Phenotypic plasticity and genotype by environment interaction for olfactory behavior in Drosophila melanogaster

Abstract

Genotype by environment interactions (GEI) play a major part in shaping the genetic architecture of quantitative traits and are confounding factors in genetic studies, for example, in attempts to associate genetic variation with disease susceptibility. It is generally not known what proportion of phenotypic variation is due to GEI and how many and which genes contribute to GEI. Behaviors are complex traits that mediate interactions with the environment and, thus, are ideally suited for studies of GEI. Olfactory behavior in Drosophila melanogaster presents an opportunity to systematically dissect GEI, since large numbers of genetically identical individuals can be reared under defined environmental conditions and the olfactory system of Drosophila and its behavioral response to odorants have been well characterized. We assessed variation in olfactory behavior in a population of 41 wild-derived inbred lines and asked to what extent different larval-rearing environments would influence adult olfactory behavior and whether GEI is a minor or major contributing source of phenotypic variation. We found that approximately 50% of phenotypic variation in adult olfactory behavior is attributable to GEI. In contrast, transcriptional analysis revealed that only 20 genes show GEI at the level of gene expression [false discovery rate (FDR) < 0.05], some of which are associated with physiological responses to environmental chemicals. Quantitative complementation tests with piggyBac-tagged mutants for 2 of these genes (CG9664 and Transferrin 1) demonstrate that genes that show transcriptional GEI are candidate genes for olfactory behavior and that GEI at the level of gene expression is correlated with GEI at the level of phenotype.

Figure 1.—

Diagram of the experimental design.

Figure 2.—

Reaction norms for olfactory behavior of adult flies reared in different larval environments among wild-derived inbred lines. Crossing over of reaction norms indicates GEI. The olfactory behavior scores are plotted on the y-axis. The x-axis designates the rearing environments. S, T, and A denote standard, tomato, and alcohol larval food sources, respectively. (A and B) Behavior scores obtained at 0.1% (v/v) benzaldehyde. (C and D) Behavior scores obtained at 0.3% (v/v) benzaldehyde. A and C show reaction norms for males. B and D show reaction norms for females. Lines are color coded according to their hierarchical clustering profile shown in Figure 3.

Figure 3.—

Hierarchical clustering for olfactory behavioral measurements at 0.3% (v/v) benzaldehyde pooled for sexes. The eight major clusters are color coded to the reaction norms in Figure 2. The numbers of the eight lines chosen from each cluster for the microarray analysis are highlighted in color.

Figure 4.—

Correlations between qRT–PCR and expression microarray signals. The y-axis shows the log transformed raw microarray signal values for the six genes. The x-axis shows the number of PCR cycles needed to reach the SYBR green detection threshold.

Figure 5.—

Quantitative complementation tests for GEI for Transferrin 1 and CG9664 mutants. Least mean squares (mutant − control) calculated for the line × genotype interaction term is plotted along the y-axis. The numbers designate the eight lines used for expression microarray analysis (see also Figure 3). Blue, yellow, and red bars indicate behavior of flies reared on alcohol, standard, and tomato food sources. Solid bars indicate female olfactory behavior and thatched bars indicate male olfactory behavior.