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Thousands of species of beetles sport horns of various sizes and shapes (shown in purple in the corner and center images). In most species, including the one Laura Corley works with (Onthophagus nigriventris, top row center), only males have horns, and then only if they grow large enough in their larval stage to trigger the genes that control horn development. While horns are handy in battle, beetles without horns fare better when resources are scarce, because they don't have to devote energy to maintaining and carrying the bulky ornaments. Photos and scanning electron photomicrographs by Douglas J. Emlen, associate professor, Division of Biology, University of Montana.

How the beetle got his horns

Walk into Laura Corley’s lab, and you won’t notice anything you couldn’t find in any other modern biology lab. But open the door to the walk-in incubator that houses her experimental animals, and you get hit by the aroma of the barnyard.

^{Robert Hubner}

Corley (left) studies dung beetles, which get their name from their reliance on the droppings of much larger animals, such as cattle or antelope, to nourish their young. Each egg is laid inside a “brood ball” of dung. The female beetle gathers the dung, chews it and mixes it with sand, shapes it into a tidy oval, and places it, with egg inside, in a sort of den she digs in the dirt. When the larva hatches out of its egg, it has exclusive access to its food supply. As it eats and grows, it hollows out the brood ball from the inside. When it finishes growing, it pupates, like a butterfly chrysalis, and then emerges from the ball as an adult.

Despite their small size and humble origins, adult dung beetles are among the most spectacular creatures on Earth. Males of various species possess an array of head ornaments that rival anything seen in the deer family. Some of the males do, that is. Whether a particular male develops horns depends not on his genes but on the ball of dung that nourishes him—how big the ball is, how much he eats, and how big he gets.

“It’s a threshold trait,” says Corley. “If they reach a critical weight, then they make horns. If they don’t reach the critical weight, they don’t.” All the male beetles have the genes to make horns; but those genes are turned on—and they grow horns—only if they get enough to eat as larvae.

Corley is investigating how the horn-development program is controlled. She’s especially interested in the insulin-signaling pathway, by which insulin and other molecules enable the animal to sense its own nutritional state and signal various parts of its body to turn specific genes on or off.

She cautions against the notion that “horns are good.” She’s not keen on the TV-nature-show version of “natural selection” in which every trait and behavior of an animal exists with direct reference to a yes/no, good/bad sort of tally sheet. The situation is more complex, and more interesting.

Corley differentiates between positive, negative, and neutral selection. The payoff in each case is which animals get to pass their genes along to future generations. Positive selection occurs when a genetically controlled trait helps its owner to spawn more offspring. You’re a salmon who can swim upstream for two months without eating, and still spawn vigorously? You’re in. Or rather, your genes are in (the next generation).

Negative selection occurs when a genetically controlled trait diminishes its owner’s chances of passing them on to the next generation. You’re a gazelle who can’t run faster than a hungry lion? You’re outta here.

Then there’s neutral selection, which is less dramatic than the other two, but may be at work just as often. “Neutral” means the trait doesn’t confer enough benefit or harm to influence the reproductive success of its owner. Such a trait may not be a great boon, but if it’s not a big negative, it won’t be selected against. A lot of traits—and the genes that control them—may be passed along this way, riding the coattails of some more essential trait.

In Corley’s beetles, the key trait—the selected-for trait—may not be those impressive horns, but the plasticity, or flexibility, to grow them or not. It’s a way for the beetle to cope with a patchy environment. Horns account for up to 15 percent of a beetle’s total weight; in an environment with minimal food, spending calories to lug them around could consume energy better used in other ways. But in an environment with abundant food, maintaining the horns is not a problem. In that situation, it might be worth having the ornaments, because males with horns have better luck with the ladies than those without them.

“What I think is just absolutely, one hundred percent cool, is that these individuals are the same, but they’re different,” says Corley. “The first time I ever found out about phenotypic plasticity—that you have the opportunity to be short or tall, or have a horn or not have a horn, or be yellow or white, or make a spot or not make a spot—and that it’s almost purely based on the environment, I completely flipped. How does that happen? And I’m still searching for the answers to that question.”
 

Presto, change-o

^{Courtesy Mike Webster}

Corley’s research is part of the emerging field of “evo-devo,” which combines evolution and embryology. Only about a decade old, the field already has provided stunning evidence about how different body plans can evolve.

“Do you need a whole bunch of genes to change and be subject to natural selection? Or do a few key regulatory genes do it?” asks Mike Webster (left), who co-teaches the course in evolution for biology majors. “Some of the most exciting results that are coming out of this area of research are that if you just change the timing of when the genes are turned on and off, you can get a very radically different body plan.”

He describes research showing that an embryonic invertebrate can be made to develop into something that looks like a spider (with two main body segments), an insect (three segments), or a centipede (many segments), depending on when a certain gene is turned on.

Still, environment can’t do the job alone. In order for a dung beetle to make horns, he must have the genes to do so. Genes remain central to evolutionary study, and changes in genes—mutations—are still thought to be the main source of differences among species. And mutations, we now know, happen disturbingly often.

“People think you have to get zapped by something,” says Charlotte Omoto, who teaches evolution as part of a genetics course for non-biology majors. “No. You know why there are mutations? Every time a new cell or new organism is produced, the genetic material has to be copied. Mother Nature’s wonderful, has all kinds of checks and balances; it’s very important to make sure things don’t change [too much]. But we have three BILLION of these letters that have to be copied every time a new cell is made. So little mistakes are made.

“It’s no different than the monk copying the Bible by hand,” she says. “And people have done this—we can see how errors have propagated in manuscripts because of writing errors. Well, exactly the same thing happens in cells.”

And in cells, those mistakes—those mutations—can boost an organism’s chances to reproduce, or ruin them completely.

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Continued