes. Unknown proteins involved in detecting salt are indicated by question marks and possibly form a hetero-trimeric complex with ppk301 (top). In the case of water sensing, it is possible that ppk301 assembles with two identical proteins to create a homo-trimeric complex (bottom). Channels from the ENaC family primarily conduct sodium (green disks; Eastwood and Goodman, 2012). (Right) Physiological studies reveal that water and salt both activate ppk301 neurons in wild-type mosquitos (black traces). However, mutant mosquitos with inactive ppk301 neurons show a strongly diminished response to water stimuli (red trace on left) and a nearly normal response to salt (red trace on right). This leads to a model where ppk301 channels are required to sense freshwater (and promote egg laying) while other mechanisms are involved in sensing salt. Figure credit: Annie Park, University of Texas at Austin (CC BY 4.0).