Genetic architecture of olfactory behavior in Drosophila melanogaster: differences and similarities across development

Abstract

In the holometabolous insect Drosophila melanogaster, genetic, physiological and anatomical aspects of olfaction are well known in the adult stage, while larval stages olfactory behavior has received some attention it has been less studied than its adult counterpart. Most of these studies focus on olfactory receptor (Or) genes that produce peripheral odor recognition. In this paper, through a loss-of-function screen using P-element inserted lines and also by means of expression analyses of larval olfaction candidate genes, we extended the uncovering of the genetic underpinnings of D. melanogaster larval olfactory behavior by demonstrating that larval olfactory behavior is, in addition to Or genes, orchestrated by numerous genes with diverse functions. Also, our results point out that the genetic architecture of olfactory behavior in D. melanogaster presents a dynamic and changing organization across environments and ontogeny.

Figure 1

Frequency distribution of mean LRI among P[GT1] insertion lines. Black bars represent LRI scores of lines tested in response to nonanol, gray bars in response to propionic acid and striped bars in response to benzaldehyde.

Figure 2

Expression of candidate genes affecting larval olfactory behavior in second instar larvae. Images of GFP expression of whole F1 larvae from a cross between P-element insertion lines and UAS-CD8::GFP line are shown. fs(1)h: ventral view of the head, expression in one of the brain hemispheres and in the cone-shaped ventral nerve cord (indicated by the white arrow). eas: ventral view of the head, expression in the anterior segment of the tracheal system (white arrows). CG32572: dorsal view of the head, expression in the region corresponding to ganglia located below the larval sense organs (arrow). Rtnl1: dorsal view of the head, expression in a longitudinal nerve (white arrow) innervating peripheral sensory organs. dnr1: lateral view, expression in the cone-shaped ventral nerve cord (white arrow). bin3: dorsal view of the head, expression in brain hemispheres and in the cone-shaped ventral nerve cord (horizontal white arrow) and the anterior segment of the tracheal system (vertical white arrows). chinmo: dorsal view of the head, expression in the anterior segment of the tracheal system (white arrow). spn / msn: ventral view of the head, expression in the brain hemispheres and the cone-shaped ventral nerve cord (white arrow). scyl: dorsal view of the head, expression in the brain hemispheres and the cone-shaped ventral nerve cord (white arrow). Contemporaneously to experimental genotypes, positive and negative controls were run; and all controls worked correctly (not shown).

Figure 3

Expression of candidate gene transcripts for D. melanogaster larval olfactory behavior. Levels of mRNA for candidate genes in P-element insertion lines (grey bars) and control Canton S-B lines (white bars) are shown. mRNA levels were measured in first, second and third instar larvae. C_T values were normalized to an internal control (Gapdh gene). Standard errors were obtained from the normalized C_T value of three independent biological replicates per line and stage. Significant differences in gene expression level are indicated by asterisks (ANOVA, *: p < 0.05, **: p < 0.01, ***: p < 0.001).

Figure 4

Comparisons between genes implicated in olfactory behavior of larvae and adult stages of D. melanogaster life cycle. Candidate genes implicated in larval olfactory behavior are bordered by the dashed line box while genes implicated in adult olfactory behavior according to Sambandan et al. (2009) are shown in the solid line box. The intersection of both boxes shows candidate genes implicated in both larval and adult olfactory behavior, these genes were identified in our study and by Sambandan et al. (2009), Moreau-Fauvarque et al. (2002) and MacDonald et al. (2006). Only genes tested for both larval and adult olfactory behavior are included in the diagram.