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WIU Department of Biological Sciences Professor and Fulbright Scholar Eric Ribbens and a few of his Opuntia fragilis specimens at the School of Agriculture's Farm near the WIU campus in Macomb. Ribbens' collection of Opuntia fragilis is probably the largest on the planet.


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Photo of flowering Opuntia fragilis, or prickly pear cactus, in the collection of WIU Department of Biological Sciences Professor Eric Ribbens. Ribbens said this year has been an exciting one for his Opuntia fragilis plants, as it is the first one during which they produced flowers. Next year, he plans to try some pollination experiments among different populations to further explore what might be preventing Midwestern plants from producing seed.

Biology Professor's Work Yields Largest Cactus Collection of Its Kind Housed at WIU Farm

November 26, 2012


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by Teresa Koltzenburg, University Relations

Last June, as Eric Ribbens and I perused his collection of his Opuntia fragilis — probably the largest collection of its kind on the planet — located near the Western Illinois University School of Agriculture's Farm in Macomb, the Department of Biological Sciences Professor and Fulbright Scholar told me about the unusual sex life of this rare and endangered prickly pear cactus.

"If you're going to go through the work of having sex, the goal is to maximize the genetic recombination. Yet, in plants, it's possible for pollen to move to the same plant. But for the Opuntia fragilis, these plants have some sort of a chemical recognition cue, and if they sense the pollen is from themselves, they shut it off and they won't let it fertilize the egg. We don't know exactly what is going on, but it turns out, in Illinois, at least — and I suspect throughout the rest of the Midwest, although we haven't studied it yet — it doesn't really matter if we take pollen from your flower or we take pollen from a flower nearby or pollen from a flower from a quarter-mile away, they all get shut off. So, somehow, the plant's mechanism is saying, 'All of this pollen is from me.' Or that pollen is a mechanism that's broken and not working right. We don't really know what is going on."

Based on his extensive research of the Opuntia fragilis species, Ribbens has provided strong evidence this species, in the Midwest at least, has forgotten how to have sex. It still tries now and then, though, he added with a smile.

With the help of students and fellow plant scientists, Ribbens has been closely studying this particular species of prickly pear cactus since 1995. He came to Western in 2000 and over the last 12 years, has continued his work researching the plant.

In 2010, he and Barbara Anderson (Burlington, IA), a former WIU biology graduate student, and Jeremy Fant, a plant scientist at the Chicago Botanic Garden, published "Opuntia fragilis (Nuttall) Haworth in Illinois: pad dynamics and sexual reproduction," in Haseltonia, the peer-reviewed Yearbook of the Cactus and Succulent Society of America. Ribbens said this particular study article was the result of their investigation of the only known natural site of Opuntia fragilis in Illinois.

"About 10 years ago, I applied for and received a grant from the Illinois Endangered Species Protection Board to study Opuntia fragilis at the Lost Mounds site, a decommissioned munitions depot with a large Opuntia fragilis population covering about 100 acres," Ribbens explained. "The grant provided funding for a graduate student research assistantship to investigate the population status and fungal infections. So, my grad student, Barbara Anderson, and I designed a project to determine turnover in pad production and to study flowering in this species of prickly pear cactus. We built a matrix model of population/pad production, and Barb determined that although the plants flower, they do not produce seed. She also showed this was due to this self-incompatibility mechanism: if the plant senses that the pollen is from itself, it will prevent the pollen tube from growing down to find the ovule and fertilize the egg. Barb showed this happens for pollen from any plant. We also collaborated with Dr. Karyri Haven's research lab at the Chicago Botanic Garden to study the genetic diversity in this population," he said. "It was moderate."

Considering its aversion to reproduction, one wonders how the Opuntia fragilis — which is sometimes referred to as the "brittle" variety — continues on? Ribbens asserts the fragility of its pads — hence, part of its eponymous scientific name, fragilis, and common name, "brittle" prickly pear—provides its survival mechanism.

"The pads break off easily, and this is actually the main way they move around. So imagine a deer walking through a site and kicking one, or a buffalo rolling in it and getting a couple of pads stuck to it. That's how we think it moved around the landscape," Ribbens explained. "Barbara and I spent about four years up in Jo Daviess County in Illinois examining the rate at which those pads break off."

The Making of a Midwestern Cactus Mission

For many, the thought of cactus plants can conjure desert scenes in drier, arid landscapes. But the Opuntia fragilis, which Ribbens began studying by accident, likes a chillier climate.

"This particular species grows on rocky outcrops, and it likes it when it's cold. It is a small, northern species of cactus, growing almost as far north as the Arctic Circle. Although it is widely distributed across North America, in the upper Midwest it is rarer, and in Michigan it is a state endangered species," he noted. "I got started studying Opuntia fragilis at my first teaching position at St. John's University in Stearns County Minnesota. An undergrad student there found them growing. He thought it was a new discovery, but it turned out that a lot of people had known about it already. Still, it caught my attention."

According to Ribbens, at the time, he was looking for a plant species to try to study a particularly difficult problem in spatial plant ecology.

"My first area of research is describing the distribution of tree seedlings around trees. And, with my computers, I can do a pretty good job of telling you where you can expect to find tree seedlings within, say, 100 feet — not 100 miles. That kind of thing is very, very hard to try to quantify," Ribbens said. "So, at the time, I was thinking what I really needed was a rare species, but one that was easy to see and to find. So I thought these plants might be interesting for my purposes. But it turned out they were totally worthless for it."

In spite of that, Ribbens continued his pursuit of the prickly pear.

"One of the fun things about it is it's rare and endangered enough that people are somewhat interested in it, and there is a little bit of grant money out there for research possibilities. But it's common enough that it's not like I'm going to accidentally wipe it out," he explained. "After I received tenure at WIU, I applied for a sabbatical, and for that, I proposed to attempt to locate every population of Opuntia fragilis in five Midwestern states. I searched herbarium records, contacted state natural resource departments or agencies, talked to cactus enthusiasts around these states and built a list of sites where we knew Opuntia fragilis had, at one point, grown. Some of those sites were gone and others certainly still existed, but for many, the status was uncertain."

Ribbens said during two summer seasons he and crews of WIU students traveled around the Midwest searching for Opuntia fragilis sites. He said they were able to investigate all but four sites they had pinpointed.

"Two of these four sites we did not travel to are on very large lakes in upper Minnesota, and I was able to verify those sites through Department of Natural Resource workers there; the other two are on private property in Wisconsin for which I was unable to get permission to search the sites," he added. "Certainly there are a few additional sites in western Minnesota, and new sites may exist elsewhere, but we found 55 sites in Minnesota, 33 sites in Wisconsin, four (now we know of five) sites in Iowa and one site in Michigan."

Over the years, Ribbens has published his findings about the Opuntia fragilis in both peer-reviewed publications (like Haseltonia), as well as in other plant publications.

"There are 15 different peer-reviewed publications that deal with this species or mostly with this species. I published four of them," Ribbens said. "I also authored a series of articles for the Cactus and Succulent Society of America's journal, which are not peer-reviewed but written in a somewhat lighter style for people interested in cactus and often know much more than I do about this group of plants."

Ribbens' research and interest in Opuntia fragilis also garnered international interest, when, several years ago, he was contacted by the editor of a Polish cactus journal, Kaktusy.

"Apparently people in Poland had noticed my recent articles in the Cactus and Succulent Society journal, and he asked if I would be interested in submitting an article to them. I now have an article in Polish (they translated it), and another one that I expect will be published soon," he said. "I probably know more about this species than anyone else in the world, but that's because nobody else cares," he added with a smile.

Ribbens' large prickly pear cactus collection near the WIU School of Ag's University Farm indeed illustrates his dedication to learning about the plant. He not only has Opuntia fragilis specimens from all the sites he traveled to and investigated — as well as specimens from South Dakota, North Dakota, Colorado, Oregon and from Canada — he has also collected specimens of two larger species of prickly pears that grow in the Midwest.

"A few years ago, a graduate student by the name of Lucas Majure from Florida contacted me about these two larger species of prickly pears in the Midwest, Opuntia humifusa and Opuntia macrorhiza. The taxonomic history of these two is long and confusing, and Lucas was studying them for his doctoral thesis. When he contacted me, he wanted to know if could I send him live pads from known locations, so I agreed, and this began what is turning out to be a very fruitful collaboration. I now have live material from 33 different locations, including 28 Midwest locations. I also sent him a number of fragilis pads, and this year, we published our paper, 'Chromosome counts of Opuntia (Cactaceae), prickly pear cacti, in the Midwestern United States and environmental factors restricting the distribution of Opuntia fragilis,'" he said.

Patterns and Pollination Plans

Over the next few years, Ribbens has specific plans to continue his prickly pear cactus research and collaborative relationships. In 2009, he traveled to Utah with a WIU undergraduate student Bill Schmidt (senior, biology, Mokena, IL) and met with cactus enthusiasts there, who showed him populations of Opuntia fragilis around the state, and in particular, locations where it seemed to be hybridizing with several other species of Opuntia fragilis.

"This was partially funded by a grant from the Cactus and Succulent Society of America. I have data, photographs and live specimens from that trip, and I am working on an analysis that I hope to publish," he said. "Also, over this past summer I spent time measuring pads from plants from each of my Opuntia fragilis specimens, assessing their pad size, shape, spine characteristics and other aspects of their general morphology. Because these are plants that have been growing in Illinois for up to five years now, differences among their pads probably are due to underlying genetic differences, not differences in local environment. Lucas Majure and I plan to analyze this data for possible patterns of variation across the Midwest landscape."

Ribbens said this year has been an exciting one for his Opuntia fragilis plants, as it is the first one during which they produced flowers. Next year, he plans to try some pollination experiments among different populations to further explore what might be preventing Midwestern plants from producing seed. He also would like to locate five or six additional possible sites in Iowa for the bigger species and write, with Majure, an analysis of the Iowa-growing Opuntia humifusa and Opuntia macrorhiza.

"Science is rarely done alone. My work would not be possible without various research grants, without all the students who have helped me and without the assistance of various departments of natural resource agencies in the Midwest," he said. "I'm also very grateful to the WIU School of Agriculture for accommodating and helping me with my cactus collection, and especially to my former WIU students Barb Anderson, Alicia Geisler, Brandon Caley and Camila Sharkey."

While Ribbens admits he doesn't necessarily enjoy providing the answer to the inevitable question about the subject of his work — "Just what are the Opuntia fragilis good for?" — he remains undaunted in his pursuit of knowledge about this rare plant.

"I have a friend who keeps saying, 'You've got to find something they are good for. They must be able to do something, be medicinal, be a food for a miracle diet cure or something.' I keep telling him the world was not made to serve people, people were made to serve the world. And just learning more about them, understanding more about them, well, that's my challenge and my joy."

Posted By: Teresa Koltzenburg (TE-Koltzenburg@wiu.edu)
Office of University Relations