The Circadian Clock: Antennal Clocks

Almost 50 years ago, Fred Urquhart offered evidence that the antennae might play a role in the flight orientation of migratory monarchs. Now, we have discovered that the antennae don’t just play a role but are required for proper time-compensated sun compass orientation (Merlin et al., 2009). We have found that monarchs have antennal clocks, whose molecular clockwork is identical to that in brain clocks, that provide the primary timing mechanism for sun compass orientation.

These findings are unexpected—it was long assumed that the circadian clock that informs time-compensated sun compass orientation resides in the brain (specifically the pars lateralis). The antennal findings do not mean that the brain clocks are unimportant.  Indeed, brain clocks are likely involved in aspects of migration (e.g. reproductive arrest and longevity) that are regulated separately from navigation and in the circadian regulation of additional events, including the daily timing of adult eclosion, the sleep-wake cycle and metabolic rhythms.  Brain clocks may also be involved in sensing decreasing day length in the fall, presumably important for signaling the migration south

We found that monarchs whose antennae have been removed or painted black to block light input do not exhibit proper time- compensated sun compass orientation.  

Even when explanted and examined in vitro, antennal clocks continue to oscillate and can still be entrained by light. This indicates that they could function independently from the brain as time-compensation components for sun compass orientation.

Thus, antennae have circadian clocks that are directly entrained by light, independent from the brain, with molecular clockwork identical to that found in brain clocks, and these clocks actually provide the primary timing component of the sun compass orientation mechanism.

The novel function of antennal clocks in sun compass orientation opens a new line of investigation into clock-compass connections that may extend widely to other insects (e.g., bees) that use this orientation mechanism.