In the mountains of central Mexico, monarchs spend winter huddled on trees and begin to stir at the start of spring. “The sun will shine on a particular part of the canopy and after a few seconds, a burst of thousands of monarchs will leave and start their migration north," says Green. Photo by Frans Lanting, Mint Images / Science Photo Library
This is an article from the spring 2020 issue of LSA Magazine. Read more stories from the magazine.
There’s this constant hum, sometimes a whoosh.”
D. André Green, assistant professor in the Department of Ecology and Evolutionary Biology, describes standing at the forests where monarchs live for the winter in the mountains of central Mexico. “You look up and think you’re just looking at bark,” Green says. “But then you see flickers of orange and realize the trees are covered with millions of monarchs.”
Green studies how complex traits, like migration, evolve. In his efforts to better understand how migration works, Green has found monarchs to be an ideal subject, allowing his research to traverse molecular to evolutionary scales and back again. “We already know a bit about the natural history of monarchs,” Green says. “What if I can start to introduce newer research methods to understand how molecules connect to this migratory phenomenon we already see and experience every year?”
People Love Butterflies
We all know the monarch’s story. Children learn the butterfly life cycle from picture books. Tourists can watch new butterflies emerge from their chrysalises and take their first flights in conservatories. But none of these rivals the real drama of the journey monarchs make each year.
Every summer, monarchs in eastern North America reproduce, search for milkweed — their host plant — and lay their eggs. Caterpillars grow and pupate and emerge as matured butterflies.
And every fall monarchs — each weighing less than a gram — fly as far as 2,500 miles to reach specific sites in the states of Mexico and Michoacán, where they spend the winter huddled together on trees. By the start of spring the monarchs stir, fall in pairs to the ground as they mate, and fly from the trees in large bursts before journeying northward. “A swirl of orange is constantly flying around you,” Green says, describing the experience of observing the monarchs firsthand. Over the span of three to four months — and several generations — the monarchs repopulate the northern United States and Canada.
Or that’s what we thought. Now Green’s research has begun to complicate this tidy narrative. One of the fundamental questions he studies lies in the fact that most monarchs in the world live outside North America and don’t migrate. “You’ll find them in different tropical regions and island systems,” says Green. “Because they’re in places where milkweed survives year-round, there’s no impetus to migrate.” Green wants to understand if these monarchs are genetically incapable of migration, or if they’re just not receiving the right environmental cues.
By bringing non-migratory monarchs from Guam to his lab in Michigan, Green studies what happens on the molecular level when these monarchs receive signals often understood to prompt migration. But, as Green explains, there’s still a lot left unknown. “We can manipulate variables like sun and temperature easily in the lab. And we still haven’t been able to create a migrant, which suggests we’re missing something.”
From Pipettes to Milkweed
In addition to traveling to Mexico and Guam, Green and his students collect monarchs from across Michigan, which are housed at the Matthaei Botanical Gardens and used for experiments. There, they feed the monarchs, provide extra heat, clean their cages, and tend to the milkweed. This labor is a vital part of his research, and one Green enjoys. “I say to my students all the time, ‘We have to pay attention to the butterflies and listen to what they’re telling us.’ Are they happy or not? People underestimate just how difficult the monarchs are to keep happy, especially through a Michigan winter.”
Green’s research combines these practices from evolutionary biology and ecology with techniques from his molecular background. This includes lab work — measuring gene expression and RNA sequencing, or transcriptomics (“pipetting a lot of clear liquids in very, very small volumes,” Green says) — and a hefty computational component to code large data sets.
Green’s desire to understand how complex biological systems evolve started when he was a kid growing up in rural Louisiana. “I loved doing research for science fairs,” he says. “I was always outside digging up random things. There were lots of bugs down there.” Photos by Austin Thomason / Michigan Photography
As Green’s research encompasses the molecular and evolutionary, his perspective has similarly evolved. “As a molecular biologist, I used to think, ‘I can study the monarch in isolation to understand its migration.’ But this work has expanded my view of how particular problems interact. Can I truly understand how migration works without understanding other animals the monarchs interact with and how those interactions change the milkweed quality? I’ve started to appreciate these questions.”
These questions reflect Green’s perspective of science more broadly, too. “We tend to pursue biology in silos,” he explains. “You can study entire populations on global scales or you can study biophysics or biochemistry. I love the idea of connecting these segments.” He stresses the slow, piecemeal nature of the scientific process, but insists opportunities exist — with the monarch and with other insects, animals, and plants — that allow for a more integrative and holistic approach to research. “We have the tools and opportunity to start to bridge different levels of biological organization and present that as a model for how we think life works, at least on this planet.”
Green is bringing this philosophy to his next project: tracking individual migrating monarchs. He is collaborating with a team from the College of Engineering to design microsensors that will be placed onto individual monarchs without disrupting their life cycles or flights. The microsensors are added in Michigan and collect data as the butterflies travel to Mexico. Though the complexity of the project poses tremendous difficulty, Green is optimistic. “What we can potentially learn by having a lens that combines my background with an engineer’s expertise would be amazing.”
Green’s research has received a lot of attention. (Remember: People love butterflies.) But as Green’s own perspective has metamorphosed, so too has his understanding of the story that initially helped catalyze his research. “I love talking about butterflies and migration, and I want this information to be accessible. But instead of oversimplifying, we need to recognize that there’s still a ton we don’t know.
“The monarch story is still sensational without sensationalizing it.”
Surprise in Diapause
At the beginning of spring, environmental cues alert monarchs that it’s time to fly north. But LSA Professor D. André Green’s study suggests that, for the monarchs, survival is a delicate balancing act.
D. André Green’s palpable excitement and integrative approach have recently led to unexpected results in his research. He started studying monarchs during a postdoctoral fellowship at the University of Chicago in 2015. At that point, there weren’t any monarchs in the lab, so Green turned to published papers to find inspiration for an experiment he might perform during winter. That’s when he found a study about diapause, the hormonally controlled state that helps monarchs survive winter.
The common story was that monarchs, once they reached Mexico, spent the entire winter in diapause, until they’re eventually roused at the start of spring. But the results of this paper showed that diapause actually ended in the middle of winter.
Green’s curiosity led him to design an experiment using gene expression to investigate the why and how of this paper’s findings. He recognized that monarchs were either responding to some environmental cue other than spring’s longer days and warmer temperatures or they had an internal timer that “wakes them” at a particular time in winter. “As these things go,” Green says, “it turned out to be both.”
Although understood in developmental biology to slow down metabolic processes, Green’s study showed, rather counterintuitively, that colder temperatures accelerated diapause. Similar results have been found with many other insect systems, and the fact that this counterintuitive result is commonly seen in this phenomenon and had no specific mechanistic explanation excited Green. The study also showed that monarchs do have an internal timer — monarchs don’t use light to measure time, so how they do experience it is still largely unknown — and provided initial insight into how a timer like that could work.
D. André Green’s research suggests that monarchs need — and might even seek out — the ideal environment to survive winter. If the conditions are either too warm or too cold, monarchs could come out of diapause too early.
What could happen to monarchs if their environment is too warm?
• They come out of diapause too early
• They could run through their fat storage too quickly, and might not have enough sustenance to re-migrate northwards
What about if it’s too cold?
• They come out of diapause too early
• They could freeze!
• They could lose important immune defense and potentially be vulnerable to infection
These findings complicate the accepted understanding of how the environment affects monarchs. “The idea was always that the overwintering sites need to be as cool as possible without the monarchs freezing, because warmer temperatures mean monarchs run through their fat storage quickly, which leads to higher mortality,” says Green. “But my study suggested that too cold temperatures could also negatively impact the butterflies. As we continue to study monarchs, we need to consider diapause timing, since it’s dependent upon temperature control.”
Diapause is a protective state. In addition to reducing metabolism, it increases immune defense. When monarchs come out of diapause too early, they lose that defense. Although there is always some waiting between the end of diapause and beginning of mating season, it’s hypothesized that if this period increases, so does the monarch’s vulnerability to infection.
It seems that monarchs may require very specific environments — not too hot, not too cold — to survive winter. “The remarkable thing about monarchs is they migrate to the exact same places every year,” says Green. “The study suggests there’s some type of specific energy balance that may be important so they don’t come out of diapause too early or too late.” Monarchs actually move around their overwintering sites a bit, changing spots throughout the season. Green’s research suggests that this movement may reflect monarchs’ continuous search for the ideal conditions.
Green is eager to explore the potential evolutionary implications unearthed by this study on the molecular level. His current experiment centers around chemical modifications to DNA that may provide the basis for the monarch’s internal timer. “For me, if you don’t understand a mechanism then you’re always going to be making a bit of a hand-waving argument as to why organisms either choose particular habitats or behave in certain ways,” Green says. “I want to know exactly how it works.”