Mating behaviour in spotted wing Drosophila species: example of coordination between visual and acoustic stimuli

Understanding the mating behaviour of an insect pest is an important condition for developing successful control strategies. In Drosophila suzukii, like in most other Drosophila, males produce several types of acoustic signals to facilitate female’s mating acceptance; some of these signals are substrate-borne and are produced by abdominal vibrations. Compared to the common model D. melanogaster, the suzukii subgroup, is further characterised by the emission of a specific vibration, the so called “toot” signals, which is characterized by an harmonic frequency structure. This peculiar signal is associated with wing movements and thus is likely produced in the thorax, rather than in the abdomen like all other acoustic signals. In this work we tested whether the “toot” signal is a specific characteristic of the D. suzukii group, and if it co-evolved with another key sexual character, the presence of spots on wings. To test this hypothesis we studied the courtship strategy and associated acoustic signals in several members of the melanogaster group, by carefully comparing the behaviour of spotted wings species (D. suzukii, D. subpulchrella, D. biarmipes, D. elegans) with the behaviour of unspotted species (D. takahashi, D. melanogaster). Our results show that all species characterised by spotted wings, including D. elegans which does not belong to the suzukii subgroup, can produce a toot signal. Conversely, the “toot” signal was never recorded from unspotted wing species. During courtship, spotted wing males combine wing exposure with sound emission so that visual and acoustic cues work together to increase female acceptance. Only in one case (D. biarmipes), wing exposure and “toot” emission appeared completely unrelated. This study advances our understanding of the toot signal in Drosophila and indicates that the “toot” signal is not a clade-specific feature, but rather an ancient Dorosophila character associated with the presence of spots on wings.