Using microarrays, Melcher and Pankratz discovered that mutant fly larvae overexpress the hug gene, which is known to encode at least two neural proteins related to growth signaling. The researchers then investigated which signals influence hug expression by exposing larvae to either high or low food levels. Because both starved and sugar-fed flies express little hug, the researchers inferred that hug levels do not solely signal internal energy requirements but respond to internal and external signals carrying information about the quality of food. The researchers also noted that the finicky pumpless (ppl) mutants, which have a feeding defect similar to klu, overexpress hug.
Behavioral studies confirmed that too much hug reduces food intake and leads to stunted growth, while too little stimulates eating. Melcher and Pankratz selected a group of flies and blocked the synapses of their hug neurons to inhibit the neurons' activity. In contrast to control flies, which start feeding on a novel food source only after an evaluation phase (they wait a while before initiating feeding), the experimental flies started eating new food right away. These hug neurons may help flies decide whether or not to eat a new food source.
Larvae express hug in only about 20 neurons, all located in the subesophageal ganglion. The axons of some of these hug neurons extend into the ring gland, a crucial metabolism and growth organ in flies. Other axons contact the protocerebrum, a structure close to brain centers for learning and remembering odors. A third set of these axons extend to throat muscles—which is surprising because most subesophageal ganglion neurons have no connection to motor function. All together, these few hug neurons can signal structures co