Monarch Butterflies Migration
The annual migration of the North American Monarch butterflies (Danaus plexippus Linnaeus) has always fascinated nature-lovers and scientists alike. Every year, in autumn, millions of Monarch butterflies from east of the Rockies and from northern Nova Scotia travel over 5000 kms, flying at the rate of 50 km per hour during the day for 8-10 weeks, to the Oyamel forests in the Neovolcanic Mountains in Mexico. Here, safe from the harsh northern winters, the butterflies rest and mate. When spring comes, they fly back north, breeding along the way. The butterflies go through three or four generations before it’s time to migrate southwards again in the fall.
Monarch butterflies have short life-spans, and the butterflies that migrate are doing so for the first time. How do these fragile creatures know winters will be harsh and food in low supply? How do they know when exactly to start migrating and where to go? How do they navigate so unerringly to Mexico? Even when they veer off-course, they can find their way to their destination, often finding the exact tree on which their forebearers roosted. How do they do this?
Genetic Origin behind Monarch Butterfly Migration
According to the behavioural and genetic analytical research work carried out by Steven Reppert and his team from the University of Massachusetts Medical School, the migratory behavior of Monarch butterflies has a genetic origin. That is, it is coded in the butterfly’s brain that at a certain time it must travel to a certain destination. This genetic make-up possibly evolved with the geographical spread of milkweed plants - Monarch butterflies love milkweed plants and lay eggs on them.
A Monarch butterfly has an internal circadian clock that is affected by the changes in the atmospheric light. The shorter hours of daylight as winter approaches tells the butterfly that it is now time to travel. The circadian clock has a molecular mechanism that helps the butterfly navigate accurately using the sun’s position in the sky.
The researchers found that these clocks are loops in which the cryptochrome proteins, CRY1 and CRY2, are made and destroyed over a 24 hour period. CRY1 is similar to the cryptochrome protein found in the fruit fly (Drosophila) and CRY2 is similar to the one found in mice. CRY1 allows light access to the circadian clock and enables the light-dark cycle. CRY2 helps with the loop function.
The presence of both these two differently functioning proteins in the Monarch butterfly indicates that perhaps the Monarch butterfly circadian clock, in the evolution process, preceded the clocks in the fruit fly and the mouse.
The researchers also noted the presence of the Juvenile Hormone (JH) in Monarch butterflies. Non-migratory or summer butterflies had high levels of JH and the migratory or fall butterflies were found deficient in it. The absence of JH leads to the migratory butterflies having smaller reproductive organs. This ensures they live longer and do not mate and lay eggs enroute to Mexico.
By experimenting with butterflies in a flight simulator, the researchers found that the JH levels are not connected to the orientated flight behaviour. It seems seasonal changes in genomic functions help define the migratory state.
The researchers compared the genes of migratory and non-migratory Monarchs, and found that 40 genes showed activity-related changes. A majority of these genes were concerned with metabolic and developmental processes, while only two appeared to be related to the migratory aspect. These two genes are vrille and tyramine beta hydroxylase; vrille occurs in the clock cells and tyramine beta hydroxylase helps with motor behavior.