The Fly Nose -Function and Evolution

Stensmyr, M.C. 2004. The Fly Nose – Function and Evolution. Doctoral dissertation. ISSN 1401-6249, ISBN 91-576-6756-X This thesis summarizes and discusses the results of four separate studies on fly olfaction. The aim of the thesis has been to investigate how odor information is decoded by the fly peripheral olfactory system, how this code has evolved and how it is used by the insects in their daily life. Here I show that insect odorant receptors (ORs), as indicated by the response profile of olfactory receptor neurons (ORNs), are narrowly tuned to specific compounds. The ligands of the system are volatile chemicals characteristic for favored or rejected traits of preferred or avoided resources. These compounds each carry important information. By relying on key-ligands even a narrowly tuned system comprising a fairly small number of ORs can be used to locate and evaluate a large number of resources in complex odor environments. This conclusion is supported by experimental data contained in the four publications comprising this thesis. In the first study we show that fruitfly ORNs respond most selectively to generic fruit and fungal volatiles, typical for favored characteristics of the flies’ preferred resources. In the second study we show how the olfactory code has evolved among the eight close relatives of D. melanogaster. The evolutionary pattern we observe illustrates how drastic alterations in odor space and food choice can have direct effects on specific ORNs tuned to important key-ligands. The concept of key-ligand tuning is further nicely illustrated in the third and fourth study by the deceptive pollination system of the dead horse arum. This plant copies in remarkable detail a cadaver in order to attract carrion blowflies. Of particular importance is the chemical mimicry, through which the plant copies three specific cadaver volatiles. Our study shows that these three compounds are the sole mean through which blowfly identify carrion, even though carrion produces a large range of volatile chemicals. The odor mimicry is accompanied by further adaptations reinforcing the carrion mimicry, among which heat is most important. Our study provides rare evidence for a direct functional role of plant thermogeny as we show that the generated heat (up to 20°C above ambient temperature) is important for fine tuning the behavior of the flies.