Sleep in Populations of Drosophila Melanogaster

Abstract

The fruit fly Drosophila melanogaster is a diurnal insect active during the day with consolidated sleep at night. Social interactions between pairs of flies have been shown to affect locomotor activity patterns, but effects on locomotion and sleep patterns have not been assessed for larger populations. Here, we use a commercially available locomotor activity monitor (LAM25H) system to record and analyze sleep behavior. Surprisingly, we find that same-sex populations of flies synchronize their sleep/wake activity, resulting in a population sleep pattern, which is similar but not identical to that of isolated individuals. Like individual flies, groups of flies show circadian and homeostatic regulation of sleep, as well as sexual dimorphism in sleep pattern and sensitivity to starvation and a known sleep-disrupting mutation (amnesiac). Populations of flies, however, exhibit distinct sleep characteristics from individuals. Differences in sleep appear to be due to olfaction-dependent social interactions and change with population size and sex ratio. These data support the idea that it is possible to investigate neural mechanisms underlying the effects of population behaviors on sleep by directly looking at a large number of animals in laboratory conditions.

Keywords: Drosophila; population; sleep.

Figure 1.

Diagrams of DAM2 and LAM25H systems. A, DAM2 apparatus. Left, Side view of DAM2 sleep tube (5 × 65 mm) for individual fly recording showing location of infrared beams and food. Right, Cross-section of the tube with the orientation of the two infrared beams. B, LAM25H apparatus. Left, Side view of LAM25H vial (25 × 95 mm) for population recording showing location of infrared beams and food. Right, Cross-section of the vial with the orientation of the nine infrared beams. Dark blue bars and light blue bars indicate transmitters and receivers. Red arrow lines indicate how pairs of infrared beam sensors work, as well as the coverage of the cross-sectional area.

Figure 2.

Populations of flies exhibit sleep patterns distinct from individual flies. A, Individual fly sleep for males and females. B, Sleep in populations of males and females. C, Quantification of total sleep from A and B. Individual males slept more than females. In populations males slept longer during the day, but less at night. D, Activity levels during wake periods. Males had more beam breaks than females in populations. E, Number of sleep episodes. Individual females had more sleep episodes than males, but populations were indistinguishable. F, Mean episode length. Females had shorter episodes than individual males, but no significant difference was detected in populations. G, Sleep Latency. Individual male flies took shorter time to fall asleep after light transitions than individual female flies, but no significant difference was found between populations of males and females. n = 32 for individuals and n = 8 groups for populations. Statistically similar groups are marked by the same letter, with different letters indicating significant differences between groups. F, female; M, male; ZT, Zeitgeber time.

Figure 3.

Populations exhibit homeostatic rebound sleep after mechanical sleep deprivation. A, Sleep profiles of individual female flies (n = 48 and n = 43 for non-SD and SD, respectively) were recorded using DAM2. B, Sleep profiles of groups of 50 female flies (n = 16 groups for both non-SD and SD) were captured using LAM25H. Red bar indicates the sleep deprivation period in both experiments. In B, the absence of data points for the SD groups during the SD period is because of the need to remove the population vials from the monitor during shaking (see Materials and Methods). C, Quantification of recovery day sleep. Day time sleep increased significantly on the recovery day after 12 h of sleep deprivation by mechanical shaking. Sleep changes were normalized to the baseline day. Δ Total sleep: total sleep changes. ZT, Zeitgeber time; SD, sleep deprivation. ***p < 0.0001; n.s., no significant difference.

Figure 4.

Suppression of sleep by starvation generates rebound sleep in populations. Sleep patterns generated by starvation in female (A) and male (B) flies in populations. C, Total daytime and nighttime sleep changes are plotted as mean ± SEM. Male flies’ sleep was reduced significantly during the day and night, but female flies’ sleep was significantly suppressed only in the night. Red bar indicates the starvation period. Twenty-four hour starvation-induced sleep loss was compensated after feeding on the recovery day. Δ Total sleep: total sleep changes. n = 8 for all conditions. ***p < 0.0001; n.s., no significant difference. ZT, Zeitgeber time; F, female; M, male.

Figure 5.

amn¹ Mutant flies housed in populations show a fragmented sleep pattern, similar to that of amn¹ individuals. Sleep profiles of amn¹ mutant flies compared with wild-type Canton S flies in individuals (A) and populations (B), respectively. C, Quantification of data. amn¹ mutant flies slept less than wild-type flies in population as well as individuals. D, Activity during waking. Populations of mutant flies were hyperactive during the light period but hypoactive in the dark compared with controls; however, no difference was detected in individual flies. E, Number of sleep episodes. Sleep episodes increased significantly in populations of amn¹ mutant flies compared with controls at night consistent with individuals, but exhibited the opposite phenotype during light period. F, Episode length. Populations of mutant flies did not show significant difference in sleep episode length where individual mutant flies decreased dramatically compared to wild-type. G, Latency. amn¹ mutant flies exhibited similar latency compared to wild type flies at night in both individuals and populations. n = 8 groups for both wild-type and amn¹ populations. n = 31 and n = 32, respectively, for wild-type and amn¹ individuals. ZT, Zeitgeber time.

Figure 6.

Populations of flies sleep better with complete food. Sleep profiles for populations of females (A) and males (B) on different food. C, Quantification of total sleep. Both female and male populations of flies slept significantly longer when on standard fly food compared with sucrose during the day, but there was no statistically significant difference during the night. D, Activity while awake. Complete food significantly increased activity levels during daytime wake periods in males, and at night in females. E, Number of sleep episodes. F, Sleep bout length. Females, but not males, had significantly consolidated sleep at night, i.e., fewer but longer sleep episodes. G, Latency. Females fell asleep faster on the complete food than on the sucrose agar food, whereas males exhibited similar latency on both food media. n = 8 groups for all conditions. ZT, Zeitgeber time; F, female; M, male.

Figure 7.

Sleep is affected by population size and social behavior. A, Sleep profiles for female populations of different sizes. B, Sleep profiles for populations of males and females (1:1 ratio of sexes). C, Quantification of total sleep. Total sleep was decreased significantly with increasing number of flies and mixed populations with the same number of total flies exhibit lower sleep than populations of female flies. D, Activity while awake. Increasing the number of flies increases population activity. E, Number of sleep episodes. The number of episodes scales with population size in opposite directions for female only and mixed populations. F, Sleep latency does not change significantly with population size. n = 5–6 groups for all conditions. ZT, Zeitgeber time.

Figure 8.

The ratio of male–female flies in mixed populations affect total sleep. Two experiments were done to test the effects of changes in sex ratio. A, Different ratios from 0 to 100% male were tested. Data are quantified in B. C, Small changes in ratios around the extremes were tested. Data are quantified in D. Mixed populations of flies had generally lower sleep than female or male same sex populations. Small changes in the number of males or females affected sleep most significantly at the extremes. To view sleep profiles clearly, error bars were omitted from A and C. n = 5–7 groups for all conditions. ZT, Zeitgeber time.

Figure 9.

Olfactory input modulates sleep amount by influencing social interactions. Sleep profiles of individual female (A) or male (B) flies. C, Quantification of data. Orco² mutants slept significantly less than w controls during the day, and male Orco² mutants slept longer than w males at night. No significant difference was detected between individual female Orco² mutants and w. n = 30–32 for all genotypes. Population sleep profiles for female (D), male (E), and 1:1 mixed-sex populations (F). G, Quantification of data. Total sleep in populations of Orco² mutant flies was similar to w controls within the male and female groups during the day and night. However, mixed female and male populations of Orco² mutants exhibited drastically elevated sleep compared with w controls during the night. n = 5–6 groups for all genotypes. ZT, Zeitgeber time; F, female; M, male.