A single male auditory response test to quantify auditory behavioral responses in Drosophila melanogaster.

Many animals utilize auditory signals to communicate with conspecific individuals. During courtship, males of the fruit fly Drosophila melanogaster and related species produce a courtship song comprised of sine and pulse songs by vibrating their wings. The pulse song increases female receptivity and male courtship activity, indicating that it functions as a sexual signal. One song parameter, interpulse interval (IPI), varies among closely related species. In D. melanogaster, a song with a conspecific IPI induces a stronger behavioral response than heterospecific songs, indicating the ability of the flies to discriminate conspecific IPI. Traditionally, the fly's response to the song is measured under grouped conditions, in which the effect of sensory modalities other than audition cannot be excluded. Here, to quantify the individual ability to discriminate a conspecific song, we systematically analyzed the auditory response of single male flies to sound with various parameters. Moreover, we applied this method, termed SMART (Single Male Auditory Response Test), to two sister species for potential application in a comparative approach. By quantifying the locomotor activity of single D. melanogaster males during sound exposure, we detected increased locomotor activity in response to pulse songs, but not to white noise or pure tone. The conspecific song evoked stronger response than the heterospecific songs, and ablation of their antennal receivers severely suppressed the locomotor increase. A pulse song with a small IPI variation evoked a continuous response, while the response to songs with highly variable IPIs tends to be rapidly decayed. This provides the first evidence that fruit flies discriminate IPI variations, which possibly inform the age and social contexts of the singer. Sister species, D. sechellia, exhibited a locomotor response to pulse song, while D. simulans exhibited no behavioral response. This suggests that auditory and other stimuli that elicit this behavioral response are diversified among Drosophila species.

Evolutionary conservation and diversification of auditory neural circuits that process courtship songs in Drosophila.

Acoustic communication signals diversify even on short evolutionary time scales. To understand how the auditory system underlying acoustic communication could evolve, we conducted a systematic comparison of the early stages of the auditory neural circuit involved in song information processing between closely-related fruit-fly species. Male Drosophila melanogaster and D. simulans produce different sound signals during mating rituals, known as courtship songs. Female flies from these species selectively increase their receptivity when they hear songs with conspecific temporal patterns. Here, we firstly confirmed interspecific differences in temporal pattern preferences; D. simulans preferred pulse songs with longer intervals than D. melanogaster. Primary and secondary song-relay neurons, JO neurons and AMMC-B1 neurons, shared similar morphology and neurotransmitters between species. The temporal pattern preferences of AMMC-B1 neurons were also relatively similar between species, with slight but significant differences in their band-pass properties. Although the shift direction of the response property matched that of the behavior, these differences are not large enough to explain behavioral differences in song preferences. This study enhances our understanding of the conservation and diversification of the architecture of the early-stage neural circuit which processes acoustic communication signals.