Fig. 1.

Differential spontaneous responses to odorants containing deuterium. The mean relative distribution of flies in the arms of the maze (% excess flies) carrying the indicated odorants ± SEM is shown in all graphs. This metric reflects the prevailing distribution within the arms of the maze of groups of 40 to 60 flies tested each time. The total number of flies tested in each group is shown and the number of groups tested is n ≥ 6 for all groups. (A) Spontaneous responses to 75 μL normal or d₃-, d₅-, or d₈-ACP, each diluted with isopropyl myristate (IPM) to 1 mL. Flies spontaneously preferred h-ACP over the solvent. In contrast, incorporation of five or eight deuterium atoms results in significantly different distribution (P < 0.001) from that toward h-ACP (attraction) to aversion. In contrast, the response to d₃-ACP was not significantly different (P = 0.012) from that of h-ACP. (B) Flies discriminate against d₈-ACP if presented with equal amount (1:1) of h-ACP (75 μL odorant/925 μL IPM). However, a 50% reduction in the amount of deuterated odorant yielded an equal distribution of the flies in the arms (% excess flies not significantly different from zero), defined as a balanced maze. (C) Similarly equal amounts (200 μL) of h-1-octanol (OCT) and d₁₇-1-octanol yielded strong discrimination against the deuterated odorant, which was eliminated upon reducing it by 75% (1:0.25). (D) In contrast, equal amounts of normal and deuterated benzaldehyde (90 μL) did not result in differential discrimination. In accord, decreasing the amount of h-BZA resulted in differential avoidance of d₅-BZA. (E) The preferential discrimination against d₈-ACP was eliminated in Or83b¹ and Or83b² mutants (P < 0.002 and P < 0.001, respectively, Dunnett test). (F) Similarly, discrimination against d₁₇-octanol was eliminated in Or83b¹ and Or83b² mutants (P < 0.001 for both, Dunnett test).