Thousands of years ago, a Red-spotted Purple butterfly (Limenitis arthemis) emerged from its cocoon without the telltale markings of its species—a white band through the middle of its wings. The creature reproduced and began to flourish.
Its offspring were very successful. They had remarkable luck in avoiding predators, and soon the Red-spotted Purple’s genetic mutation had spread extensively. It turns out that without a white band on its wings, Limenitis arthemis looks a lot like another species, the Pipevine Swallowtail (Battus philenor). The Swallowtail is toxic, and predatory birds learn to avoid it and anything that looks like it.
The Red-spotted Purple’s appearance is an example of adaptive mimicry—the development of an advantageous trait similar to that of another species—and also of convergent evolution—one species developing an appearance similar to another species’ independently, despite the two being separated by millions of years of evolution.
Sean Mullen, an Arts & Sciences assistant professor of biology, studies mimetic butterflies such as Limenitis to better understand the mechanisms underlying convergent evolution. He uses a combination of traditional techniques and new technology to figure out exactly which genes are at work in determining butterfly wing patterns.
“I want to understand how diversity arises,” he says. “Where does variation come from at the genetic level? How does that translate into variation in how genes are expressed during development, and how do those differences lead to variations in adult wing patterns?”
He is particularly interested in finding out whether different species exhibiting similar traits evolved those traits through the same genetic mechanisms. Are the same genes being turned on and expressed in the same way in order to produce the same wing patterns? If the answer is yes, it suggests that evolution is predictable and follows certain rules. If the answer is no, it lends support to the idea that evolution is flexible, with species evolving similar traits using different genetic pathways.
“To answer this question, you need to know at the level of the DNA if the causative mutations are the same,” says Mullen. “But you also need to know how the rest of the genome responds to that change.”
Mullen has discovered that the Wnt-A gene is responsible for wing pattern variation in North American Limenitis, and that the convergent aspects of color-pattern that Limenitis shares with another group of butterflies, Heliconius, are caused by the same gene. However, there is a difference in how the Wnt-A gene is expressed in each species—so in this case evolution doesn’t work in an entirely predictable way.
“We think that during development, the actual mechanism that produces that [particular wing pattern] might be a little bit different,” explains Mullen.
Agarose gel used to separate DNA.
To find out exactly how the gene is expressed in each butterfly, Mullen has devised a new strategy to leverage scientists’ decades of work on another insect species—the lowly fruit fly. Over the past century, biologists have painstakingly mapped every little quirk of the fruit fly’s anatomy.
What makes these flying pests so valuable? Their short life span allows scientists to observe quickly the effects of gene manipulation on successive generations. Fruit flies also have simple physiologies and similar bodily systems to those of other animals. Because they have been studied for so long, scientists have a thorough knowledge of their genomes and a good understanding of their gene expression network.
Mullen plans to transplant different versions of the Wnt-A gene from Limenitis butterflies into fruit flies, and then observe how the gene regulatory network in the flies reacts to these genetic manipulations. This approach is systems biology in its purest form—observing how a complex system reacts to a change in one variable.
“Systems biology perspectives were not really available until recently,” Mullen explains. “It is the advent of these incredible new genomic and bioinformatics tools that allows us to step back and take this holistic look at things.”
Thanks to Mullen’s work, with an assist from gene-sequencing technology and the fruit fly, biologists are that much closer to understanding how convergent evolution works.