Come fall, monarch butterflies feel the need for a change in latitude. A new study shows that changes in the activity of a suite of genes in the butterflies’ brains help the insects find their way to overwintering grounds in Mexico.
Steven Reppert, a neurobiologist from the University of Massachusetts Medical School in Worcester, leads a team of scientists on a mission to uncover the monarch’s migratory signals. The team describes a new genetic analysis of stationary summer monarchs and fall migratory monarchs in the March 31 BMC Biology.
At least 40 genes are involved in keeping the monarchs Mexico-bound once they start migrating, the researchers report. Reppert and his colleagues analyzed 9,000 of the monarch butterfly’s genes, about half of the genes in its genome.
Each fall, hundreds of millions of monarchs in the eastern United States and Canada begin flying south for the winter. The butterflies navigate with internal clocks and use the sun as a compass to find their way to overwintering grounds in oyamel fir forests in central Mexico. No one knows what environmental signals flip the switch that causes butterflies to forgo reproduction and start migrating.
Reppert and his colleagues collected monarch butterflies in the summer and fall. As researchers expected, stationary summer butterflies had high levels of a reproductive hormone called juvenile hormone, while migratory fall butterflies are deficient in the hormone. Summer butterflies stay near the place they hatched and reproduce, laying their eggs on milkweed plants. And summer butterflies don’t orient themselves according to the sun, so it was possible that the hormone could block navigation. The researchers gave some of the fall butterflies a chemical that mimics juvenile hormone and then placed them in a flight simulator to see whether this was the case. But the fall butterflies still had accurate compasses, indicating that reproduction and navigation are controlled by separate systems.
Next, the scientists analyzed gene activity differences in summer and fall butterflies. The researchers found 40 genes showed changes in activity between the summer and fall butterflies. Of those genes, 14 were more active in fall butterflies and 26 were more active in summer butterflies. Only two of the genes had any obvious, known connection to migration — a gene called vrille, which is part of the butterfly’s circadian clock, and the tyramine beta hydroxylase gene, which is involved in motor behavior. The other genes are involved in metabolic processes, brain development and other processes, and 15 genes have unknown functions.
“Nothing stood out and said, ‘I’m a migration gene,’ ” Reppert says.
The 40 genes Reppert’s team found are involved in helping the butterflies orient to the sun while migrating, the team concludes. Though some may also participate in flipping the migratory switch, this study didn’t address that question.
While it is disappointing that none of the genes is definitively linked to navigation, Reppert’s group is on the right track to figure out how migrating monarchs find their way south, says Orley “Chip” Taylor, an insect ecologist at the University of Kansas in Lawrence, and director of Monarch Watch.
“I think they’re doing exactly what needs to be done to unravel all of this,” Taylor says.
Reppert says he plans to analyze activity of the remaining genes in the monarch genome and to pinpoint where in the brain gene activity changes.
