A Quest to Understand the Bizarre Talent of the Bombardier Beetle

A Quest to Understand the Bizarre Talent of the Bombardier Beetle

By Emily LitvackResearch, Discovery and Innovation
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Wendy Moore with a Cactus longhorn beetle. (Photo by J. Schaller)
Wendy Moore with a Cactus longhorn beetle. (Photo by J. Schaller)
A dorsal beetle. (Photo courtesy of Wendy Moore)
A dorsal beetle. (Photo courtesy of Wendy Moore)

Terrestrial arthropods – animals with external skeletons, no backbones, segmented bodies, and jointed appendages – are some of the most successful and diverse forms of multicellular life on Earth. They live virtually everywhere on the planet, and in vast numbers. Insects, spiders, scorpions, and even mites, are terrestrial arthropods. They are truly a phylum to be reckoned with.

Wendy Moore, associate professor of entomology and curator of the University of Arizona insect collection, studies the evolution and ecology of these critters. One in particular, the Bombardier beetle, has long captivated her – and for good reason. The Bombardier beetle explosively blasts boiling hot, noxious chemicals onto its predators, and entomologists have yet to figure out how that came to be, and what mechanisms make it possible.

By studying its genome, Moore believes, she and a team of researchers will finally get to the bottom of the Bombardier's bizarre talent. Moore and four co-principal investigators are among five finalist teams in Pacific Biosciences' Single Molecule Real-Time grant competition to sequence the "most interesting genome in the world." Between now and April 5, anyone can vote for the genome of his or her choice.

Here, Moore discusses her research, and why the Bombardier beetle is truly worthy of the designation of "most interesting genome in the world."

What led you to become an entomologist?

Growing up in the pristine barrier island country of South Carolina, with unspoiled beaches and seagrass meadows, and the endless expanse of the north Atlantic, I think I was always in love with nature, so I didn't think twice about getting my first college degree in biology at Vanderbilt University. There, I was inspired by a remarkable scientist and mentor named Gary Polis. I spent a summer with Polis, and a crew of other students, in remote regions of Baja California studying the island ecology of small arthropods. It was then that I realized my passion and commitment to fieldwork. 

My master's degree followed shortly thereafter, when I worked with another well-known invertebrate biologist, Rick Brusca, at the College of Charleston. There, I was fortunate to be part of his National Geographic-funded project to study the systematics and biogeography of marine crustaceans in Polynesia. My love of fieldwork grew even stronger as we traveled to most of the island nations of the South Pacific, scuba diving and collecting specimens.

It had become obvious to me that I wanted a research career, but I wanted to expand my expertise in the world of arthropods, which, by the way, comprise 82 percent of all living animal species – most of which are insects. The field of systematics thrilled me with its combination of fieldwork, discovery and analytical approach to understanding the history and diversity of life.  As I explored and scanned the arthropod horizon, I stumbled upon Bombardier beetles – and I was hooked immediately.

One of the top beetle researchers in the U.S. was David Maddison, at the University of Arizona.  I joined his lab for my Ph.D. studies and then completed a two-year postdoc with David Kavanaugh at the California Academy of Sciences. In a remarkable twist of fate, a few years later David Maddison moved to Oregon State University, and I was lucky enough to be the person who filled his position here at the UA. To this day, David and I continue to collaborate on beetle projects. And, in fact, I also continue to collaborate on invertebrate projects with Rick Brusca, who is now my husband.

What is it that interests you about terrestrial arthropods?

Their ubiquity and unarguable success is a big draw. Any group of animals that can live virtually everywhere on the planet and in such huge numbers is inherently interesting. They also afford limitless opportunities to ask research questions. Their radiations into all kinds of niches have produced often bizarre and diverse lineages. Some of the Bombardier beetles I work on, for example, are ant-nest inhabitants. They actually trick their ant hosts into thinking they're ants, and move into the nest, where they feed on the young ants.

Why is it important that we understand not only the ecology, but also the evolution of terrestrial arthropods?

Understanding how the biosphere came to be, what the ancestral lineages were and where they lived, and how they came to be what they are in today's world, gives us critically important context. We cannot ask even the most basic questions – like "why do these beetles live with ants" or "how did they get these bizarre adaptations that allow them to trick ants into accepting them into their nest" – unless we understand their evolutionary history, the steps along the way, that allowed such fantastic life histories to come about.

You go about this by using molecular genetics and morphological methods. How can genes and morphology tell us about a given species evolution?

All organisms on Earth are related to one another and this pattern of relationship is commonly represented by a branching tree, or evolutionary tree. Evolution leaves a pattern of past changes in an organism's anatomy and DNA. So, by comparing the morphology and DNA from many organisms we can reconstruct the pattern of their relatedness. It's like building a family tree, or genealogy, but for species instead of for your relatives and ancestors. Once we have the branching pattern that depicts the relationships among organisms, we can then overlay that with whatever we might be interested in. We might be interested in the evolutionary history of a body feature or the places in the world the organism lives (biogeography), or we might want to understand how a particular behavior came about in the history of a species (such as living in ant nests). And, if we have evolutionary trees for both Bombardiers and their ant hosts, we can look for congruence in time and space and branching patterns, thus revealing their shared history or coevolution.

Can you talk a little bit about the Bombardier beetle specifically?

Bombardier beetles are my specialty. This is the common name for two lineages – brachinines and paussines – of ground beetles, the family Carabidae, that are able to explosively blast boiling hot chemicals with uncanny precision directly at their enemies. Remarkably, they make the chemicals themselves, store them within their body, and violently discharge them without injuring themselves.

Many species of Bombardiers live right here in Arizona, and they can be especially common in riparian habitats. They spend the days hiding under rocks along stream sides, emerging at night to scavenge for food and find mates.

What makes it the "most interesting genome" in the world?

Chemical weapons are uncommon in the animal world, and Bombardier beetles have been particularly perplexing. How can they store these volatile chemicals in their bodies, mix them on demand, and then direct the resulting explosion where they want without injuring themselves? What is the genetic basis of such a mechanism, how did it evolve, and could the features and details of this be mechanically replicated by humans? An understanding of the genetic basis of this phenomenon will give us great insight into how chemical defenses evolved in other carabid beetles and in the animal kingdom in general.

What can we gain by sequencing its genome?

We propose to sequence the Bombardier genome to learn how this remarkable beetle makes these toxic chemicals, stores them within its body, and discharges them without self-injury. And to understand the steps along the way during the evolution of such a remarkable system.

Our team has an ongoing National Science Foundation-funded research project to investigate what genes might be involved in producing the chemical precursors of Bombardier beetle explosions.

What can those interested in seeing this genome sequenced do to help support your project?

Vote for us every day between now and April 5. Our candidate is a native of the Sonoran Desert. Brachnius elongatulus is an explosive Bombardier beetle that lives along the riverbanks of Madera Canyon in the Santa Rita Mountains less than 50 miles from the UA campus.

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