Connecting the circadian clock and the time-compensated sun compass to the brain of Danaus plexippus. (a) Top left: molecular mechanism that underlies the circadian clock in the Monarch within neurons of the Pars lateralis (PL). The positive transcription factors CLOCK (CLK) and CYCLE (CYC) bind to E-boxes (CACGTG) on the promoters of the period (per), and probably timeless (tim) and cryptochrome 2 (cry2) genes and activate transcription. In the evening, the PER/CRY2/TIM (P/C2/T) proteins form a complex in the cytoplasm, with PER being phosphorylated during the night phase. Further phosphorylation leads to PER/CRY2 entering the nucleus late at night and CRY2 repressing CLK-CYC mediated transcription. At dawn, light activates the CRY1 blue light photoreceptor with resulting degradation of TIM in the cytoplasm. It is not known whether TIM enters the nucleus in these neurons as CRY1-mediated TIM degradation would also provide a derepression mechanism at dawn. However, CRY2 also cycles and is at low levels during the day so it is conceivable that CRY1 also acts in the nucleus to send a degradation signal to CRY2 at dawn. Top right: light stimulates receptors to UV polarized light in the dorsal rim (DR) of the retina. A CRY1 expressing pathway (double headed, orange) connects the DR to the circadian clock in the PL (and perhaps vice-versa), which in turn uses a CRY2 pathway (blue arrow) to connect with the central body (CB), which may house the sun compass that mediates navigation. There may also be a direct pathway (pink arrow) between the DR and the compass. (b) Schematic representation of neurons and fibers expressing different circadian clock proteins in monarch butterfly brain. Regions expressing TIM, PER, CRY1 and/or CRY2 are highlighted in red. In these areas the four clock proteins partially colocalize. Areas expressing TIM or CRY1 are indicated in green. In these regions the two clock proteins do not colocalize. CRY1 positive fibers are represented by continuous orange lines. Projections of dorsal rim area photoreceptors are indicated by dotted orange lines. Neurons and fibers expressing exclusively CRY2 are represented in blue and within the central body are shown as blue circles and blue hatching. Areas positive exclusively to TIM and PER are indicated in light blue and brown, respectively. PL, pars lateralis; PI, pars intercerebralis; SOG, subesophageal ganglion; CB, central body; LO, lobula; ME, medulla; LA, lamina; RE, retina (Modified from [4,6]).