<i>Drosophila</i> Avoids Parasitoids by Sensing Their Semiochemicals via a Dedicated Olfactory Circuit

<div><p>Detecting danger is one of the foremost tasks for a neural system. Larval parasitoids constitute clear danger to <i>Drosophila</i>, as up to 80% of fly larvae become parasitized in nature. We show that <i>Drosophila melanogaster</i> larvae and adults avoid sites smelling of the main parasitoid enemies, <i>Leptopilina</i> wasps. This avoidance is mediated via a highly specific olfactory sensory neuron (OSN) type. While the larval OSN expresses the olfactory receptor Or49a and is tuned to the <i>Leptopilina</i> odor iridomyrmecin, the adult expresses both Or49a and Or85f and in addition detects the wasp odors actinidine and nepetalactol. The information is transferred via projection neurons to a specific part of the lateral horn known to be involved in mediating avoidance. <i>Drosophila</i> has thus developed a dedicated circuit to detect a life-threatening enemy based on the smell of its semiochemicals. Such an enemy-detecting olfactory circuit has earlier only been characterized in mice and nematodes.</p></div

Larvae and ovipositing flies are repelled by parasitoid odor.

<p>(<b>A</b>) Larval choice assay and preference indices when larvae were exposed to the wash of <i>L</i>. <i>boulardi</i>. (<b>B</b>) Different choice assays (T-maze, Trap assay, Oviposition assay) for adult flies and resulting preference indices when exposed to the wash of <i>L</i>. <i>boulardi</i>. PI = (number of larvae, flies, or eggs in odor side − number in control side) / total number. Bar plots indicate minimum and maximum values (whiskers), the upper and lower quartiles (boxes) and the median values (bold black line). Deviation of the indices against zero was tested with Wilcoxon rank sum test.</p

The ab10B neuron is necessary and sufficient to govern oviposition avoidance and larval avoidance behavior in <i>D</i>. <i>melanogaster</i>.

<p>(<b>A</b>) Preference indices of ovipositing wildtype flies, flies expressing <i>Shibire</i>^<i>ts</i> in ab10B neuron, and corresponding parental lines at restrictive (30°C) and permissive (23°C) temperature when tested with wash of <i>L</i>. <i>boulardi</i>. (<b>B</b>) Preference indices of the same fly lines when tested in the larval assay. Attraction to ethyl butyrate (grey bars) depict that loss of odor-guided behavior in larvae expressing <i>Shibire</i>^<i>ts</i> in ab10B neuron is odorant specific. (<b>C</b>) Light preference of ovipositing wildtype flies, flies expressing channelrhodopsin in ab10B neuron, and corresponding parental lines. (<b>D</b>) Light preferences of the same fly lines when tested in the larval assay. (<b>A–D</b>) Bar plots indicate minimum and maximum values (whiskers), the upper and lower quartiles (boxes), and the median values (bold black line). Groups were compared by the Kruskal Wallis test with a Dunn’s multiple comparison for selected pairs. For calculation of preference indices, see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002318#pbio.1002318.g001" target="_blank">Fig 1</a>.</p

The ab10B neuron detects parasitoid odors.

<p>(<b>A</b>) Example spike traces of GC-coupled SSR with all <i>D</i>. <i>melanogaster</i> OSN types and the headspace of <i>L</i>. <i>boulardi</i> (note that the amount of odors within headspace is too low to be detected and analyzed by GC, but is still detected by ab10B). FID, flame ionization detector. (<b>B</b>) GC-coupled SSR with the ab10B neuron and the wash of <i>L</i>. <i>boulardi</i> (1st panel), as well as the identified active compounds (2nd–4th panel). (<b>C</b>) SSR dose-response curves of the ab10B neuron tested with active compounds. (<b>D</b>) GC-coupled SSR with mutant ab3A neuron ectopically expressing either Or49a or Or85f. Blue, green, and red lines indicate active compounds. (<b>E</b>) Tuning breadths of Or49a and Or85f. 232 odorants are displayed along the <i>x</i>-axis according to strengths of responses they elicit from each receptor. Odorants eliciting strongest responses are placed near the center of distribution. Negative values indicate inhibitory responses. For a list of compounds, see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002318#pbio.1002318.s005" target="_blank">S4 Fig</a>; for raw data see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002318#pbio.1002318.s001" target="_blank">S1 Data</a>. (<b>F</b>) Identification of glomeruli activated by parasitoid odors (-)-iridomyrmecin, (<i>R</i>)-actinidine, and nepetalactol (a mixture of 1S4aR7R7aS, 1R4aS7S7aS-nepetalactol and their enantiomers). 1st to 3rd columns, false color-coded images showing odorant-induced calcium-dependent fluorescence changes in OSNs expressing Or49a or PNs labeled by GH-146-Gal4 at the antennal lobe (AL) level. Flies express UAS-GCaMP3.0 under control of either Or49a-Gal4, or the GH146-Gal4 driver line. (<b>G</b>) GC-coupled extracellular recordings from larval dorsal organ and wash of <i>L</i>. <i>boulardi</i>. (for more GC-SSR traces of wildtype ab10B neurons and mutant ab3A neurons expressing Or49a or Or85f see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1002318#pbio.1002318.s004" target="_blank">S3 Fig</a>)</p

Innervation patterns of DL4 and DA2 PNs in MB and LH.

<p>(<b>A</b>) Reconstruction of two DA2 PNs. (<b>B</b>) Reconstruction of two DL4 PNs. (<b>C</b>) comparison of DA2 and DL4 domains after registration of datasets into a common reference space. DA2 and DL4 PNs overlap in the base of the MB and ventroposterior LH. a: anterior, d: dorsal, l: lateral, p: posterior v: ventral.</p