The neuropeptide pigment-dispersing factor (PDF) is a key transmitter in the circadian clock of Drosophila melanogaster. Here we studied the rhythmic behavior of neural mutants with modified arborizations of the large PDF neurons. In sine oculis1 (so1) mutants we found a higher density of PDF fibers in the fly's pacemaker center, the accessory medulla. These flies exhibited a significantly longer period (24.6 h) than control flies. When PDF levels were elevated to very high levels in the dorsal brain as true for somda mutants and small optic lobes;so1 double mutants (sol1;so 1), a short-period component split off the long period in behavioral rhythmicity. The short period became shorter the higher the amount of PDF in this brain region and reached a value of ~21 h. The period alterations were clearly dependent on PDF, because so1;Pdf 01 and somda;Pdf 01 double mutants showed a single free-running component with a period similar to Pdf 01 mutants (~22.5 h) and significantly longer than the short period of somda mutants. These observations indicate that PDF feeds back on the clock neurons and changes their period. Obviously, PDF lengthens the period of some clock neurons and shortens that of others.
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