The survival advantage of olfaction in a competitive environment

Abstract

Olfaction is generally assumed to be critical for survival because this sense allows animals to detect food and pheromonal cues. Although the ability to sense sex pheromones [1, 2, 3] is likely to be important for insects, the contribution of general odor detection to survival is unknown. We investigated the extent to which the olfactory system confers a survival advantage on Drosophila larvae foraging for food under conditions of limited resources and competition from other larvae.

Figure 1. Anosmic Or83b mutant larvae show reduced survival and fail to exploit a secondary food source

(A–B) Cumulative eclosion rates of Or83b functional (green), Or83b mutant (magenta), and Or42a functional (blue) flies from either 10 (A) or 50 (B) embryos in the absence (left) or presence (right) of a second food source are plotted against days AEL (mean ± S.E.M., n=6). Half-maximal eclosion rates are indicated with an arrowhead. The cumulative eclosion rate did not differ across genotypes with 10 embryos (A), but the cumulative eclosion rate of Or83b mutants was significantly lower than that of the three other genotypes when the second food source was supplied to 50 embryos (B) (ANOVA and post-hoc Tukey’s HSD test; * 99% confidence level). (C) Distributions of Or83b mutant (n=7), Or83b functional (n=4), and Or42a functional (n=5) animals from 4 to 8 days AEL are shown in stacked bar plots. (D) Percentage of larvae accumulating on the second food source at 5 days AEL (mean ± S.E.M.). Bars that are statistically different are labeled with different letters (ANOVA and post-hoc Tukey’s HSD test; 99% confidence level).

Figure 2. Survival of anosmic Or83b mutants is further reduced by competition with larvae that detect odors

(A–C) Cumulative eclosion rates (mean ± S.E.M.) from competition of Or83b functional vs. Or83b mutants (n=6) (A), Or42a functional vs. Or83b null mutant larvae (n=6) (B) and Or42a functional vs. Or83b functional larvae (n=4) (C) for 10 (left), 50 (middle), and 80 (right) total embryos. Values of cumulative eclosion that differ statistically between genotypes are indicated with black asterisks (Student’s t-test; **P<0.01). Overlaid bar plots indicate the total number of adults eclosing on each day from 10 to 20 days AEL. Half-maximal eclosion rates for each genotype are indicated with an arrowhead and eclosion distributions that differ statistically between competing strains are indicated with gray asterisks (Kolmogorov-Smirnov; **P<0.01). Unmarked comparisons are not significant by the same tests (P>0.05).