A Life with Lycaenids
At the first lab she attended in a course on terrestrial arthropods, Naomi Pierce was expected to dissect a cockroach. Not the familiar kind we...
At the first lab she attended in a course on terrestrial arthropods, Naomi Pierce was expected to dissect a cockroach. Not the familiar kind we find in kitchens, but the Madagascar hissing roach, a blackish-brown insect "the size of a baby's fist," which hisses by driving air out of its spiracles when disturbed.
"I loitered at the end of the line, and when the time came, stood for a long time looking into the bin of cockroaches," she wrote years later in a scholarly article. "The Teaching Assistant wasn't impressed: 'Well, go ahead--just pick it up!' I made a quick lunge, but my target scuttled deftly out of my grasp, its tibial spurs rasping against my palm. That was that. The Teaching Assistant was by now completely exasperated. 'Come on, everyone is waiting.' I burst into tears, seized my books, and ran the entire six blocks down Science Hill to my dormitory."
Pierce was a sophomore at Yale when that roach got the better of her. Now married and the mother of twin four-year-old daughters, she is Hessel professor of biology at Harvard and a world authority on butterflies. (She designed and organized the exhibition on display through September 3 at the Museum of Comparative Zoology [MCZ].) She might well have been lost to biology forever had she not taken a course on evolution in her junior year, even though she was by then a humanities major. The course was taught by ecologist and evolutionary biologist Charles Remington, who talked in one of his lectures about studying butterflies in Colorado--which happened to be where Pierce had grown up. The idea of doing field work there appealed to her and so, finding him alone in his office one day, she asked him about it. "His eyes lit up," she remembers, "and he told me about the Rocky Mountain Biological Lab where he did research every summer. He was very encouraging and suggested I take courses there, so that's what I did that summer. I took four courses and returned that fall to begin my senior year as a biology major. I took all science courses, including a course in entomology that really got me hooked on butterflies."
|Scholar Naomi Pierce and subjects--a display of lycaenids at the Museum of Comparative Zoology exhibition.|
|Portrait by Stu Rosner|
Pierce has been hooked on butterflies ever since. After graduating from Yale in 1976, she spent several months, with Remington's encouragement, collecting and studying butterflies in Japan, the Philippines, Thailand, Java, Bali, and Australia. When she arrived at Harvard as a graduate student in 1977, the MCZ's entomology department was headed by ant specialists Edward O. Wilson (now Pellegrino University Research Professor) and Bert Hölldobler (now at the Theodor Boveri Institute for Biological Sciences in Würzburg). To take advantage of their myrmecological expertise, Pierce decided to concentrate on butterflies of the family Lycaenidae (lie-see-nid-ee), whose caterpillars are entangled in the ant world in varied and often bizarre ways.
One of the largest and most diverse of all butterfly families, the lycaenids--to which Vladimir Nabokov devoted special attention--include the familiar coppers, hairstreaks, and blues. About half the species in the family associate with ants. These associations are often mutualistic: caterpillars tended by ants get protection from predators and parasites; in return, the caterpillars secrete food for the ants from specialized glands. In some lycaenid species, particularly those in North America, the association is loose: caterpillars may or may not be tended by ants, and when they are, several species of ants may be involved. With other lycaenids, notably those found in Australia and South Africa, the association is obligate and complex: caterpillars of a given lycaenid species are almost always tended by ants of a single, specialized species. Almost all lycaenid caterpillars are herbivorous, but in a few unusual species they have become carnivorous, using chemical mimicry to infiltrate the nests of their attendant ants and prey on their broods.
Pierce has been fascinated by such interactions since she first learned about them, and chose to study the evolutionary costs and benefits such associations entailed. The benefits were pretty obvious, she says--food for the ants, protection for the caterpillars. The costs were harder to get at.
With Wilson's backing, she got a grant that allowed her to return to Australia, where she embarked on a study of the species Jalmenus evagoras, the "imperial blue," whose association with ants has served, she writes, "as a model system for the study of interspecific interactions, chemical communication, mutualism, and the evolution of complex life history traits." In simple but elegant experiments, Pierce raised some caterpillars under field conditions with their attendant ants and others without ants. The results were spectacular. About one in a hundred caterpillars raised with ants survived to maturity, compared to only one in 10,000 of those raised without ants.
Pierce didn't stop there. She went on to show that the food the caterpillars provided their attendants served as an important currency of exchange in their interactions with the ants, and that the quality of the food they provided depended on the kind of plant they fed on in the first place. Many butterflies protect themselves from predators by tasting bad. They accomplish this by ingesting secondary compounds, such as alkaloids, from the plants they feed on: monarch butterflies that ingest cardiac glycosides from milkweed plants are the best-known example. In contrast, caterpillars of the lycaenid family get protection not from plants directly, but from the ants they associate with--but the kind of food they supply the ants is important.
In the case of Jalmenus, Pierce showed first that the food the caterpillars were secreting for their ant guards was rich in amino acids. Amino acids happen to be the building blocks of proteins whose essential element is nitrogen, so Pierce showed next how important nitrogen was to the caterpillar-ant exchanges. Using the same approach as before, she raised some caterpillars on plants that had been treated with nitrogen fertilizers and raised others on untreated plants. The caterpillars she raised on treated plants attracted more ants and consequently had significantly higher survival rates. What's more, female butterflies preferred to lay eggs on treated plants. Indeed, the more Pierce studied the situation, the higher the correlation she found between caterpillar survival and the quality of the plants they fed on (in combination with their ant associations). The plants that conferred the highest survival rates--legumes and other nitrogen-fixing plants--were also the protein-rich ones. "When I found that the demands of feeding an ant guard could constrain the host-plant choices of the lycaenids, and that this pattern seemed to be true for hundreds of species," she says, "I was just over the moon."
By raising both ant-tended and non-ant-tended caterpillars in her lab, Pierce also showed more clearly the evolutionary cost of the interspecies association. Ant-tended caterpillars never attained full size, mostly because they shared their food with the ants. That made the males less attractive as mates when they became butterflies and resulted in fewer matings. Females could compensate to some extent for their loss in size by investing more energy in egg production and laying almost as many eggs as normal sized females, but they were still less fecund.
Pierce's work raised questions she has been trying to answer ever since. Some are relatively narrow, such as determining how associated species of ants and caterpillars recognize each other. She has shown that virtually every aspect of the imperial blue's biology is tied in some way to its attendant ants--the males look for mates by seeking pupae tended by the right ants, and the females use ants as cues in deciding where to lay eggs--and that much of the communication is olfactory, although Jalmenus caterpillars and pupae also use vibrations to recruit ants in their defense. But Pierce is also pursuing larger questions. How do mutualistic associations arise in the first place? What forces drive their evolution? How are they reflected in the evolutionary tree?
|Clockwise from top left: Female Jalmenus evagoras (imperial blue), an Australian butterfly, laying eggs, which are safeguarded by an attending ant; Hypochrysops ignitus (Australian fiery jewel); lupines, the host plant for Glaucopsyche lygdamus; and symbiosis in action, as an ant drinks from the gland of a larval Glaucopsyche lygdamus.|
|Lycaenid images by Naomi Pierce|
Pierce talks about her work with quiet modesty and good humor, though her ready laughter cannot mask her passion and intensity. Words such as exciting and fascinating turn up frequently in her accounts. She likes to tell stories and does not hesitate to speak her mind. She recalls, for example, her interview for a fellowship at Christ Church, Oxford: being closely questioned about her work by one of 15 dons all dressed in academic gowns ("It was worse than my thesis defense"). Finally the Very Reverend Dean who was then head of Christ Church lowered his spectacles and asked, "Well, young lady, you've traveled so far to see us. Do you have any questions for us?"
"Aren't there any women at Christ Church?" she asked.
"And there was this awful silence," she says, until the dean laughed and said, "Well, maybe we won't let you ask any questions after all." Only later did she learn that there had been a fight that very year about admitting women to the college, which the old guard vehemently opposed. "So I'd really put my foot right into it," she says.
Foot or no foot, Pierce won the fellowship and, two years later, an assistant professorship at Princeton, although her commitment to an academic track wavered briefly while she was there. Discouraged by what she calls "academic politics," she applied to medical school and was accepted, but decided to wait a year before starting, partly because she wasn't sure she could afford it, partly because she felt obliged to see a couple of doctoral students through to their degrees. Then, while on a field trip to Japan in the summer of 1988, she learned she had won a MacArthur Fellowship.
"I'd been there about two weeks," she recounts, "and had been out in the field every day until one day a pouring rain kept me in the lab. Then the phone rang. So I picked it up and someone called Ken Hope came on the line saying, 'Guess what, you just won a MacArthur grant.' It was completely out of the blue. In fact, I thought he was joking. Then I realized he couldn't have gotten the phone number except through my office back home, where I'd left it in case of emergency. Later my father wrote me a postcard saying: 'Dear Naomi. Congratulations. Now you can afford to go to medical school.'"
The fellowship not only put an end to thoughts of medical school, it also brought Pierce recognition, as well as offers from Berkeley and Harvard. After careful consideration, she chose Harvard, which agreed to let her set up a systematics laboratory. That enabled her to shift the focus of her research from behavioral ecology to the phylogenetic histories of the butterflies she studied and the relationship between their evolutionary descent and the behaviors they displayed.
The aim of modern biological classification is to find out how species are related by descent--to reconstruct the tree of life, so to speak, and see how all its branches link together. Nowadays it's done by comparing genes (consisting of lengths of DNA) across species to see how closely they resemble each other. From such comparisons systematists infer what the genes of a common ancestor must have been like, and from that they try to reconstruct the tree.
"The point," in Pierce's words, "is to be able to look at caterpillar behaviors and map them onto the evolutionary tree. The ants are the template against which the lycaenids have diversified, so we would expect to see their effects reflected in the tree." Harvard's offer gave her a chance to study such questions. She might not have been able to do so otherwise, she says, for she had no track record in systematics and would have had difficulty getting a grant for it.
Thus, her current work is a departure from, but closely related to, the work she began with Jalmenus on the nature of caterpillar-ant associations and their relation to host plants. Now she's asking how evolution brought those associations about: whether they evolved only once and then radiated to produce the many diverse forms known today, or whether they evolved separately several different times. She is testing her ideas by reconstructing a family tree based on molecular patterns and similarities that can be traced back to ancestral species, and by calibrating rates of divergence within and between ant-associated lineages. That's a tall order, she admits, considering that the lycaenid family comprises about 6,000 species, but she has been at it since she returned to Harvard in 1991 and thinks she can almost see the light at the end of the tunnel. Moreover, by using DNA, she hopes finally to resolve evolutionary questions originally raised by her predecessor, Nabokov, in his seminal work on lycaenid systematics.
"I'm not known for most of this work yet, because it's still in the process of coming out," she says, but the note of confidence and optimism in her voice is unmistakable. Perhaps it's just as well that she ran in tears down Science Hill to escape the hissing roach, for what the roaches lost the butterflies have decidedly gained.
John de Cuevas '52 is a contributing editor of this magazine.
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