Odorant receptor polymorphisms and natural variation in olfactory behavior in Drosophila melanogaster

Abstract

Animals perceive and discriminate among a vast array of sensory cues in their environment. Both genetic and environmental factors contribute to individual variation in behavioral responses to these cues. Here, we asked to what extent sequence variants in six Drosophila melanogaster odorant receptor (Or) genes are associated with variation in behavioral responses to benzaldehyde by sequencing alleles from a natural population. Sequence analyses showed signatures of deviations from neutrality for Or42b and Or85f, and linkage disequilibrium analyses showed a history of extensive recombination between polymorphic markers for all six Or genes. We identified polymorphisms in Or10a, Or43a, and Or67b that were significantly associated with variation in response to benzaldehyde. To verify these associations, we repeated the analyses with an independent set of behavioral measurements of responses to a structurally similar odorant, acetophenone. Association profiles for both odorants were similar with many polymorphisms and haplotypes associated with variation in responsiveness to both odorants. Some polymorphisms, however, were associated with one, but not the other odorant. We also observed a correspondence between behavioral response to benzaldehyde and differences in Or10a and Or43a expression. These results illustrate that sequence variants that arise during the evolution of odorant receptor genes can contribute to individual variation in olfactory behavior and give rise to subtle shifts in olfactory perception.

Figure 1.—

Molecular variation in six Or genes. The gene structure for each receptor is schematically represented with a horizontal line denoting genomic DNA and exons represented by shaded boxes. The number of SNPs in the coding and noncoding regions is shown within each exon or within hexagons, respectively. Open boxes denote 5′-UTRs and 3′-UTRs. The number of indels is indicated by numbers within inverted triangles.

Figure 2.—

Linkage disequilibrium between polymorphisms in each of six Or genes. Segments below and above diagonals reflect r² values and P-values, respectively, for all possible marker pair combinations. P-values are determined by Fisher's exact test (not corrected for multiple tests).

Figure 3.—

Associations between polymorphisms in each of three Or genes and variation in olfactory behavioral responses. Polymorphic markers are depicted on the x-axis, and log(1/P) values for marker-trait associations on the y-axis. Marker–trait associations for benzaldehyde and acetophenone are indicated by the red and gray lines, respectively. The dashed red and gray horizontal lines indicate the corresponding permutation thresholds.

Figure 4.—

Haplotype analysis of SNPs/indels significantly associated with variation in behavioral responses to either benzaldehyde (red bars) or acetophenone (gray bars) for Or genes Or10a, Or43a, and Or67b. Significant phenotypic differences among haplotypes were determined by ANOVA and haplotypes that differed significantly in olfactory behavior were determined by post hoc Tukey's tests at P < 0.05 and are indicated by different letters above the bars. Specific haplotype sequences for each of the odorant receptors and their frequencies are presented in Table S3.

Figure 5.—

Correspondence between behavioral response to benzaldehyde and differences in Or10a and Or43a expression. Expression of (a) Or10a and (b) Or43a in male and female high- and low-responder haplotypes is shown. Odorant receptor expression is shown relative to the expression in the low responder. Data are shown as mean ± SEM, *P < 0.05; ****P < 0.0001.