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Cloning—Long Before Dolly the Sheep, There Was 'Bartlett' the Pear
Plants & Gardens News | Volume 12, Number 3 | Fall 1997
by Janet Marinelli
Next time you're browsing at the local garden center, consider Dolly. During the recent brouhaha over this chubby little lamb, the first mammal ever cloned from a single adult cell, not one politician or pundit noted that in horticulture cloning has been a routine procedure for decades. In fact, most of the plants found at nurseries today are named cultivars, which are propagated asexually, generally from cuttings (as opposed to sexually, from seed). The reason? That's the only way to preserve choice characteristics of the parent plants, whether a lanky growth habit, giant flowers, or an unusual leaf color. So when you buy, say, Aster novae-angliae 'Purple Dome', a New England aster known for its compact, mounded habit as well as its vibrant purple flowers, you're buying a clone—a plant that is genetically identical to every other specimen of 'Purple Dome' in existence. (Clones are those plants that have additional names in single quotes after the two-part botanical names.)
One reason the cloning of mammals came later than that of plants is because there are more biological roadblocks to the former. Chop off a human thumb, for example, and you're left with nothing but a stump. But cut off a shoot of the plant, and it can form adventitious roots; cut off some roots, and they can generate a new shoot system. Leaves can regenerate both roots and shoots. Nevertheless, the implications of plant cloning, though less obvious than those of cloning mammals, especially humans, are still worth pondering.
At first glance, asexual propagation would seem to be, as critics claim, an unmitigated disaster for genetic diversity, the most basic level of the diversity of life. But it's not quite so simple. Perhaps we humans are prone to such oversimplifications because we're large animals, and large animals tend to be rather drab in the bedroom department. Whether Homo sapiens or giant tortoise, we come in two genders: male and female. We must pair up to reproduce. Period.
Gender Benders
But plants! Plants display a (to us) staggering assortment of sexual techniques. For decades scientists—and scientific journalists—intent on observing evolutionary oddities on far-flung oceanic islands have helped publicize Charles Darwin's voyage in the Galapagos. The Galapagos may have made for better headlines, but truth is, Darwin actually spent most of his time exploring the sexual relations of plants at his home in the Kentish village of Downe. Darwin, and Linnaeus before him, were fascinated by the sex lives of plants. And for good reason, I think. Some plants, such as hollies, are dioecious, meaning that like large animals, each plant is either male or female. Ho-hum. Other plants, the common cane begonia, for instance, are monoecious—that is, individual specimens have separate male and female flowers. Still other plants are hermaphroditic, which is to say that their flowers have both male and female organs. Peer into a corn poppy and beyond the four, typically red petals you'll see a tiny green urn, the female organ called the pistil, surrounded (it figures) by scores of male organs called stamens—long, slender, sticklike filaments topped by pollen-bearing anthers shaped like little coffee beans.
And that's not the least of it. Darwin devoted an entire book, The Different Forms of Flowers on Plants of the Same Species (1877), to the distinctions between what he called two "interesting" subgroups of hermaphroditic plants: heterostyled and cleistogamic. Interesting indeed. I like to look at heterostyly as a kind of coquettishness in plants. Some species produce styles (structures connecting the stigma, or pollen-trapping surface, to the ovary) of two or more lengths so they are taller or smaller than the male parts, the anthers. This serves to discourage self-fertilization and promote pollination by other flowers from different plants—or, to put it another way, to keep the neighborhood boys at bay and give some handsome strangers a chance. Of course, some plants couldn't care less about handsome strangers; these are the cleistogamous, or self-copulating, plants. Their flowers don't even bother to open, but rather pollinate themselves. Some violet flowers, for example, are cleistogamous. In fact, many plants are capable of "selfing" to one degree or another—that is, fertilizing the female part of the flower with pollen from the same flower. What's more, plant species can reproduce sexually, or asexually, or both.
Who needs to go to the Galapagos when all this is going on in the backyard?
Whatever Works
Evolutionary biologists believe that sexual reproduction provides the genetic variation that enables a population to adapt to changing conditions. However, for a population that is already well adapted, asexual reproduction can have its advantages; in effect, it's Mother Nature's version of the adage "If it ain't broke, don't fix it." Grasses, for example, reproduce asexually by producing new shoots from their crowns. Eventually, the connection between the offshoot and the parent plant disintegrates, and a new grass plant—a clone—is established. In some plants, seeds regularly develop from egg cells unfertilized by pollen, in a process called apomixis, representing the ultimate form of single parenting: not even a sperm donor is necessary. The resulting plants are clones as well. Many other kinds of cloning can also be seen in the plant world, including the formation of "daughter" bulbs and corms, the rooting of arching stems that touch the ground, and the growth in kalanchoes and other succulents of tiny plantlets along the edges of leaves.
Lately, cloning has come to be seen as the forbidden fruit of biotechnology, but many horticultural clones have long been important food plants. The 'Bartlett' pear clone originated as a seedling in England around 1770, and the 'Delicious' apple clone was born about a hundred years later as a chance seedling in Jesse Hiatt's orchard in Peru, Iowa. Mealtime as we know it would be very different without clones, but clones have been equally common in ornamental horticulture. They guarantee that we'll get the compact New England aster instead of a tall, spindly one for a particular spot in a perennial border, or a highly fragrant, yellow-flowering form of Carolina allspice (Calycanthus floridus 'Athens') instead of the species proper with its reddish brown blooms.
A few cultivars for the flower border or vegetable patch can be a good thing. Even in the natural garden, clones have a place: wouldn't it be wonderful, for example, if someone could find a cultivar of native flowering dogwood that would not be susceptible to the anthracnose fungus that has been decimating wild populations? Then, too, it's a safe bet that named cultivars of native trilliums, orchids, and other species threatened by unscrupulous collectors have been propagated vegetatively, not dug up from the wild. So why not use them? Nature employs cloning in certain instances, so why shouldn't we?
Vastly Overdone
The real problem with cloning is that it's vastly overdone. As a rule, open-pollinated vegetables and ornamentals propagated from seed are a wiser choice. The overuse of clones in agriculture and horticulture is risky because if a major food crop, say, is based on only one or two cultivars, a disease or insect attack can wipe it out. A clone can perpetuate itself successfully in nature, sometimes even better than a plant resulting from a sexual union, but only so long as the environment remains reasonably constant. If the environment changes drastically, a clonally reproduced or highly selfing species may be at a disadvantage because it's less likely to be able to evolve forms better adapted to the new conditions. And in the twenty-first century, change, whether global warming or a spate of new imported insect pests, is undoubtedly in store.
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."