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. 2015 Sep 2;87(5):1036-49.
doi: 10.1016/j.neuron.2015.07.025. Epub 2015 Aug 13.

Multimodal Chemosensory Circuits Controlling Male Courtship in Drosophila

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Free PMC article

Multimodal Chemosensory Circuits Controlling Male Courtship in Drosophila

E Josephine Clowney et al. Neuron. .
Free PMC article

Abstract

Throughout the animal kingdom, internal states generate long-lasting and self-perpetuating chains of behavior. In Drosophila, males instinctively pursue females with a lengthy and elaborate courtship ritual triggered by activation of sexually dimorphic P1 interneurons. Gustatory pheromones are thought to activate P1 neurons but the circuit mechanisms that dictate their sensory responses to gate entry into courtship remain unknown. Here, we use circuit mapping and in vivo functional imaging techniques to trace gustatory and olfactory pheromone circuits to their point of convergence onto P1 neurons and reveal how their combined input underlies selective tuning to appropriate sexual partners. We identify inhibition, even in response to courtship-promoting pheromones, as a key circuit element that tunes and tempers P1 neuron activity. Our results suggest a circuit mechanism in which balanced excitation and inhibition underlie discrimination of prospective mates and stringently regulate the transition to courtship in Drosophila.

Figures

Figure 1
Figure 1. P1 neuron chemosensory tuning correlates with mate preference
(A–B) Male courtship behavior towards virgin, conspecific females plotted as the male’s distance to female (red) and his unilateral wing extension (grey) for 10 individual pairs (A) and population average, smoothed by 10 second sliding window (B). The top 3 individuals have courtship latencies of less than 2 minutes; the last pair mate within two minutes of courtship onset. (C) Courtship behaviors plotted as in (B) toward moving magnet in males expressing ReaChR in P1 neurons using P1-Gal4 (solid lines) or no-Gal4 controls (dashed lines), n=7 each genotype. See Figure S1A. (D) P1 neuron anatomy revealed by photoactivation of PA-GFP expressed under FruGal4; box indicates P1 processes in the lateral protocerebral complex imaged in subsequent figures. Scale bar here and throughout 10 µm. All animals imaged in this study are male. Autofluorescence from the glial sheath and basal fluorescence from non-photoactivated structures have been masked. (E) Schematic of preparation used for in vivo functional imaging. See also Figure S1B and Extended Experimental Procedures. (F–G) Representative P1 neuron GCaMP responses recorded in the same male as he contacts the abdomens of female (F) or male (G) stimulus flies. Top row shows still images from the video used to guide stimulus presentation (illuminated by infrared light, male’s eye pseudocolored red) and heat map of fluorescence increase in P1 neurons for a single tapping bout. Bottom row shows normalized fluorescence traces for six bouts of tapping, indicated by tick marks (left), and zoomed in view of a single bout (right). Here and throughout, stimulating females are sexually naïve unless otherwise indicated. (H–I) Summary of P1 neuron responses to male and female stimuli across repeated stimulus bouts in 17 subject animals. Individual stimulus bouts (H) and paired intra-animal averages (I) are shown. Significance, unpaired (H) or paired (I) t-test. Here and throughout, *: p<.05, **: p<.01, ***: p<.001, ****: p<.0001. Red lines in (H) indicate median and quartiles and in (I), means. (J) Mean P1 neuron responses to indicated fly stimuli measured by in vivo imaging as in F-G. One-way ANOVA with Tukey’s correction for multiple comparisons. Here and throughout, different statistical letter groups indicate p<.05. n=40–46 flies for D. melanogaster male and virgin female stimuli, n=9–12 flies for other stimuli. See also Figure S1C–F. Error bars here and throughout, SEM.
Figure 2
Figure 2. Gustatory pathways transmitting pheromone signals from the foreleg to courtship centers in the higher brain
(A) Schematic (top) and two-photon stacks (bottom) of sensory innervation in the first thoracic ganglion (T1) of the ventral nerve cord (VNC). Second (T2), third (T3) and abdominal ganglia (Ab) denoted. (B–C) Maximum intensity two-photon stacks with vAB3 neurons labeled by photoactivation of PA-GFP expressed in FruGal4 (B) and the genetic intersection of AbdBLDN-Gal4 and FruLexA (C). Autofluorescence from the glial sheath, basal fluorescence from non-photoactivated structures, and out-of-plane Fru+ soma have been masked. (D) Models of pheromone circuit depicted in this figure; boxed area indicates central brain region imaged in subsequent panels. (E–F) Representative multi-plane imaging of GCaMP responses in Fru+ neurons (E, n=3) or all neurons (F, n=7) in response to acetylcholine iontophoresis onto ppk25+ terminals in the VNC of an explant preparation. Controls and quantification, Figure S2. (G) vAB3 neurons labeled by electroporation of Texas Red Dextran and mAL neurons labeled by photoactivation of PA-GFP overlap in the sub-esophageal zone (SEZ). Autofluorescence from the glial sheath and basal fluorescence from non-photoactivated structures have been masked. (H–I) Representative multi-plane imaging of Fru+ neurons expressing GCaMP in response to stimulation of ppk25 gustatory neurons (H, n=7) or vAB3 neurons (I, n=3) expressing the P2X2 channel and activated by local application of ATP in the VNC.
Figure 3
Figure 3. Excitatory vAB3 neurons and inhibitory mAL neurons anatomically and functionally converge onto P1 neurons
(A) Models of pheromone pathways depicted in this figure. (B–C) P1 projections interdigitate with vAB3 and mAL axons in the lateral protocerebral complex. (B) shows P1 neurons labeled red by R71G01-Gal4 expressing Tomato and vAB3 projections labeled green by photoactivation using PA-GFP expressed in FruGal4. (C) shows P1 neurons labeled green using P1-Gal4 to express GFP and mAL neurons labeled red by electroporation of Texas Red Dextran into their axonal tract. Autofluoresence from the glial sheath has been masked. (D–F) Functional imaging of P1 neurons in response to stimulation of vAB3 neurons by acetylcholine iontophoresis, prior to (D) and after (E) severing the mAL axonal tract. P1 neurons express GCaMP under R71G01-Gal4 and mAL axons were labeled using FruLexA to express Tomato. Representative GCaMP responses and results of photodamage are shown in (D–E); summary of paired P1 responses in individual animals (n=13) shown in (F). Significance, paired t-test.
Figure 4
Figure 4. Differential pheromone tuning of vAB3 and mAL neurons
(A–D) Functional responses of vAB3 expressing GCaMP under AbdBLDN-Gal4 (A, C) or mAL expressing GCaMP under R25E04-Gal4 (B, D) to foreleg contact with male or female stimuli. (A–B) Representative heat map and fluorescence traces. (C–D) Individual bout responses for all animals (n=46–104 bouts) and paired mean responses (n=10 animals for vAB3, 6 animals for mAL). See also Figure S4E–G. (E) Circuit models depicting potential routes for pheromone signaling in response to a female stimulus (left) or male stimulus (right). (F) P1 activity evoked by female stimulation with the number of times the male tapped the female in each bout shown. (G) P1 responses evoked by foreleg contact to a female stimulus are not correlated with the number of taps within a bout. 45 bouts scored from 8 experiments. Red line indicates linear fit. See also Figure S4H, I. (H–I) P1 response prior to or after severing of the mAL tract to increasing stimulation of vAB3 by acetylcholine iontophoresis. (H) shows representative experiment. (I) plots inferred relationship between vAB3 and mAL responses (n=7, grey) and linear fit (red). vAB3 activity at each voltage was inferred from the P1 response after mAL severing (orange points, H). mAL activity at each voltage was inferred from P1 response in intact circuit subtracted from the P1 response after mAL severing (blue points, H)
Figure 5
Figure 5. Olfactory signals inhibit but do not excite P1 neurons
(A–B) Representative traces of P1 neurons expressing GCaMP in response to stimulus approach and touch (A) and mean P1 responses aligned to first touch (B, n=38–39 tapping bouts from imaging 7 males). Gray traces here and below define distance between imaged and stimulus flies. (C) Representative responses of P1 neurons to indicated stimuli before and after severing foreleg tarsi. (D–F) Representative P1 responses to indicated fly stimuli in intact males (D) or males without antennae (E). (F) Pooled P1 responses (n=4 tapping bouts per stimulus in 7–16 imaged males). Two-way ANOVA with Sidak correction for multiple comparisons.
Figure 6
Figure 6. P1 neurons integrate gustatory and olfactory signals to encode mate desirability
(A) Circuit model of cVA pathway to P1 neurons (B–D) DC1 neurons (green, B, D) or LC1 neurons (green, C) were labeled by photoactivation of PA-GFP expressed under FruLexA; P1 neurons (red, B, C) were labeled by expression of Tomato using R71G01-Gal4. DA1 projection neurons (PNs) were labeled by Texas Red Dextran electroporation (red, D). Arrows indicate lateral horn (site of DA1 projection neuron synapses with DC1 and LC1), arrowheads highlight overlapping neurites in the lateral protocerebral complex. Boxed areas indicate anatomic sites imaged in (E, F). Fluorescence from nonphotoactivated structures has been masked. (E–F) Responses of DA1 neurons (E) and DC1 neurons (F) to approach of a single fly. GCaMP was expressed using FruGal4. Representative responses (left) and average of ~18 stimuli from 5–7 animals, aligned to when the stimulus is 4mm from the male’s antennae (right). (G) Representative P1 neuron responses to interleaved touch of virgin females or virgin females perfumed with cVA, in males with or without an intact Or67d olfactory receptor (left). Pooled tapping responses (2–4 per animal from 5–6 animals of each genotype) and median response shown at right. Letter groups show results of two-way ANOVA with Tukey’s correction. (H) Imaging of P1 neurons expressing GCaMP in response to stimulation of vAB3 and/or DA1. Representative multi-plane GCaMP image (left, 3 planes, ~10um) and paired comparison across 8 flies (right). mAL was severed to maximize P1 responses to vAB3. Significance: paired t-test.
Figure 7
Figure 7. Circuit mechanisms regulating P1 neuron response to social stimuli
Olfactory and gustatory afferents converge functionally on P1 neurons in the lateral protocerebral complex. Dashed lines indicate potential functional connections not explored in this study. P1 neurons drive male courtship and are active during the male’s enactment of courtship.

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