Koustav Maiti, Ramakrishna Mission Vivekananda Centenary College, Rahara
The monarch butterflies ( Danaus plexippus ) are very special migratory insects, traveling a vertebrate-like long distance of 4000 km, each year to reach their overwintering ground in Central Mexico. These butterflies use various survival strategies during migration. At low temperatures, they roost in the tree branches in cluster form and use Batesian mimicry to save themselves from predators. But, the most interesting fact is how they migrate from generation to generation, following the same path.
The Sun plays a vital role in extracting the right directional information for the migration and is used as a sun compass by the monarch butterflies. Decreasing day length and fall of temperature can affect some environmental events, which stimulate the migration from generation to generation. Ultraviolet opsin-expressing photoreceptors, present in the monarch’s eyes, are responsible for detecting the polarized light at the time of migration. Due to Earth’s daily rotation on its axis every 24 h, the sun also seems to move from east to west, which hinders the monarch butterfly from reaching its destination. So to compensate the time for the sun compass system, the monarch uses another biochemical oscillator, known as the circadian clock. Together, the process of using a sun compass and a circadian clock is called a time-compensated sun compass.
The monarch butterfly has 2 different types of Cryptochrome (CRY) proteins – Type 1 Drosophila-like CRY (CRY 1), and Type 2 Vertebrate-like CRY (CRY2). 4 cells found in the dorsolateral region of the central brain in the butterfly plays a key role in circadian clock regulation. These brain cells cycle the transcriptional factor CRY 2 to regulate the feedback loop of the circadian clock. One type of CRY 1 –expressing fibers connected to pars lateralis (PL) may regulate diapause and aging and the other type of CRY 1-staining fibers (PL to optic medulla) may indicate a brain clock modulation of polarized light input. This clock may also help to determine the decreasing day length that insists them migrate towards the South.
The compensation clock is also found in the antennae of the monarch. If the antennae are removed or twisted, they can’t migrate in the proper direction. So, the antennae are also provided with another circadian clock apart from the brain that is entrained by light. This provides the primary timing component of the sun compass.
Like many long-distance migrants, the monarch also uses the Earth’s magnetic field (Magnetic compass) for proper navigation during migration. The components like polarity, intensity, and the inclination angle can be used to determine their current position concerning their final destination. Magnetite (Fe3O4), iron oxide particles are found in monarch as substrates to sense the Earth’s magnetic field. Both the types of CRY present in monarch i.e. CRY 1 and CRY 2, can be used as a transducer to collect the magnetic information.
There may be either a genetic or epigenetic basis of migration as all the monarchs can migrate during summer and spring if the same environment can be seen. Epigenetic events are involved in histone modification or DNA methylation.
If we understand the molecular mechanism of navigation of monarch butterfly migration, we can easily know the molecular and neural basis of other long-distance migrants. But there are much more to know about the molecular and genetic basis of migration of monarch butterfly.
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Reference
1. Zhan S, Merlin C, Boore JL, Reppert SM. The monarch butterfly genome yields insights into long-distance migration. Cell. 2011;147(5):1171-1185. doi:10.1016/j.cell.2011.09.052
2. Steven M. Reppert, Robert J. Gegear, Christine Merlin. (2010). Navigational Mechanisms of Migrating Monarch Butterflies. Trends in Neurosciences 33, 399-406; doi: https://doi.org/10.1016/j.tins.2010.04.004
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