Newswise 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 pearprovides 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.

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Biology Professor's Work Yields Largest Cactus Collection of Its Kind