Sleeping upside-down: Knockdown of a sleep-associated gene induces daytime sleep in the jellyfish <i>Cassiopea</i>

The conservation of sleep among diverse animals provides clear evidence for its physiological importance, but the extent of its regulatory conservation is unknown. The upside-down jellyfish <i>Cassiopea xamachana</i> sleeps, and this behavior is controlled by radially spaced marginal ganglia. After defining a sleep-wake threshold, we compared gene expression profiles of ganglia from animals deprived of sleep and found differential expression in many sleep-related genes including GABAergic, melatonergic, and cholinergic receptors. In particular, a nicotinic acetylcholine receptor alpha subunit-like (Chrnal-E) was strongly differentially expressed in sleep deprived animals. Animals deprived of sleep under both mechanical and light perturbations suggested <i>chrnal-E</i> as a key gene regulating sleep. We found that chemical cholinergic neuromodulators regulate pacemaker activity. Visualization of <i>chrnal-E</i> mRNA revealed it is expressed primarily within the ganglia, and that the area of expression expands after sleep deprivation. In order to confirm the functional role of <i>chrnal-E</i>, we developed RNAi for use in <i>Cassiopea</i> and determined that Chrnal-E promotes wakefulness. Finally, we field-sampled control and sleep deprived animals and found <i>chrnal-E</i> has lowest expression late at night in controls, while in sleep deprived animals, <i>chrnal-E</i> peaks at this time, supporting a link to wakefulness. Our finding that <i>Cassiopea</i> sleep is regulated by the cholinergic-like system underscores that some components of sleep regulation are deeply conserved even in an animal with a distributed nervous system.