Missed Part 1? Have a gander here
For researchers attempting to unravel the story of the Monarch migration, the fact that the circadian clocks in the brain played no part in migration came provided a surprising twist. The functions that did seem to be lost when the brain clock proteins were silenced highlighted that while they were not involved in orientation, they were important in Monarch migration. To facilitate their long journey, migrants undergo a series of changes: they lose their ability to reproduce, known as reproductive diapause, see increased longevity and alter their metabolism to use fat reserves as sources of energy. All of these, as well as other normal circadian rhythms crucial to Monarch life cycles seem to be mediated by the brain clocks.
This still left the hunt for the time-compensation clocks very much on. There was, however, only one real candidate. Given the varied and crucial sensory roles played by the antennae, it was little surprise when cells located there exhibited similar oscillations in circadian proteins to those in the central complex in the brain. Interestingly, when the function of the antennae was blocked, by painting them black, Monarchs appeared disorientated, unable to use their sun compass properly.
Slowly, the increasingly complicated picture of just how Monarchs completed this vast journey was beginning to be pieced together. Researchers knew how Monarchs compensated for the movement of the Sun in their sun compass and further probing the Monarch’s eyes yielded clues as to just how the position of the Sun was interpreted. It appeared that two sections of the eye performed two different tasks that together delivered information regarding the position of the Sun in the sky. It appears that the main retina is responsible for detecting the Solar azimuth, the direction the Sun is in, with 90° at due East and -90° at due West, and that the outer section of the eye, known as the dorsal rim which contains specialised light-detecting molecules known as pteropsin, is responsible for receiving directional cues from the angle to which ultraviolet light from the Sun is polarised.
There were now three established migratory mechanisms: the circadian clocks in the brain and the antennae, which control general circadian processes and sun compass compensation and the dual-functioning eyes, that receive direction cues from the position of the Sun (these processes are summarised in the image below). What wasn’t yet clear, and still remains to be determined, is how and where all of these signals are integrated. What is known as that these architectures do interact with one another with the end result being a signal to the motor system to facilitate orientated flight.
At this point researchers were greeted with a new problem – though an integrated system of cellular processes underpinning this migration had taken shape, it was clear that this was not the whole story. It had long been known that migrants could correctly orientate themselves under thick cloud – this needed little research, given that stretches of the 4000 km journey were undoubtedly not going to be cloudless skies. It was clear that there were more mechanisms at play than just the time-compensated sunlight orientation.
Certain migratory species have been shown to be able to use the Earth’s magnetic field for geolocation. The familiar depiction of the Earth’s magnetic field shows that it originates in the Southern hemisphere, before looping and re-entering the planet in the Northern hemisphere. Therefore, at different latitudes on the planet, the field can be detected as originating or returning at a certain angle (see below). Detection of this angle, known as the inclination angle, could give an accurate determination of where exactly the Monarchs were – a map sense complimentary to the sun compass.
The role of magnetism in the orientation of Monarchs can be exquisitely demonstrated using flight simulators. Monarchs orientate away from the North Pole when exposed to conditions mimicking positive inclinations and orientate in the reverse direction in negative inclinations (see below). Essentially, these experiments suggested that the further North in the Northern Hemisphere Monarchs are, the more likely they are to orientate South, while the further South they are, the more likely they are to orientate North (mimicking their migratory behaviour). A role for magnetism in this process would add the final piece in the puzzle of monarch migration.
The story of these small creatures’ mighty migration concludes tomorrow…
‘Til next time,
1. Reppert SM, Gegear RJ, Merlin C. Navigational Mechanisms of Migrating Monarch Butterflies. Trends Neurosci [Internet]. 2010 Sep 2;33(9):399–406. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2929297/
2. Zhan S, Merlin C, Boore JL, Reppert SM. The Monarch Butterfly Genome Yields Insights into Long-Distance Migration. Cell [Internet]. Elsevier; 2016 Jun 1;147(5):1171–85. Available from: http://dx.doi.org/10.1016/j.cell.2011.09.052
3. Guerra PA, Reppert SM. Sensory basis of lepidopteran migration: focus on the monarch butterfly. Curr Opin Neurobiol [Internet]. 2015 Oct;34:20–8. Available from: http://www.sciencedirect.com/science/article/pii/S0959438815000185
Image sources are given in figure legends. All rights and credit goes to original authors.