Researchers have for the first time identified a set of genes involved in long distance migration. The turn of the seasons from summer to fall flips at least 40 genetic switches in the brains of monarch butterflies, causing the autumn generation to put off mating and hightail it for Mexico, according to a study published March 31 by the journal BMC Biology.

 

    "We've been able to find a relatively small collection of genes that are important" for migration, said Steven Reppert, a neurobiologist at the University of Massachusetts Medical School in Worchester and the study's main author. "People can now focus on those as potentially being involved in orientation behavior."

 

    Monarchs that crawl out of their chrysalises during the summer live and reproduce for two to six weeks. Butterflies that emerge in the fall, however, make less of a hormone required for reproductive development. Coupled with the suspended sexual maturation is what Reppert calls "this incredible urge to fly south." Using the sun as a guidepost, and keeping track of the time of day as their point of reference crosses the sky, the insects head for warmer weather. Autumn butterflies spend the winter in Mexico, then mature and mate before flying north again.

 

    Reppert and his colleagues treated fall monarchs with a chemical mimic of the reproductive hormone to induce sexual development. Then, the researchers analyzed their flight direction with the help of a home-made flight simulator, mounting the butterflies on a vertical wire in a 10-gallon drum, which was open on top so the insects could see the sun. Fall butterflies, even when they were made reproductively active, spent much of their time in the simulator facing south, suggesting that their instinct to head south had not abated. Summer monarchs, in contrast, fluttered in all directions.

 

    Next, Reppert and colleagues compared gene expression in summer and fall generation monarchs to a library of genes active in butterfly brains. Between reproductive and non-reproductive animals, there were 23 genes differentially activated. They also found 14 genes that ramped up expression in migratory monarchs, and 26 that were more highly expressed in non-migratory butterflies. The results suggest that reproduction and migration are separable events governed by distinct gene expression changes.

 

    "What's really exciting about this paper is it really clearly separates two important aspects of monarch biology" -- their reproduction and their migration, said Karen Oberhauser, an ecologist who studies monarchs at the University of Minnesota in St. Paul and was not involved in the study. She has noticed monarchs that emerge on the cusp of fall sometimes reproduce, but also migrate, and the current study shows how this apparent mismatch is possible. For these seasonally borderline butterflies, Oberhauser said, it makes sense to head south before laying their eggs: the offspring will have a better chance for survival in warmer climes.

 

    Among the genes Reppert and his team identified were two that stood out as relevant to migration. Autumn butterflies downregulate the expression of a gene called vrille (vri), which is a negative regulator of the circadian clock -- necessary for using the sun as a compass during their southward journey. The gene tyramine beta hydroxylase (Tbh), which plays a role in synthesizing a neurotransmitter involved in movement activity such as muscle contraction, also showed lower expression levels in fall monarchs.

 

    Fifteen of the migration genes tagged in the study had no known function, so the study gives them a tentative label as regulators of southward orientation, the authors wrote. "In fact, these may be the most exciting genes because they may have functions unique to migration," Reppert wrote in an email.