“Selection for Long and Short Sleep Duration in Drosophila Melanogaster Reveals the Complex Genetic Network Underlying Natural Variation in Sleep”, Susan T. Harbison, Yazmin L. Serrano Negron, Nancy F. Hansen, Amanda S. Lobell2017-11-01 (, , ; backlinks)⁠:

[available as the Sleep Inbred Panel] Why do some individuals need more sleep than others? Forward mutagenesis screens in flies using engineered mutations have established a clear genetic component to sleep duration, revealing mutants that convey very long or short sleep. Whether such extreme long or short sleep could exist in natural populations was unknown.

We applied artificial selection for high and low night sleep duration to an outbred population of Drosophila melanogaster for 13 generations.

At the end of the selection procedure, night sleep duration diverged by 9.97 hours in the long and short sleeper populations, and 24-hour sleep was reduced to 3.3 hours in the short sleepers. Neither long nor short sleeper lifespan differed appreciably from controls, suggesting little physiological consequences to being an extreme long or short sleeper.

Whole genome sequence data from seven generations of selection revealed several hundred thousand changes in allele frequencies at polymorphic loci across the genome. Combining the data from long and short sleeper populations across generations in a logistic regression implicated 126 polymorphisms in 80 candidate genes, and we confirmed three of these genes and a larger genomic region with mutant and chromosomal deficiency tests, respectively. Many of these genes could be connected in a single network based on previously known physical and genetic interactions. Candidate genes have known roles in several classic, highly conserved developmental and signaling pathways—EGFR, Wnt, Hippo, and MAPK.

The involvement of highly pleiotropic pathway genes suggests that sleep duration in natural populations can be influenced by a wide variety of biological processes, which may be why the purpose of sleep has been so elusive.

One of the biggest mysteries in biology is the need to sleep. Sleep duration has an underlying genetic basis, suggesting that very long and short sleep times could be bred for experimentally. How far can sleep duration be driven up or down? Here we achieved extremely long and short night sleep duration by subjecting a wild-derived population of Drosophila melanogaster to an experimental breeding program. At the end of the breeding program, long sleepers averaged 9.97 hours more nightly sleep than short sleepers. We analyzed whole-genome sequences from seven generations of the experimental breeding to identify allele frequencies that diverged between long and short sleepers, and verified genes and genomic regions with mutation and deficiency testing. These alleles map to classic developmental and signaling pathways, implicating many diverse processes that potentially affect sleep duration.

…Sleep was measured in 100 virgin males and 100 virgin females of each population each generation. The 25% most extreme long (short) sleepers were chosen as parents for the next generation of the long (short) sleeping populations. Control populations were maintained by choosing 25% of the males and females at random to be parents for the next generation. Night sleep duration, defined as sleep during the lights-off period, ranges 0–12 hours (720 minutes).

Unselected control populations averaged 495.9 ± 11.71 (replicate 1) and 364.9 ± 11.99 (replicate 2) minutes of night sleep at generation 13 and were not statistically-significantly different from night sleep in the outbred population prior to selection (Figure 1A; S3 Table).

…Flies responded rapidly and dramatically to 13 generations of artificial selection (Figure 1A; p = 0.0002; S1 and S2 Tables). Night sleep in the short-sleeping populations was reduced to 111.9 ± 10.74 minutes (replicate 1) and 54.8 ± 5.66 minutes (replicate 2) by generation 13. In contrast, night sleep in the long-sleeping populations was increased to 685.0 ± 3.35 (replicate 1) and 678.5 ± 3.46 minutes (replicate 2) in the same generation. Night sleep differed by 598.4 minutes (9.97 hours) on average between long sleepers and short sleepers. The phenotypic response was moderately asymmetrical in the direction of decreased night sleep (p = 0.0344; Figure 1A).

Figure 1: Phenotypic response to artificial selection for night sleep duration. (A), combined-sex average night sleep duration ± SE is plotted for each generation of selection; (B), combined-sex night sleep coefficient of environmental variation (CVE) is plotted for each generation of selection; (C) & (D), combined-sex cumulative selection differential (ΣS) versus combined-sex cumulative response (ΣR) for (C) long sleep and (D) short sleep populations; (E), combined-sex cumulative differential (ΣD) versus combined-sex cumulative response (ΣR) for the control populations. Light blue and dark blue triangles indicate Replicate 1 and Replicate 2 populations selected for long sleep; Light red and dark red squares indicate Replicate 1 and Replicate 2 populations selected for short sleep; and light gray and black circles indicate Replicate 1 and Replicate 2 control populations.

…The estimated realized heritabilities h2, which indicate the degree to which the animals responded to the selection procedure, were relatively high for long-sleepers;65, 68–70 h2 = 0.310 ± 0.022 and h2 = 0.238 ± 0.032 (all p < 0.0001) for replicates 1 and 2, respectively (Figure 1C). For short sleepers, the realized heritabilities were h2 = 0.179 ± 0.026 and h2 = 0.215 ± 0.017 (all p < 0.0001) (Figure 1D). In addition, the regression of the control after 13 generations of breeding with random parents was not statistically-significant, {-0.108 ± 0.312 (p = 0.7368) and −0.271 ± 0.206 (p = 0.2161) for replicates 1 and 2 (Figure 1E)}, suggesting that inbreeding depression did not impact these populations.67 Thus, the outbred population, which was derived from DGRP lines with the largest mean differences in night sleep duration responded rapidly to artificial selection for long or short night sleep. This heritable response indicates that these populations will be informative for identifying genes and pathways involved in night sleep duration.

…Response of life history traits to selection for long or short night sleep duration: Sleep is crucial for life, yet its relationship to important life history and fitness traits is not well understood. Several previous mutagenesis screens have noted reduced lifespan in mutants with short sleep duration,44, 45, 49, 51, 77, 78 though there are exceptions.51, 79 We measured lifespan in all 6 selection populations; in contrast to the reduced lifespan seen in short-sleeping mutants, we found no statistically-significant differences in lifespan for either sex in any of the selection populations (Figure 4A; S5 Table).

If we assume that sleep is associated with fitness, an asymmetrical response to selection would indicate reduced fitness in the direction of the greater response to selection.67 Thus, we would predict that short-sleeping flies would be less fit than long-sleeping ones. To investigate this possibility, we measured egg-to-adult viability as a proxy for fitness. We found no differences among selection populations (Figure 4B; S5 Table). However, we noted a propensity for flies to die during sleep monitoring in the latter generations of the experiment (Figure 4C). Over the course of the entire experiment there were no statistically-significant differences among populations in the numbers of flies surviving, but there were statistically-significant differences in survival at generations 3 (p = 0.0429), 9 (p = 0.0352) and 10 (p = 0.0455). Short-sleeping females were the most vulnerable, though flies of all populations were less likely to survive the sleep monitoring. Thus, any physiological consequences of being an extreme long or short sleeper did not manifest themselves in either lifespan or egg-to-adult viability, but the reduced survival of short sleepers during the later generations of selection suggests that they might be more susceptible to stress.

Figure 4: The response of life history traits to selection for long or short night sleep duration. (A), percentage flies surviving versus lifespan; (B), number of flies surviving to the adult stage versus generation of selection; (C), percentage of males and females surviving sleep assay. Light blue and dark blue triangles indicate Replicate 1 and Replicate 2 populations selected for long sleep; Light red and dark red squares indicate Replicate 1 and Replicate 2 populations selected for short sleep; and light gray and black circles indicate Replicate 1 and Replicate 2 control populations.

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