Southbound genes

A genome study in monarch butterflies pulls out a set of 40 key players in long distance migration

By | April 8, 2009

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.
Image: David R. Weaver
"We've been able to find a relatively small collection of genes that are important" for migration, said linkurl: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 linkurl: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.
**__Related stories:__***linkurl:Clocking monarch migration;
[7th January 2008]*linkurl:On the move;
[7th December 2007]


Avatar of: Ruth Rosin

Ruth Rosin

Posts: 117

April 8, 2009

According to Reppert : "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".\n\nMonarchs in the Eastern U.S., that migrate long-distance to the South, in autumn, are undoubtedly attracted to warmth;which is why the direction of their migration is affected by the direction of the sun (though not by any sun-compass). They are also attracted by the odors of the specific type of plants on which they feed, before, as well as during migration. And they are carried by major seasonal winds. \n\nThey do not, however, have any "instinctive" urge to fly South, and they do not "instinctively" plot a flight-route that would lead them to Mexico, or any other "goal" they had never visited, and know nothing about. \n\nHad they lived in the Southern hemisphere, , instead of South, in the Southern autumn. In the Western part of the U.S., e.g. in California, they never migrate to Mexico, and they do not migrate long-distance at all. Instead, in autumn they start gradually migrating further South, through several generations. And in spring they start gradually migrating back North.\n\nThe belief that Monarchs have "instincts' that govern whether they will migrate and to which "goal" they will migrate, is the product of a totally unwarranted and misguided belief in the existence of "instincts". "Instincts" were initially believed to be "heritable" behavior, which, "genetically predetermined" behavior, presumably identifiable by not being learned behavior. Instead, all individual traits (including behavioral traits) of all living organisms, develop ontogenetically (in each individual organism), under inseparable (!) effects of both (!) genes & environment; which means that "instincts" do not exist at all! In fact, by now, even those who still believe in the existence of "instincts", cannot provide any usable definition of "instinct". The term has, thus, become a meaningless word, that not a few scientists still accept as a proper solution to scientific problems.\n\nIf you read the published reports of a years-long project of capturing & tagging migrating Monarchs as they pass through Cape May, you will find that observers once saw a swarm of migrating Monarchs flying South, and shortly afterwards, they saw at the same site another swarm flying in exactly the opposite direction! You can find reports about Monarchs drowning in the Atlantic ocean. And there is even a report about a tagged Monarchs accidentally found in the Bahamas, where no one was looking for tagged Monarchs. Obviously the Northern Monarchs that migrate very long distances, are not heading to Mexico, or any other "genetically predetermined goal". Scientists who believe in the existence of "instincts", however, claim that such cases are due to the Monarchs having made errors. The Monarchs did not make any errors. Instead, it is only those very silly scientists who err!\n\nOf course, you can "prove" any hypothesis, no matter how silly, or crazy it may be, by excluding as "errors" all cases that contradict the hypothesis. Resorting to such practice, however, is outright counter-scientific.\nAs long as many scientists working in the Behavioral field , still remain faithful to their totally misguided, unwarranted faith, in the existence of "instincts", they will continue to inundate the whole field with an ever-growing amount of rubbish. Garbage in, garbage out!

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