Genomics of Mimicry
Heliconius mimicsThe Butterfly as Copycat
This article orginally appeared in the March 2005 Issue of GenomeLife.
Some butterflies like to blend in with the crowd. Way in.
In some 28 geographically distinct regions in South and Central America, two species of the butterfly genus Heliconius have taken on 28 or more distinct appearances, each resembling one or more other species of local, unrelated butterflies. The disguises are convincing: in many cases, the regional varieties of Heliconius look so different that they were originally described by taxonomists as belonging to different species altogether.
Few would dispute that the myriad wing patterns of Heliconius butterflies are pleasing to the eye. But would evolution go to all this trouble simply for aesthetics? Not likely. What's actually happening is a phenomenon known as Müllerian mimicry (after the 19th-century German zoologist Fritz Müller). In this type of evolutionary copycatting, several species unpalatable to predators in one location evolve to share a similar appearance that serves as a warning signal. In effect, the butterflies' wing pattern says to hungry birds: "We are not edible. We are toxic to you." A predator need only be exposed to a single butterfly with that pattern before learning to avoid all of them. As a result, all of the mimic species benefit.
An Array of Possibilities
Both Nijhout and Postdoctoral Fellow Bob Reed would like to understand the genomic basis of Heliconius butterflies' ability to change their look over the course of evolution. Reed is taking two complementary approaches to the problem. The first is to find individual genes that control color patterns and clone them in order to understand what they do. Much of this work involves identifying the butterfly equivalents of genes in the more fully-characterized fruit fly genome that control pigment synthesis and pattern formation.
The other approach is a broader, more systems-oriented one: to use genomics to understand the developmental networks underlying the range of color patterns that can be seen in the various geographic regions. "We want to look at the genes and regulatory elements implicated in the development of these patterns," says Reed, "and see how expression of those has been moving across geographic boundaries over time."
Heliconius gene expressionColor patterns are critical to the Heliconius survival in any given region. Any shuffling of pieces of the genome harboring genes that control wing color, or any movement of butterflies from one region to another where color patterns differ, will have major consequences. For example, one region may be populated by butterflies that have been protected from predation by virtue of their white color pattern. "If a yellow color pattern moves into that region containing a white color pattern," says Reed, "then those yellow butterflies are going to get picked off by birds."
To attack the network problem, Nijhout and Reed are collaborating with Owen McMillan at the University of Puerto Rico. The two labs are constructing a microarray that will contain thousands of genes expressed in the wing. It will include genes expressed at different developmental stages and from different Heliconius morphs.
"With microarray data," says Reed, "we can discover which genes are expressed in relation to the different color pattern elements and which genes are turned on or off in the different morphs. We'll know what developmental networks we're looking at and how they're modulated. Then we can start asking a whole new set of questions about the population biology of the networks and the color pattern genes."
Harmonic Convergence
The most exciting aspect of studying the molecular evolution of Heliconius, says Nijhout, is that they are evolving right before our eyes.
"Here we have a chance, for the first time, to see a genetic network in an ongoing evolutionary radiation that has been underway since the last ice age. Except for bacteria or viruses, there is no other group of organisms I know of where we have our hands on an actual evolving, diversifying system."