Fig. 2.

Limited discrimination of different sugars. (A) Two-choice drinking assay was used to monitor the innate preference of fructose (fru), glucose (glu), and maltose (mal). The graph shows the Preference Index for different sugar pairs (100 mM, 1:1 ratio). Each bar represents one sugar pair (i.e., the first bar shows the choice between glucose and fructose, with a preference toward fructose) (n = 3–4 experiments, ~40 flies per experiment) [*P < 0.05, **P < 0.01, Student's t test (one population)]. (B) Plots of the DI for experiments shown in C–H (n = 3–4 experiments, 5 flies per experiment) [*P < 0.05, **P < 0.01, Student's t test (one population)]. (C–H) Plots of proboscis extension before, during, and after training for different sugar pairs. Each plot represents three to four experiments, with five flies per experiment. (C) Flies showed conditioned aversion to 250 mM glucose but still extended to the unpunished 250 mM fructose. (D) Flies reduced the PER to 250 mM fructose as well as to the unpunished 250 mM glucose. With 250 mM fructose and 250 mM maltose, flies showed selective aversion to 250 mM fructose paired with heat (E) and generalized aversion when 250 mM maltose was paired with heat (F). With 250 mM glucose and 250 mM maltose, flies showed selective aversion to 250 mM glucose paired with heat (G) and generalized aversion when 250 mM maltose was paired with heat (H).