Sex and the Prairie
Oak groves once floated like ships on a sea of wildflowers, every plant sending and receiving the stuff of life by wind, insect, or bird. But these days, finding a mate ain’t so easy.
By Ryan Chew
Photo: Ray Mathis
Even if you had a little botany in school long ago, and vaguely remember terms like pistil and stamen, it can come as a jolt to be reminded that plants breed — that almost all plants inherit their genetic make-up from a mother and a father, and that, in the words of University of Illinois at Chicago biology professor Mary Ashley, some plants, seemingly so rooted, could still be described as “getting around.”
Yet plants do choose mates, offering pollen to wind, insects, or birds in the hope it will find a compatible flower, in a demure courtship that Erasmus Darwin called “the vegetable passion of love.” The suitability of each plant for the soil where it takes root is determined by the genes it gets from paternal pollen and maternal ovule.
The grasses and wildflowers of our region evolved a variety of reproductive strategies in the unbroken expanse of tallgrass prairie. Common plants like wild bergamot and prairie dock had thousands of potential mates for miles around, and some developed ways of excluding the pollen of closer relatives to avoid inbreeding. Naturally rarer plants, like the eastern prairie fringed orchid, adapted to the longer distances that their pollen had to travel.
Today, with the prairie fractured into remnants, the terms of plant relationships are changing dramatically. Without an infusion of genes from distant colonies, plant populations on small prairies may grow ever more closely related with each generation, until the loss of variety in their genes leaves them unable to adapt to the cycles of rainfall, disease, and insect infestation, and they die out. This spiral of decline, the loss of fitness as beneficial genes fall out of the gene pool, is known as “inbreeding depression.”
To prevent inbreeding depression, preserve managers sometimes “outcross,” bringing seed from outside sources to renew the gene pool. But this can backfire, because plants also adapt to particular local circumstances. The grasses of the southern prairie may lack the hardiness needed in an Illinois winter. Plants also develop genetic combinations that only work when found together. For example, short, thin-stemmed flowers and tall, strong-stemmed flowers might each survive, but if you crossbreed the two, you may wind up with tall, thin-stemmed plants that droop. This potential for decline when naturally separate populations are crossed is called “outbreeding depression.”
To learn the genetic ramifications of our now fragmented landscape, Ashley and a cohort of rising biologists are using techniques familiar from TV’s CSI series to trace the parentage of plants. They find that some species continue to hook up over surprising distances. Others only pair with nearby neighbors. For these species, biologists are measuring the risk of outbreeding depression so we can decide which inbred populations might survive being outcrossed, and which would do better if left alone.
The common oak of the prairie is the bur oak, a wind-pollinated tree with thick bark adapted to survive searing prairie fire, living in open groves known as savannas. When Mary Ashley began studying bur oak relationships in Illinois savannas, little was known about the distance at which these trees could pollinate. “The savanna landscape is naturally fragmented,” she said. “Bur oaks grew on less than 5 percent of the land, in groves separated by grassland. We assumed that most pollination was from trees right next door, but how would you know how far pollen is moving?”
Ashley, along with then graduate students Beverly Dow and Kathleen Craft, set out to map the exact relationship between every bur oak in a savanna and the acorns produced by pollinated oak flowers. At an isolated grove in McHenry County at least a quarter mile from other bur oaks, they gathered leaves from each of 67 specimens in the stand. For taller trees, the adventurous grad students climbed up to the flowers, or lassoed branches to pull leaves within reach. They collected acorns by placing tarps under trees, competing with squirrels that saw the tarps as picnic blankets. “At one study, the local sheriff took out a gun and shot off branches for us,” Ashley said. “But that was in a less populated area.”
Photo: Kathleen Craft
The researchers ground up the leaves, using chemical reactions to separate DNA and zero in on paternal markers — genetic signatures for each tree that are passed only by pollen into the genes of acorns. Since the maternal tree contributes nothing to this segment of DNA, it can be used to identify the tree that the pollen came from.
More than half of Ashley’s acorns had paternal markers not found in the DNA from the leaves. “For these acorns, the father wasn’t even in our sample, so the pollen had to be coming from somewhere outside the grove,” Ashley said. “There are no other oaks for a couple hundred meters. You’ve got this cloud of pollen from the trees right next door, so how is it that so much other pollen gets in? It was the first time anyone had shown this.”
She was also surprised that pollen reaching the acorns could be traced to trees in all directions around the maternal tree, despite prevailing winds. “The chance that pollen came from any other tree in the grove was nearly random,” she said.
Ashley’s team confirmed their findings in other groves. At a remote stand of bur oaks at Goose Lake Prairie, 60 percent of acorns were pollinated from outside the preserve. Surveying 14 preserves in northeastern Illinois, she found similar genetic variation in each, despite differences in size and distance from other groves. She theorizes that wind pollination in bur oaks allows gene flow, replenishing genetic variability in small savannas. Across these distances, her oaks act not like isolated groups of trees, but like a single population, maintaining healthy genetic diversity.
The eastern prairie fringed orchid has two disadvantages compared to the bur oak. First, while some oak savanna survived European settlement (as grazing land and woodlots, and then as parks), much of the orchid’s wet prairie habitat was intentionally drained and destroyed. Second, the orchid’s pollinator is not the swift wind, but the nocturnal hawkmoth, with a range measured in meters, not kilometers. This orchid was never common, but it appeared regularly throughout its range, including much of Illinois. Now, it is limited in Illinois to just a few sites, and most are more distant from each other than the flight of a hawkmoth. Although thousands of individual orchids survive, the few on each site only receive pollen from each other, leaving the species at risk of inbreeding depression.
Photo: US Fish & Wildlife Service
Tim Bell, a professor of botany at Chicago State, has been crossing fringed orchids from different sites in Illinois to determine whether the species would be helped by importing pollen from site to site. Over several years, he pollinated plants in June, then collected fruit pods a couple months later. The half-inch pods hold thousands of tiny orchid seeds, each no larger than a speck of dust. He uses a solution of water and bleach to break open the surface of the seed, mimicking a process that can take a year of weathering in natural settings, and then watches for the seeds to plump up and sprout tiny roots. There, Bell runs into a hurdle: eastern prairie fringed orchids can only grow in soil with a special root fungus. The balance between the fungus and the plant is delicate, tipping in the wrong direction when greenhouse seedlings are transplanted. No one has grown a mature orchid of this species from seed, though hand-pollinated seeds sprout when scattered on soil near mature orchids.
“Since we can only look at viability and germination in the first generation, the conclusions are all qualified,” Bell says. “Outbreeding depression may not show up till a later generation. Still, we haven’t seen any evidence of it.” The plumpest seeds most likely to germinate came from distant outcrosses. Crosses within one preserve were less successful. Outcrossing seems to eliminate inbreeding problems the isolated orchids already suffer. “I suggest hedging bets,” Bell said. “Gathering seeds from more than one source and mixing it up.”
The least viable seeds in Bell’s study came from flowers pollinated by pollen from the same plant. This form of reproduction, called “selfing,” is risky, since the offspring don’t gain the benefit of genetic variation. Still, selfing allows reproduction in settings where no other plants are nearby, so some species with narrow niches, like this orchid, have evolved to allow it.
Purple coneflowers adapted to avoid the inherent genetic loss, by excluding pollen from plants with which they share one particular segment of the DNA. This prevents selfing and crosses between closely related plants. The purple coneflower was once common across an unbroken range, and few coneflowers grew up in such isolation that they needed their own pollen.
Now, they remain common in scattered remnants, but many colonies are genetically isolated. Coneflowers that exclude closely related pollen may be passing up viable relationships. To test the effects of this pollen limitation strategy, Stuart Wagenius, a conservation scientist with the Chicago Botanic Garden, cross-pollinates coneflowers, focusing on a species of western prairies, Echinacea angustifolia. He paints a small leaf on each “floret” to indicate which of its neighbors he used to pollinate it, and then shrouds the blossom with a “pollen exclusion bag” — a flower condom — to keep other pollen out. If the floret is pollinated, it withers and goes to seed. Florets that stay open must have received incompatible pollen.
Photos left to right: Carol Freeman, Rob Curtis/The Early Birder
In large preserves, Wagenius finds that most coneflowers produce seed with most of their neighbors. The smaller the population, the fewer compatible neighbors. In the smallest fragments, 30 to 40 percent of crosses are incompatible. Wagenius has also monitored coneflowers grown from these seeds, finding similar results at other stages of growth. In a common garden where they share the same soil and rainfall, plants grown of seeds from larger preserves produce twice as many flowerheads as plants from small patch seedstock. Coneflowers in isolated colonies are hitting a double genetic barrier: they suffer from inbreeding, and the very strategy that once maintained their genetic diversity is now preventing some plants from making seeds at all.
Other species face different genetic challenges. Dan Gustafson of the Citadel in South Carolina cites local adaptations in big bluestem to caution against blindly importing seed. At Goose Lake Prairie, he looked at side-by-side fields of bluestem, one from Illinois seed and the other from Nebraska seed. The Nebraska grass was two feet shorter and suffered insect damage, while the Illinois grass was untouched. Illinois bluestem likely has chemical defenses against local insects that Nebraska bluestem lacks. Crossing Illinois bluestem with other strains would probably lead to outbreeding depression as these traits were diminished or lost. Fortunately, Gustafson believes big bluestem here maintains healthy genetic diversity, perhaps because it is so abundant, with thousands of individual plants even in small preserves.
Photo: William C. Miller III
Kay Havens, a scientist at the Chicago Botanic Garden, offers another perspective on outbreeding. She is studying two species of lobelia, moisture-loving flowers of wet prairie and woodland. The red blossoms of the cardinal flower attract hummingbirds, while the flowers of the great blue lobelia bring in bumblebees. Havens brought seeds of both species north to the Botanic Garden from outside Joliet, and they grew well despite their new environment. She crossed cardinal flowers from the southern stock with Botanic Garden plants, and they also did well. But when she outcrossed the distant colony of great blue lobelia with local plants, the successive generations were “smaller and wimpier” — classic outbreeding depression.
Cardinal flower and great blue lobelia are close in evolutionary terms, but she thinks their different pollinators caused the divergent results. “Bumblebees make frequent stops, and even if they get a few miles down the road, there is probably little pollen carried over,” Havens said. The short-hop bees don’t hold distant blue lobelia populations in genetic contact. Hummingbirds, however, bear pollen far enough that the cardinal flowers developed as a single population over long distances, much like Mary Ashley’s bur oaks.
Havens says there is no one answer to the genetic problems prairie plants face. “We have to develop species-specific strategies,” she says. Our fragmented landscape has left a complicated legacy. Plants that once had a deep reservoir of genetic variation distributed among thousands or millions of individual plants now swim at the shallow end of their gene pool. Only wise management will prevent those pools from drying up.
Where a restoration group gets its seeds
Photo: Carol Freeman
Members of the North Branch Restoration Project (NBRP) knew early on that habitat restoration work would involve preserving and restoring local gene pools. Toward that end, with the Forest Preserve District and other experts, the group has been honing its seed policy for 30 years for its stretch of preserves in northeastern Cook County. Some principles:
In the early years, the group collected seed from “spontaneous populations” at nearby prairie remnants in peril of development. Thus, restoration areas currently “contain the genetic information from a wide variety of local populations, many of which are now extinct.”
The group typically doesn’t introduce seed into its highest quality areas. Exceptions are certain rare species, when their success depends on high-quality associates.
NBRP annually documents how much seed is collected for each species. These days, most seed comes from the large populations thriving at the restored areas.
The end goal is to “restore the natural community to a healthy state, one where the community becomes self-renewing and self-sustaining yet dynamic, allowing for evolution and other natural processes to occur.”