User:Itb4030/Monarch butterfly migration

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Time-compensated sun compass
The sun plays an integral role in the monarchs’ migratory patterns: monarchs travel during the day and use a circadian clock based on the position of the sun in the sky as a compass to orient themselves in the proper migratory direction. Because the position of the sun changes over the course of the day, to maintain a proper flight bearing regardless of time of day at which they travel, monarchs use a circadian clock to compensate for the changes of the position of the sun in the sky; they use what is known as a time-compensated sun compass. Various studies have shown this behavior both in natural systems and laboratory settings, yet there remains much to be researched about the underlying mechanisms for interpreting the orientation and timing cues that lead to the migratory patterns of the monarchs. Even with a time-compensated sun compass, it remains unclear with this model alone how monarchs effectively navigate to a single shared migratory location from variable starting locations.

Experimental evidence
When monarchs entrained to laboratory light-dark cycles were placed in flight simulators, or recording containers in which tethered butterflies are allowed to freely fly in the horizontal plane in all directions, migratory monarchs could integrate current sunlight conditions with their internal time of day to determine and consistently show a southward preferred direction of travel. However, when these monarchs were placed into flight simulators with six hour clock advances or delays, preferred direction of travel changed due to interference with the time-compensated sun compass. Monarchs orient to the sun based on their internal time of day, so drastic changes to the position of the sun at the same perceived time results in disrupted navigation. In such an instance, monarchs can no longer accurately identify southward travel, and, depending on the light conditions, may begin to migrate in other directions.

Molecular basis of circadian clock
The importance of the circadian clock in the function of this time-compensated sun compass system has led to investigating the molecular basis of the clock mechanism in monarchs, resulting in a well-defined model of both central and peripheral clocks. Similarly to how circadian clocks operate in Drosophila and mammals, the monarch circadian clock uses a transcription translation feedback loop (TTFL) to drive rhythms in the mRNA and protein levels of its core circadian clock components. However, the monarch mechanism has been found to be ancestral because it diverges from other clock mechanisms in the functions of its elements, some which reflect that of a Drosophila clock and some which reflect that of a mammalian clock. The most unique aspect of the monarch clock mechanism is that it involves two cryptochrome (CRY) proteins – CRY1 and CRY2 – with differing functions. CRY1 functions similarly to the CRY protein in Drosophila as a blue light photoreceptor that allows for the circadian clock to entrain to a light-dark cycle. CRY2 functions similarly to the mammalian CRY1 and CRY2 proteins in that it functions as one of the major repressors in the monarch TTFL.

In the core loop of the monarch clock mechanism, the proteins CLOCK (CLK) and BMAL1 function as heterodimeric transcription factors that drive transcription of the period (per), timeless (tim), and cry2 genes. When translated, the PER, TIM, and CRY2 proteins form complexes in the cytoplasm and, after a delay, translocate back into the nucleus, allowing CRY2 to repress transcription. After a certain amount of time passes, the PER,TIM, and CRY2 protein complex will degrade and no longer repress CLK and BMAL1, causing the TTFL to restart. Alternatively, blue light photoreception in the CRY1 protein can induces degradation in the TIM protein, which restarts the TTFL, and is how CRY1 in the monarch circadian clock gives rise to the ability to entrain to the Earth's 24 hour day cycle.

In addition to the core feedback loop, a second modulatory feedback loop has also been identified in monarchs. This feedback loop is much like the Drosophila second feedback loop and includes genes that encode orthologs of VRILLE and PDP1, which are known to regulate CLK transcription in Drosophila.

Neurobiological basis of circadian navigation
Among the better understood areas of the sun compass is the neural and anatomical layout of the underlying mechanism in the butterfly. Polarized light is first perceived by the monarch's compound eyes. This polarization, which is used by various insects for navigation, is then detected by the dorsal rim area, a specialized feature of the compound eye. These cues are then passed on to the central complex of the brain, where they are interpreted. Here, single neurons combine the azimuthal location of the sun and the e-vector angle (angle of polarized skylight). This information is then processed and further combined with other locational and orientational cues, as well as input from the monarch's circadian clock, in order to produce the oriented flight that is necessary for migratory behavior. Further research is needed in order to model the neuronal network and fully understand how spatial cues are modeled and integrated in the brain.

While neural processing occurs in the monarch's brain, research indicates that the actual circadian clock underlying the migratory patterns is located in the butterfly's antennae. Butterflies with their antennae removed showed no consistent group orientation in their migratory patterns: first exposed to a consistent light-dark cycle prior to release, antennae-less monarchs would show consistent individual directional flight, but no clear cardinal directionality as a group, unlike intact monarchs. Examination of various genes and proteins involved in circadian rhythms showed that the antennae exhibited their own circadian fluctuations, even when removed from the butterfly and studied in vitro, demonstrating that the antennae are sufficient for the generation of circadian rhythms. Further investigation into the role of the antennae has shown that even one functioning antenna is sufficient for correct orientation during migratory flight. However, two antennae with conflicting inputs to their respective circadian clocks will lead to incorrect orientation. Overall, the study of antennae-less Monarchs as well as the in vitro analysis of the antennae indicate that the antennae are both necessary for the proper functioning of the time-compensated sun compass and contain their own circadian clocks that function even without the butterfly's brain.

Bi-directionality of sun compass
Monarchs are known to use their time-compensated sun compass during both the southern migration in the fall and the northern remigration in the spring. The change in directionality necessary to re-orient the monarchs has been shown to depend on the cold temperatures that the monarchs experience while overwintering in the coniferous forests of Mexico. The change in sun compass direction does not depend on the change in photoperiod experienced during the winter months, but this change is likely to affect the timing of the northern remigration in the spring.

An experiment demonstrating the importance of cold exposure for remigration utilized fall monarchs with and without cold temperature exposure in the laboratory. The monarchs that experienced cold temperatures during the winter months successfully changed the direction of their sun compass and oriented north in the spring. In contrast, the monarchs that never experiences the cold temperatures during the winter months oriented south in the spring, and thus did not experience a change in sun compass direction to accompany their migration. Therefore, the cold exposure experienced while overwintering is required for the monarch's migration cycle.

During the northern remigration of monarchs in the spring, the time-compensated sun compass uses the same substrates as used in the fall. However, the mechanistic differences in these substrates that allows for a switch in the directionality of the compass is still unknown. RNA-sequencing differences found between the fall and spring butterflies is one avenue of research that could locate the mechanism responsible for the recalibration, which may utilize a temperature sensor to start the switch.