To characterize Drosophila mating drive, 3-day-old, WT Canton-S males were tested in a 2-D satiety assay. Over the course of the assay (4.5 h), males mate an average of 4.8 ±0.3 (mean ±standard error of mean, SEM) times. Matings initiate mostly in the first 2 h (78%) (Figure 6A, 6B) and become less frequent as the assay progresses (Figure 6A, 6B). This decrease is not due to the lack of mating partners (74% females remain unmated throughout the assay) or physical fatigue (Figure 6F). Rather, this effect is likely explained by a decrease in the male's courtship during the assay, from 42.6 ±8.0% (mean ±SEM) (1st h) to 2.0 ±0.7% (mean ±SEM) (last hour) of nonmating time (Figure 6C). This decline in courtship is also seen when the males are tested in courtship assays with new females (Figure 6D, 6E), and recovers after the males have been isolated from females for 3 d (Figure 6G, 6H). These results show that internally maintained mating drive in male flies can be satiated in a 2-D satiety assay and can recover over time.
The 2-D satiety assay can also be easily combined with Drosophila neural manipulations. We recently reported that thermogenetic stimulation of dopaminergic neurons reverses mating drive in satiated male flies25. Using 2-D satiety assays, we find that dopaminergic stimulation (TH>TrpA1, 28.5 °C) at the end of the satiety assay increased both courtship (Figure 7A, 7C, red) and copulation (Figure 7A, 7B, red). The reversal effects are not observed with parental control genotypes (Figure 7A - 7C, black and gray). All of these results are consistent with the recent study25 and suggest that the 2-D satiety assay, along with the auxiliary assays (courtship and locomotion), can be used to dissect molecular and neuronal components of mating drive.

Figure 1. Designing and Assembling Behavioral Arenas. An 8-chamber arena (used for 2-D satiety assays) consists of 6 layers held together by thumb nuts and hex screws (A). A 32-chamber arena (used for courtship assays) is manufactured similarly, but without the food wall (layer 4). The arena parts are cut out with a laser cutter and the layers are numbered as (B) for 8-chamber and (C) for 32-chamber. The assembled 8-chamber arenas are shown in (D) (front view) and (E) (side view with numbered layers). The assembled 32-chamber arenas assembled in a similar fashion (F, G), but Layer 4 (food wall) is omitted (G) since no fly food is in the arena for courtship assays. Please click here to view a larger version of this figure.

Figure 2. Preparing 2-D Satiety Assays with Food. Standard Drosophila food is placed in a microwave-safe jar with water (A). The food is microwaved until melted (B). A blunted pipette tip (C, arrow) is used to transfer food to chambers of a partially assembled behavioral arena (D). The food is re-solidified at 4 °C (E) before the behavioral arena is completely assembled (F). Please click here to view a larger version of this figure.

Figure 3: Generating Virgin Females from the w1118(X)/hs-hid(Y) Stock. Fly bottles should be heat shocked when 50 - 80% of the pupae are uneclosed as indicated by their opaqueness (A). The inset image shows samples of uneclosed (top) and eclosed (bottom) pupae (A). Bottles are weighed down and submerged in 37 °C hot water bath for 1 h with water level just above the bottom of the bottle stoppers to ensure even heating (B). See the black arrow in (A) for waterline. Please click here to view a larger version of this figure.

Figure 4: Loading Flies into a Behavioral Arena. Gently aspirate and hold 15 - 20 females in the aspirator (A). The pipette tip is used to open the rotating door (B), and the flies gently released into the chamber (C). Use the aspirator to close the rotating door (D). Please click here to view a larger version of this figure.

Figure 5: Performing and Recording a 2-D Satiety Assay. (A) A standard consumer camcorder (a) is used to record the assay in an incubator set to the desired temperature. The incubator contains a flask of water (b) and a humidity detector (c) to ensure the appropriate humidity level (>30%). The prepared 2-D satiety assay is filmed under the camcorder (B). Please click here to view a larger version of this figure.

Figure 6: Satiation and Recovery of Fy Mating Drive. In a 2-D satiety assay, male flies mate frequently during the first 2 hours of the assay (A, B) but decrease their courtship (C) and matings (B) as the assay progresses. The progressive decrease in courtship behavior is maintained when males are transferred to a courtship assay with new females after completing a 2-D satiety assay of 0 h, or 1 h, or 4.5 h (D, E). Red arrows point to males exhibiting courtship and mating behaviors, while orange arrows point to non-mating, non-courting flies (A, D). The decline in sexual behaviors is not a result of physical exhaustion, as males show equivalent levels of locomotor activity before and after the 2-D satiety assay (F). Males gradually recover their mating (G) and courtship (H) levels over 3 d of isolation from females. In this figure, ***p <0.001, **p <0.01, n.s. not significant for t-test (C, F) and one-way ANOVA with Tukey post-test (E, G, H). N = 15 - 16 for each condition for all experiments. Error bars represent standard error of the mean (SEM). Please click here to view a larger version of this figure.

Figure 7: Thermogenetic Satiety Reversal in Male Flies. As in the standard 2-D satiety assay, males show a decrease in matings over the course of the experiment, but thermogenetic stimulation of dopaminergic neurons (TH>TrpA1, red), but not in parental-control genotypes (black and gray), reinstates mating drive in satiated males (A). No matings are scored when the X-axis is broken in (A). This reversal of mating drive can be quantified using either mating numbers (B) or courtship (C). X-axis numbers in (B) and (C) refer to time in (A). Orange background color indicates thermogenetic stimulation (A - C). In this figure, ***p <0.001, n.s. not significant for interactions between genotype and temperature in two-way ANOVA with Bonferroni post-test (B, C). N = 8 for each genotype (B, C). Error bars represent SEM. Please click here to view a larger version of this figure.
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