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Cultivar Crazy—Preserving the Genetic Heritage of Plants
Plants & Gardens News | Volume 13, Number 3 | Fall 1998
by Janet Marinelli
If the purple coneflower Echinacea purpurea 'Magnus' were reincarnated, it would reappear as Dolly Parton.
Dolly, the singer/ celebrity, is known as much for her big blonde hair and—how shall I put this?—ample upper anatomy as her Smoky Mountain country trill. 'Magnus' is the horticultural equivalent of the typical overdone, slightly floosy country music star: Declared Perennial of the Year by the Perennial Plant Association (PPA), 'Magnus' has showy big blooms with glowing orange cones and deep pink flower petals. But what truly distinguishes it from the rest of its purple coneflower kin, lovely denizens of North American prairies and meadows, are its petals, which are not reflexed—they don't curve downward but rather alluringly (in the opinion of the PPA) stick straight out.
The fixation on size and glitz in some gardening circles—dahlias as large as dinnerplates, $900 clivias—is nothing new. Lately, because native plants are chic, there has been a proliferation of celebrity wildflower cultivars. The growing popularity of indigenous species stems in large part from the desire of ordinary gardeners to help preserve the continent's beleaguered native plant life. Consequently, it's worth exploring whether the new native cultivars are likely to have such a salutary effect.
With the advent of high-tech tools, researchers have discovered that plant species generally have high levels of genetic variation, and that if we're serious about saving plants we need to preserve their genetic diversity. Before your eyes begin to glaze over, I should point out that this isn't as abstruse as it sounds. Individual members of a species, whether magnolia or human, differ. Some of us have blonde hair; some, brown. A few of us are endowed with Dollyesque proportions; the rest of us purchase push-up bras. The rare purple coneflower has horizontal petals instead of the delightfully droopy ones. Biologists believe that genetic diversity provides species with the wherewithal to overcome the ecological pitfalls thrown in their path—a new insect pest, a drought, a warming planet—what Darwin would call natural selection pressures, the assumption being that a few of these diverse individuals are likely to have the genetic repertoire needed to cope. And the way most species get genetic variation is via sexual reproduction, with its random recombination of genes that occurs with each new generation.
Now consider the nature of cultivars. Cultivars (the plants that have additional names in single quotes after the two-part botanical names) are prized for their unusual characteristics, and the surest way for a grower to guarantee that you'll get the desired traits of the parent plants is to propagate them asexually, typically from cuttings (as opposed to sexually, from seed). The bottom line is that when you buy, say, Aster novae-angliae 'Purple Dome', a cultivar of the native New England aster known for its compact, mounded habit as well as its flashy deep purple flowers, you're buying a clone—a plant that is genetically identical to every other specimen of 'Purple Dome' in existence. No genetic variation there.
Even seed-propagated cultivars such as Echinacea purpurea 'Magnus' do little to promote their species' genetic health. In order to insure that these plants "come true" when grown from seed—that you'll get those unusual horizontal petals—breeders must eliminate most variation in the gene pool through repeated back-crossing.
Plant Pets
You could say that cultivars, like Holstein cows, quarter horses, and chihuahuas, are domesticated organisms, "plant pets." They're created when human beings apply artificial selection pressures to control the species' evolutionary course.
Since our neolithic ancestors began domesticating wild plants 10,000 or 15,000 years ago, we humans have been encouraging certain traits and eliminating others. Our proclivities are so predictable that researchers have come up with a fancy moniker for the process: the domestication syndrome. For example, the plants become easier to grow, and produce the desired product more reliably or abundantly, whether it is flashier flowers or a larger potato tuber. The valuable (to us) parts of the plant—flower, seed, fruit, root—grow huge; scientists call this phenomenon gigantism.
Problem is, characteristics that appeal to humans may also make a plant less able to slug it out in the wild. Horticultural fashion may select traits such as double flowers, which typically are sexually dysfunctional. Conventional cultivation practices may favor individuals or varieties that flourish in a garden, but not in the natural landscape. Domesticated plants can become utterly dependent upon people for their growth, reproduction, and evolution. Not what most natural gardeners have in mind when they plant a purple coneflower in their little patch of prairie, hoping to augment the gene pools of shrinking wild populations.
If half a dozen gardeners in Missouri plant 'Magnus' in their meadows, is this the end of the world? Not likely. What is scary, though, is the prospect of 'Magnus' rocketing to stardom of country music giant Garth Brooks magnitude, with zillions of specimens suddenly for sale at Plant Depots throughout the East and Midwest. Unique local gene pools could be swamped by 'Magnus', with potentially devastating effects on the species' genetic diversity. The resulting offspring of wild plants could be the botanical equivalents of the 98-pound weakling.
Genetic swamping is especially detrimental for plants whose natural populations have been tattered by human development. The large, previously continuous "metapopulations" of these species have been splintered into small, more or less isolated subpopulations. Small populations mean small gene pools. 'Magnus' or its equivalent comes along, swamping the species' already shrinking genetic base. Chance—what researchers call genetic drift—can take over, and more beneficial traits may be lost in the random recombination of genes that occurs during sexual reproduction. In species that don't typically grow in small, far-flung populations, so-called inbreeding depression, or interbreeding between closely related individuals, can drastically reduce genetic variation, causing still more reproductive problems for the plants and further reducing population size—a downward spiral that has been termed "genetic meltdown."
Now, don't get me wrong—I'm not knee-jerk anti-cultivar. A soft spot for the latest gigantic pompom chrysanthemum is pretty harmless: They're not invasive, so they won't escape into the wild and bully other plants; and since they originated in Eurasia, they're not going to harm any North American gene pool. I like trying tasty new tomato (or other crop) cultivars, though I continue to grow heirlooms, too, to help preserve the diversity of our food plants. I'd be the first in line to buy a disease-resistant selection of the native flowering dogwood (Cornus florida), which is currently being ravaged by anthracnose.
Using Our Noodles
To make matters still more complex, whether or not inbreeding is a potential problem for a species depends, among other things, on its preferred reproductive technique. Some plants are naturally inbred because they tend to reproduce asexually, or their flowers tend to pollinate themselves instead of relying on pollen from flowers on other plants and the genetic recombination that that brings. Any deleterious traits in these species would have been largely purged over time by natural selection.
In short, if we're ever to be effective natural gardeners, we need to start using our noodles. We need to know a lot more about the reproductive preferences, genetic profiles, and possible ecological effects of plants we decide to grow. When in doubt, a good rule of thumb is to buy native species that have been propagated sexually, by seed collected from diverse nearby natural populations.
Without a Dolly or a 'Magnus', the world would be a poorer place. But on a planet replete with Dolly Partons, there would be no Pam Tillis, no Deanna Carter, no Shania Twain.
Brooklyn Botanic Garden Director of Publishing Janet Marinelli is editor of BBG's renowned series of quarterly gardening handbooks and the author of Your Natural Home and The Naturally Elegant Home. Janet is a champion of the gardener's role in the preservation of the planet, a philosophy that informs her P&G News column, "Down to Earth." It's a philosophy that also serves as the bedrock for her latest book, Stalking the Wild Amaranth: Gardening in the Age of Extinction. In Stalking the Wild Amaranth, Janet tells of her quest for a landscape art that protects disappearing species, both flora and fauna. It's a gardening journey marked by humor—ecologically sensitive gardening needn't be a dreary affair, Janet insists. "We can do our part," she says, "and still have flair and fun."