Chemical communication in the genus Leptopilina (Hymenoptera: Figitidae), a parasitoid of Drosophila

Animal communication systems employ a variety of different modalities, including visual, acoustic, electrical, and chemical stimuli. In insects, the use of chemicals for information transfer, the so-called semiochemicals, is a common phenomenon. In this doctoral thesis, I investigate the chemical communication of the genus Leptopilina, a genus of solitary parasitoids of Drosophila. I mainly study the sexual communication of the closely related species L. heterotoma, L. boulardi, and L. victoriae, with particular emphasis on mate attraction, mate recognition, and receptiveness elicitation. In chapter 2, I identify the female mate attraction pheromone of L. heterotoma. The pheromone is composed of several iridoids; its main component is (-)-iridomyrmecin, which is also shown to mediate competition avoidance in host-searching females. Because (-)-iridomyrmecin has previously been identified as defensive compound of L. heterotoma, an evolutionary route can be hypothesised, leading from the defensive function to competition avoidance and mate attraction pheromone accompanied by an increase in specificity and complexity of the semiochemical blend. In chapter 3, I investigate the mating frequency and post-mating attractiveness of L. heterotoma females. Previous studies by different authors mentioned that females are monandrous and lose their attractiveness after mating. These reports, however, rely on incidental observations rather than dedicated experiments. I show, through a series of mating trials that L. heterotoma females are indeed monandrous, but do not lose their attractiveness. The found monandry implies the need for species-specific mate recognition in L. heterotoma, and the post-mating attractiveness is interpreted as a side effect of the parsimonous use of iridoids in L. heterotoma. In chapter 4, I study the species specificity and chemical diversity of the female mate recognition pheromones of L. heterotoma, L. boulardi, and L. victoriae. Mating trials with conspecific and heterospecific females show that mate recognition is highly species specific in the three species. I then continue to investigate the chemical composition of the female mate recognition pheromones. In L. heterotoma, mate recognition is mediated by iridoids alone, whereas in L. victoriae, cuticular hydrocarbons are of major importance. The picture is yet different in L. boulardi: iridoids and cuticular hydrocarbons are equally important in mate recognition. The chemical diversity of the mate recognition pheromone very likely ensures its species specificity. In chapter 5, I show the species specificity of the putative male antennal aphrodisiac pheromones in L. heterotoma, L. boulardi, and L. victoriae. For this, I devise a setup, in which the odour profile of heterospecific females is manipulated, so that the females are recognized as conspecifics by males. Males readily court heterospecifics in this setup, but can elicit receptiveness only in conspecific females. The identity of the putative aphrodisiac pheromones is not investigated, but the setup can likely be used to test candidate compounds for their behavioural activity. The results presented in this thesis provide new insights into the evolution of chemical communication in Leptopilina. The suggested evolutionary route from defensive compound to mate attraction pheromone in L. heterotoma strongly supports the hypothesis that communicative functions can evolve for non-communicative compounds---and that thus the composition of chemical signals may be partially predestined by an inventory of non-communicative compounds. A similar inventory of such non-communicative compounds, however, does not necessarily lead to the same pheromone composition. In the investigated Leptopilina species, the female mate recognition pheromones differ greatly between the species. This supports the hypothesis that signals under strong stabilizing selection can evolve through saltational shifts, thus allowing rather drastic changes despite strong stabilisation. Building upon the new insights presented here, the chemical communication of more Leptopilina species should now be investigated to further advance our understanding of the evolution of chemical communication