Neuroanatomy of cells expressing clock genes in Drosophila: transgenic manipulation of the period and timeless genes to mark the perikarya of circadian pacemaker neurons and their projections.

Visualization of per and tim-expressing neurons revealed a previously unknown neuronal cluster in the adult brain.

• Neurons expressing the clock genes period (per) and timeless (tim) contribute to circadian rhythms.
• The GAL4-expression system was used to visualize all per and tim-expressing neurons in the central nervous system.
• New projections of these neurons were identified, including a previously unknown neuronal cluster located dorsally.
• The development of these neuronal projections appears to begin during the larval stages.
• Other neurons not previously identified as PER- or TIM-immunoreactive were also visualized, suggesting clock gene activity in these cells.

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