Solving the Decades-Long Mystery of Manu

As a herbarium botanist, I love the detective work of identifying plant specimens. So it makes me crazy to find a perfectly good specimen, with flowers and fruit and all the prime identifying features, sitting nameless in a folder of “Unknowns.” If it’s a tough one, I may spend a few weeks staring at leaf veins and hairs, flower parts and fruit cross-sections, before stumbling on a correct match. It’s a sweet electric ping, and for days afterward I can’t get the image and the name of the thing out of my mind. As I’m riding the train home or chewing my dinner, I’ll relive the moment, and name it again and again. The pleasure of it seems so silly it’s embarrassing.

This time, though, victory evaded me.

I had met my match in what we called the “Manu Mystery Plant.” It was a small tree, with tiny white flowers and bright orange fruits ribbed like a “Chinese lantern” plant, first collected in 1973 by now-retired Field Museum ecologist Robin Foster in Manu National Park in Peru. Manu, at the base of the Andes in the Peruvian Amazon, is home to one of the richest, most ecologically intact rainforest fragments left in the world. It is, for the most part, closed to all but certain Indigenous groups and a few researchers at the Cocha Cashu Biological Station.

At Cocha Cashu, Foster and other scientists carefully inventoried the forest over the years with long-term plot studies, but one little understory tree was too small to be included. We knew which monkey ate its fruits and even which fungus grew on its leaves; its uses by local people were noted by an ethnobotanist, but the little tree itself, collected again and again, went without a scientific name. Over the years, several experts in tropical tree identification pondered the specimens, but no one took on the task of classifying it.

Most taxonomists who classify plants specialize in one or two families. (If you need a taxonomy refresher, it goes: family, genus, species. Check out The Brain Scoop’s “Taxonomy of Candy” video for more.) This means that if a plant isn’t already placed in a family, it can fall through the cracks. Fieldworkers might give it a local nickname—“Chinese Lantern,” “like Orthion” (a violet relative), or “Mystery Plant”—and set it aside indefinitely. 

The creeping sense that the Manu Mystery Plant might face a similar fate haunted me for 25 years. I would pick it up from time to time, trying various methods to gather clues: boiling and slicing the flower's ovaries to see the inner structure, examining its microscopic pollen. I’d become temporarily afflicted by shimmering-mirage-like hypotheses and run to the herbarium only to have them proven wrong.

Then, in 2015, my colleague Nigel Pitman prodded me to take this on in earnest and submit a Field Dreams grant application to the Field Museum Women’s Board. The next thing I knew, some wonderful people dug deep in their pockets and we were embarked. This terrifying show of confidence meant we finally had to get the job done.

The journey took years longer than anyone expected: five, to be exact. When we began, we knew we’d have to analyze its DNA, but our dried specimens didn’t yield good results. So the intrepid Peruvian ecologist Dr. Patricia Álvarez, Field Museum research associate and long-time field biologist at Manu, got us a fresh DNA sample. Field Botany curator Rick Ree sequenced the DNA and determined that its closest genetic relatives were in the small family Picramniaceae. 

None of us who knew the plant could believe it. Picramniaceae always have compound leaves, or several leaflets attached to a single stalk. They have flowers in long pendulous spikes, and the fruits are round berries or slender winged samaras, dry and papery like ash seeds. But most significantly, Picramniaceae do not have stipules: seemingly useless, often scale-like blips that occur in some plants where the leaf attaches to the stem. Stipules are usually very telling; most plant families consistently have them or don’t. As students, we joked that the adaptive function of stipules was that they help plants get identified.

The Manu Mystery Plant had textbook stipules, and simple leaves, and flowers in small tight clusters held close to the main stem. In our case, the stipules helped the plant not get identified: no identification key in existence would have placed our plant in the Picramniaceae. 

We sent the specimens off to Wayt Thomas, a botanist at the New York Botanical Garden and a specialist in Picramniaceae. He was as flummoxed as we were, but saw the Picramnia family resemblance in the plant’s tiny (2–3 millimeter) flowers. To continue drilling down, we had to obtain a couple more rounds of DNA samples from related species in Peru, with more help from Patricia, as well as from Manu researchers Varun Swamy and Yuri Huillca.

The months dragged on, but the tree finally clicked into place. It turned out the plant was in a genus all its own—which we decided to call Aenigmanu (the enigma of Manu) to reflect how many seasoned botanists it had stumped. We chose the species name Aenigmanu alvareziae for our colleague Patricia Álvarez, who combed the rainforest for this misfit time and again, and who has spent 15 years in Manu studying and documenting its trees and fungi and its wildlife, producing field guides, and teaching. 

My part in all this was small: I’ve never been to Peru, or collected the plant, monitored its populations, sequenced its genes, or written its technical description. I only kickstarted the process, with help from Field Dreams. But I’ve known this plant’s face for almost 30 years, puzzled over it, dissected its flowers, boiled them, sliced them, sketched them, photographed its pollen and leaf hairs, and nursed occasional revelations of what it surely was(!) that had me flipping frantically through folders in the herbarium, trying in vain to persuade myself I was finally on the right track.

It’s good to know that, after 48 years, Aenigmanu is finally out of limbo, its elegant name and portrait announced to the world, and its specimens at home in the herbarium with their relatives.  

Now, let’s see what else we have in the “Unknowns” case…