The millions of monarch butterflies (Danaus plexippus) that flit on fragile wings from North America to fir forests in Mexico have evolved a slew of special adaptations to allow this arduous flight, which can be as far as 4,000 kilometers. Now the draft genome of the species, published in the November 2011 Cell, suggests how genetic adaptations allow these lovely insects to survive their long journey.

Butterflies’ circadian clocks help them sense decreasing day length and trigger the migration, says study co-author Steven Reppert, a neurobiologist at the University of Massachusetts. The genome reveals new information about the molecular control of these mechanisms.

The butterflies have a large number of olfactory receptor genes, which, when activated in the antennae, might help them interact with other monarchs to find their destination.

Genes involved in eye development might help the butterflies detect fine changes in the sun’s position, as well as patterns of polarized light. These subtle differences most likely assist them in staying on track to their faraway wintertime destination.

Reproductive organs
Not all monarchs migrate, but those that do lack a key enzyme that produces the juvenile hormone, which stimulates the reproductive organs. Lacking this keeps the butterflies underdeveloped and disinterested in sex so they can focus on their flight.