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Engineering the apple.
Author: Hubbell, Sue. Source: Natural History v. 110 no8 (Oct. 2001) p.
44-53 ISSN: 0028-0712 Number: BGSI01175457 Copyright: The
magazine publisher is the copyright holder of this article and it is reproduced with permission. Further reproduction of this article in violation of
the copyright is prohibited.
One autumn morning not long ago, I was walking down a row of espaliered
apple trees near Geneva, New York. I was visiting the biggest living
library of apple trees anywhere in the world--the Plant Genetic Resources Unit of the U.S. Department of Agriculture (USDA), based at Cornell
University. The day was cold and the sky leaden, promising an early snow, but maples in full autumn color ringed the field and echoed the
cheerful reds, russets, and yellows of the apples. Some of the apple trees had drooping limbs; some grew straight and stiff. The shape of the
leaves and the color of the bark varied, as did the fruit--some in clusters, some dangling independently. Some apples were huge, others not
even bite-sized. The names of some apples were unfamiliar to me, yet they tasted so good that I wondered why they weren't in markets.
Others were so sour or bad-tasting that I quickly understood what apple growers mean by the word "spitters." The trees--and the look and
taste of the fruit they bore--were so dissimilar that it was hard to believe the entire group was botanically related. But my guide for the
morning, Philip Forsline, curator of apples and sour cherries for the USDA, told me that even bad-tasting ones could be of interest because of
their manner of growth, time of bearing, hardiness, or resistance to disease and pests.
Commercial apples are a serious business in the United States, the world's
second-largest apple-producing country (after China). Putting in a
commercial orchard or replanting an old one with a new variety takes money, time, and labor. Years pass before new trees bear enough fruit
to pay back the orchardist for the investment, so apple developers need to be sure of the qualities being packed into a new variety before they
promote it. And this is where apples present a real challenge. Nearly every apple tree grown from a seed is a new variety, whose fruit may not
be at all like that of the mother tree. Such unpredictability is a serious problem for orchardists. Their most common solution--invented long
before there was a sheep named Dolly--has been a type of cloning known as grafting, an ingenious way that humankind discovered to make
an end run around the intricacies of apple genetics. Orchardists take a shoot (called a scion) from a tree that bears good eating apples and
bind the shoot to the trunk of a tree that doesn't produce good apples but has other desirable properties, such as vigor, resistance to disease,
or the ability to stay a manageable size. The grafted shoot will grow up to produce the same good apples as the tree it was taken from. It is a
clone of that tree, growing on another root system.
A hands-on solution to a practical problem, grafting offers little insight
into apple genetics, which are so complex that bewildered botanists
used to think that apples did not obey Mendel's laws. It is true that the progeny grown from the seeds of an apple tree do not sort themselves
out into the neat, predictable pattern that Gregor Mendel first laid out in the 1860s with his pea plants. But we now know that Mendel was
lucky. The pea plant traits he studied (flower color, for instance) are controlled by single genes (or "factors" of inheritance, as he called them).
More often, though, a single trait is affected by more than one gene. In addition, certain genes are expressed only when environmental
conditions activate them (such as the darkening of Siamese kittens' paws due to the lower temperature of their extremities). And that's just for
Like cats and people, most apple trees are diploid--that is, their genes
occur on pairs of chromosomes. Typically, apples have seventeen
pairs of chromosomes, for a total of thirty-four. But some varieties are haploid, with seventeen single chromosomes. Others, especially among
crab apples, are polyploid, which means that their chromosomes are not paired but tripled, quadrupled, quintupled, or even wadded up into
bundles of six. In fact, some apples have as many as eighty-five chromosomes. And each of the genes on each of the chromosomes can
have different alleles (alternative forms). A single seed may thus contain a lot of genetic variation that has accumulated down through the
Orchardists have unquestionably benefited from the apple's easygoing acceptance
of extra chromosomes. Many familiar varieties of apples
are polyploid: Stayman, Jonagold, Baldwin, and the beloved old pie apple Rhode Island Greening. The Jonagold is a modern, contrived cross
between Jonathan and Golden Delicious parents. But the Stayman sprang up all on its own, from a Winesap seedling in Kansas; the Baldwin
and Rhode Island Greening, too, were spontaneous polyploids.
The desire for a predictable product, however, has led most growers to
focus on a very limited portion of the apple's tremendous genetic
diversity. In fact, the vast majority of contemporary apples are the progeny of just a few varieties--a dozen at best. As a result, today's
available genetic base is meager compared with that in the nineteenth century, when an estimated 7,000 or more kinds of apples were
cultivated. In addition, many of the varieties commercially grown today "present well" (that is, have a uniform, unblemished appearance), but
they no longer have much flavor. This is why many horticulturists have recently been devoting themselves to tracking down old varieties on
abandoned farms and in old cow pastures. And it is why, since 1989, the USDA has been sending scientists on collecting expeditions to the
presumed birthplace of eating apples: the high-altitude forests of Kazakhstan, in central Asia.
The apple brought back to the Cornell facility from Kazakhstan is Malus
sieversii, a tree with no common name. Many researchers, including
Philip Forsline, believe that M. sieversii is the principal ancestor of all the varieties of apples we buy at the supermarket, which are collectively
known as M. (times) domestica. The M. sieversii seeds gathered during the first expedition to this species' homeland--in the region of the
Kazakhstani industrial town of Almaty (formerly Alma-Ata, "father of apples")--have produced trees that are now mature enough to bear
apples. Trees at the facility are grown in tight, close rows, which keeps them more compact and uniform than they would be in their native
forests. Nevertheless, even to an untrained eye the individual trees show obvious differences. The leaves are of various shapes and hues; the
trees branch and twig in different ways. Some send up many stems, giving the trees a shrubby appearance.
Also in the USDA collection are younger M. sieversii trees, grown from
seeds gathered more recently and a long way from Almaty, in places
that have never been farmed. Too young to bear fruit, these trees are still thorny, betraying their relationship to roses. (Roses and apples
belong to the large family Rosaceae, which also includes strawberries, pears, and the stone fruits, such as plums, cherries, and apricots.)
Some of the new seedlings are already showing resistance to various pests and diseases. According to Forsline, "The interesting thing is that
since the trees in the apple forests grow up from seeds, each is different from its neighbor. You can see a healthy tree growing right next to a
diseased one. Resistance is genetic. We think it would be good if we can incorporate resistance into commercial varieties of apples.".
Apples of the species M. sieversii were first described in modern times
by P.S. Pallas, a German naturalist who in 1786 saw them growing in
the Caucasus, where, he noted, apple trees competed for dominance with oaks. In 1911 Frank Meyer, an American who traveled the world
looking for unusual plants, took note of the apple forests in the Tien Shan ("mountains of heaven"), a range straddling the border where China
meets Kazakhstan and Kyrgyzstan. Meyer had crossed into those mountains from China in early spring and had spent the next few months
exploring the range, struck by the unusual forests of apples he found, sometimes at altitudes of 10,000 feet. No one followed up on his
discovery until the 1920s, when Russian agronomists started investigating the area.
People who think the apple originated in Kazakhstan theorize that M. sieversii
probably hybridized rapidly with crab apples native to central
Asia. According to some botanists, it is those hybrids, not pure M. sieversii, that became the ancestors of what we now think of as proper
eating apples. But others disagree, seeing the wide distribution of crab apples--which are native not only to Asia but to northern latitudes
around the world--as evidence for apples having had other places of origin. Through genetic change, crab apples might, on their own, have
come to produce apples that were sweeter and bigger--the ancestors of domestic apples.
The question of whether apples began in one place or several may now have
been settled by Barrie Juniper and other scientists at the
University of Oxford, who have been analyzing the chloroplast and nuclear-DNA sequences of cultivated apples in order to untangle their
origins. Their work to date has uncovered no evidence that M. sieversii hybridized as it moved westward--supporting the claim that the wild
apple forests of Kazakhstan did indeed give rise to the sweet eating apple.
Wherever apples originated, by the middle of the third millennium B.C.,
tasty eating apples were being cultivated far to the west of the Tien
Shan range, even as far as Persia. By the first millennium B.C., apples had become a standard part of the diet of the well-to-do in the
Mediterranean world. The Romans spread the knowledge of apples and their cultivation throughout the territories they conquered. By medieval
times, orchardists in Europe had become so skilled that the privileged classes could offer apples to their guests every month of the year by
growing some varieties that ripened early and others that were "good keepers," retaining their freshness through the winter. To own an apple
orchard was as much a source of pride as having a good wine cellar is today.
Yet apples were not universally admired during the Middle Ages, especially
among the less privileged. Popular wisdom had it that apples
caused "bad stomachs" and fever, as well as "ill humors." This belief may well have been reinforced by the apple's association with the Fall in
various translations of the Bible, including the King James Version (though in the oldest Hebrew and Greek texts, the Tree of Knowledge
bears merely a generalized "fruit").
For several hundred years, apples were treated with some suspicion, but
by the seventeenth century the fruit was back in favor, and many
varieties were being grown and grafted throughout Europe. Settlers in the New World, finding the place lacking in sweet apples, were quick to
import them from Europe. New Englanders grew apples not so much for eating as for animal feed and cider making. (At the time, access to
potable water was not a simple matter of turning on the tap, and the newcomers needed to produce a beverage that would satisfy thirst
without making them ill. Cider, fresh or hard, was such a drink.) For these purposes, good eating apples were not necessary. In addition, the
first settlers--often urban dwellers unskilled in rural crafts--may simply have been ignorant of grafting.
For whatever reasons, most apple trees in America grew for a couple of
centuries without benefit of grafts. And without grafting to produce
only selected strains, the rich genetic heritage locked up inside the wild apple trees was allowed expression--serving as what one collector of
wild apple seedlings called "the biggest genetic experiment the planet has ever seen." This genetic experiment was taken westward by John
Chapman, better known to generations of Americans as Johnny Appleseed, who traveled more than 100,000 miles across the Midwest,
handing out and planting apple seeds along the way. Since he was a follower of Emanuel Swedenborg, an eighteenth-century Swedish
scientist and religious thinker who condemned tampering with the will of nature, Chapman eschewed grafting.
By the time of Johnny Appleseed's death in 1845, however, grafting was
becoming the preferred method of propagation for serious commercial
orchardists in the United States, as had long been the case in Europe. Greater control over apple production led to a growing emphasis on the
appearance, shipping, and keeping qualities of apples rather than on their flavor. And by limiting the genetic base of the apples they grew,
orchardists increasingly found the fruit susceptible to scab, mildew, brown rot, and a host of other diseases--not to mention coddling moths,
aphids, and spider mites, three of the hundreds of insects and mites that plague the modern grower. Nowadays those diseases and pests are
fought with a whole witches' brew of chemical sprays, dusts, and powders--which gets us back to the USDA's interest in the apples from
Kazakhstan, many of which show innate resistance to bugs and blights.
When resistance comes from several genes--that is, when it is polygenic--it
is longer lasting, but such resistance is harder to achieve in
crossbreeding. Single-gene crossbreeding for resistance can be done by first making the original cross and then backcrossing to eliminate
traits such as small size and sour taste that may have come along with the desired gene. But when the trait is polygenic, it may take five or
six backcrosses to eliminate the unwanted genes that have hitchhiked along with those helping to confer resistance. One apple tree
generation is about four years (that's how long it takes, on average, before a tree bears fruit), so the development of polygenic resistance
would take something like twenty years, as well as a plot of land big enough to grow and try out many crossbred trees.
This explains why many apple breeders are interested in genetic engineering--in
the new sense of what is called transformation. If the genes
responsible for a certain trait can be identified and directly transferred, the process of transformation is quick, efficient, and in some ways
easier than cross-breeding, because it eliminates the need for back-crossing. Resistance can even be transferred from animal species. The
cecropia moth, for example, produces a peptide that attacks many kinds of bacteria. When the moth gene responsible for producing that
protein was added to the DNA of apples used for rootstocks, the apples grafted onto them became resistant to fire blight, a devastating
bacterial disease that turns apples, pears, and other plants black. And at least for Gala and McIntosh apples, the same cecropia moth gene
seems to help confer scab resistance, too.
Whatever the promise (or, in the opinion of some, the threat) of genetic
engineering, everyone agrees that preserving the natural diversity of
apple trees should be a high priority. And on this score, there is cause for alarm: the apple forests of Kazakhstan are in danger of
disappearing. "During the Soviet period," says Forsline, "the area was held as national parkland, but with the breakup of the Soviet Union, the
mountains are no longer protected. Wealthy people are having many remote areas bull-dozed and cleared in order to build vacation homes.
The apple forests are disappearing, and the groves nearest to Almaty are 90 percent gone.".
At the end of my tour through the Geneva orchards, I asked Forsline which
apples would continue to grow if humans disappeared from the
planet. He reflected for a moment and said, "Well, the eating apples that we've grown on this continent would soon be gone. Those need our
care, and besides, they aren't native here. I don't think any of them would survive here." He paused, then added, "But apples would still grow
in central Asia, and they'd be better.".
In what way? I asked.
"The places where they grow now would no longer be threatened by development,
and they would continue to evolve, growing hardier, more
resistant to disease and pests, and better fitted to their particular surroundings.".
Adapted from Shrinking the Cat: Genetic Engineering Before We Knew About
Genes, by Sue Hubbell. Copyright 2001 by Sue Hubbell.
Published by Houghton Mifflin and reprinted by permission.
Braeburn apple, Wenatchee, Washington Text incomplete in journal.
A Roman mosaic, below, shows farmers grafting apple trees. Opposite page:
The harvest at an organic orchard in Washington State. ERICH
Legendary apples, opposite page: The Hesperides guard a tree bearing the
golden apples given to Hera as a wedding present. Below:
Temptation of Eve, by the sculptor Gislebertus, ca. 1130. ERICH LESSING/ART RESOURCE.
Ready for the harvest: Three-year-old Gala apple trees in a commercial
orchard, right. Below: Grafting involves inserting and sealing "scion
wood" into slits in a tree's trunk. JOHN MARSHALL.
Above: Apple specialist Philip Forsline in a Kazakhstani forest of Malus
sieversii. Map: The town of Almaty is known as the "father of apples."
Isaac Newton's tree-shaded worktable, below. Did a falling apple inspire
his concept of gravity? Opposite page: Fluctuating summer weather
resulted in three stages of bloom at one time for a Braeburn apple. ERICH LESSING/ART RESOURCE.