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Gall midge olfaction and its role in speciation

Tina Boddum

With the swede midge (Contarinia nasturtii) as our main model species, we study two types of olfactory cues that are of importance for gall midges: 1) the pheromones emitted by the female to attract the male; 2) and the host plant volatiles that the females use when finding a host for oviposition.

Structures of the swede midge pheromone components

We found that both the blend of compounds and the enantioisomeric form are important for male attraction in the wind tunnel and in the field. For pheromone reception, the gall midges use the sensillum type that display sexual dimorphism, male swede midge use the gall midge specific sensilla circumfila while the Hessian fly (Mayetiola destructor) use s. trichodea.

Transmission electron microscopy of male swede midget sensilla circumfila, the innervating cell (A and B), the sensilla pores (C1) and the area in which the two adjacent sensilla fused (C2). Olfactory cells in Hessian fly s. trichodea (D), the sensilla pores (D2) and their support cells (E).

In a detailed study of the female host finding behavior, we found that olfactory cues are important for the swede midge host selection. However, this "first impression" can be modulated by later plant characters, such as the physical defense of the host plant, or which host plants are available. By using the electrophysiological technique GC-EAD, we compared the response of 12 gall midge species, including the swede midge, to a blend of 45 plant volatiles to explore the relative impact of host plant chemistry, life-history strategies and the midge phylogeny on the gall midge host plant recognition system.

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