HOW PLANKTON GETS JET LAGGED
A hormone that governs
sleep and jet lag in humans may also drive the mass migration of plankton in
the ocean, scientists at the European Molecular Biology Laboratory (EMBL) in
Heidelberg, Germany, have found. The molecule in question, melatonin, is
essential to maintain our daily rhythm, and the European scientists have now
discovered that it governs the nightly migration of a plankton species from the
surface to deeper waters. The findings, published online today in Cell,
indicate that melatonin's role in controlling daily rhythms probably evolved
early in the history of animals, and hold hints to how our sleep patterns may
have evolved.
In vertebrates,
melatonin is known to play a key role in controlling daily activity patterns --
patterns which get thrown out of synch when we fly across time zones, leading
to jet lag. But virtually all animals have melatonin. What is its role in other
species, and how did it evolve the task of promoting sleep? To find out, Detlev
Arendt's lab at EMBL turned to the marine ragworm Platynereis dumerilii. This
worm's larvae take part in what has been described as the planet's biggest
migration, in terms of biomass: the daily vertical movement of plankton in the
ocean. By beating a set of microscopic 'flippers' -- cilia -- arranged in a
belt around its midline, the worm larvae are able to migrate toward the sea's
surface every day. They reach the surface at dusk, and then throughout the
night they settle back down to deeper waters, where they are sheltered from
damaging UV rays at the height of day.
"We found that a
group of multitasking cells in the brains of these larvae that sense light also
run an internal clock and make melatonin at night." says Detlev Arendt,
who led the research. "So we think that melatonin is the message these
cells produce at night to regulate the activity of other neurons that
ultimately drive day-night rhythmic behaviour."
Maria Antonietta
Tosches, a postdoc in Arendt's lab, discovered a group of specialised motor
neurons that respond to melatonin. Using modern molecular sensors, she was able
to visualise the activity of these neurons in the larva's brain, and found that
it changes radically from day to night. The night-time production of melatonin
drives changes in these neurons' activity, which in turn cause the larva's cilia
to take long pauses from beating. Thanks to these extended pauses, the larva
slowly sinks down. During the day, no melatonin is produced, the cilia pause
less, and the larva swims upwards.
"When we exposed
the larvae to melatonin during the day, they switched towards night-time
behaviour," says Tosches, "it's as if they were jet lagged."
The work strongly
suggests that the light-sensing, melatonin-producing cells at the heart of this
larva's nightly migration have evolutionary relatives in the human brain. This
implies that the cells that control our rhythms of sleep and wakefulness may
have first evolved in the ocean, hundreds of millions of years ago, in response
to pressure to move away from the sun.
"Step by step we
can elucidate the evolutionary origin of key functions of our brain. The
fascinating picture emerges that human biology finds its roots in some deeply
conserved, fundamental aspects of ocean ecology that dominated life on Earth
since ancient evolutionary times," Arendt concludes.
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