CLOCK expression identifies developing circadian oscillator neurons in the brains of Drosophila embryos

CLK expression in the larval central nervous system is limited to circadian oscillator cells, beginning at embryonic stage 16.

• CLK is initially expressed in presumptive small ventral lateral neurons, dorsal neurons 2, and dorsal neuron 1 during early embryonic development.
• This CLK expression pattern persists throughout larval development.
• PER accumulates in all CLK-expressing cells except for presumptive dorsal neurons 2 during late embryonic stages 16 and 17.
• PER is expressed in non-CLK-expressing cells starting from embryonic stage 12, indicating a CLK-independent mechanism.
• Oscillator neurons begin their development during embryogenesis, suggesting that their intrinsic phase characteristics are established early.

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