에덴의 사과: 현대 사과의 야생종을 보존하기

Central Asia's fruit tree species are invaluable storehouses of genetic diversity with the potential to secure the future of global crops.

This story is part of National Geographic's special eight-month Future of Food series.

An epiphany came to Adrian Newton in the form of an afternoon tea. In 2009, the British forest conservation ecologist was surveying threatened fruit trees in the forests of the western Tien Shan mountains, in the Central Asian Republic of Kyrgyzstan, when local residents invited him into their tapestry-bedecked home in the heart of the woods to share a ceremonial meal.

"They sit you down and make you this lovely cup of tea, and then you're served a whole range of different jams and preserves, and all of these are local. They're all made from the forest and [are] absolutely delicious," says Newton, a professor at Bournemouth University in the United Kingdom. "That's when it really hit home to me what a fantastic cultural value these forests are. You do feel in a small way that you are in a land of plenty." (Related: "Beyond Delicious.")

The ancient woodlands of Kyrgyzstan—and of the four neighboring former Soviet republics of Kazakhstan, Tajikistan, Turkmenistan, and Uzbekistan—are home to more than 300 wild fruit and nut trees. They include walnut trees, eight to ten species of cherry, up to ten species of almond, four or five plum tree species, and four wild species of apple, according to a 2009 report co-authored by Newton, The Red List of Trees of Central Asia.

According to that same report, 44 species of trees and shrubs in the region are "critically endangered, endangered, or vulnerable." They've been menaced for decades by overgrazing, pests, diseases, timber—felling for fuel, and most recently, climate change.

One of these threatened species, Malus sieversii—a wild apple that Newton describes as "small but highly colored with a very nice sweet flavor"—is one of the key ancestors of all cultivated apples grown and eaten around the world. So rich and unique is this species, Newton says, that on one wild apple tree, "you can see more variation in apple form than you see in the entire cultivated apple crop in Britain. You can get variation in fruit size, shape, color, flavor, even within the tree, and certainly from tree to tree."

Several thousand years of selective breeding have mined that diversity to give us the varieties we know today, from the Golden Delicious to Cox's Orange Pippin to the improbably named Winter Banana. Just 10 of the 3,000 known varieties account for more than 70 percent of the world's production.

But in the process many traits that might still be valuable—genes for disease resistance, say, or heat tolerance—were left behind. For breeders of apples and other fruits today, tapping the riches of the original Garden has become a practical strategy—and saving it from destruction, Newton says, an urgent necessity.

Malus sieversii has been identified as the wild ancestor of domestic apples.

Apple Knowledge

The Latin noun malus can mean either "apple" or "evil," which is probably why the "tree of knowledge of good and evil" in the Garden of Eden is often depicted as an apple tree, even though the biblical book of Genesis does not say what sort of fruit tree it is.

In 2010, a research team led by Riccardo Velasco of the Edmund Mach Foundation in Trento, Italy, took knowledge of apples themselves to a new level: They sequenced the complete genome of the domesticated apple Malus domestica. It has the highest number of genes—57,000—of any plant genome studied so far, and about 36,000 more genes than humans have.

Velasco's team also identified M. sieversii as the wild ancestor of domestic apples, reporting that it was domesticated in Central Asia some 3,000 to 4,000 years ago. But it turns out not to be the only grandma of the Granny Smith.

A 2012 study led by Amandine Cornille, now an ecologist at Uppsala University in Sweden, showed that the domesticated apple acquired genes from other wild varieties as it spread west along the Silk Road. Traveling traders, Cornille explains, unwittingly dispersed cultivated apples by consuming them and excreting their seeds en route, as did their camels and horses.

Cornille and her colleagues in France, Armenia, China, and Russia sampled and sequenced rapidly evolving DNA regions from wild apple species in Siberia and the Caucasus, as well as from Malus sieversiiand Malus sylvestris, the wild European crab apple. Some of these wild apple trees, they note, bear "small, astringent, tart fruits," yet had more valuable traits as well, including pest and disease resistance or longer storage capacity. The genetic analysis showed evidence of frequent hybridization of domestic apples with wild species. Many of those crossings were probably done deliberately by farmers.

The wild crab apple in particular was a "major secondary contributor" of genes to the modern domesticated apple, according to Cornille, beginning about 1,500 years ago. In fact, the domesticated apple is now more closely related to M. sylvestris than to its original ancestor in the Tien Shan mountains.

According to tree experts from Fauna & Flora International, around 90 percent of the fruit and nut forests of Central Asia have been destroyed in the past fifty years.

Cultivating Diversity

Modern breeders at the U.S. Department of Agriculture and at Cornell University in Ithaca, New York, are using both traditional grafting techniques and genetic engineering to continue the work begun by farmers along the Silk Road, melding wild apple genes into domesticated varieties.

In the 1990s, horticulturalist Phil Forsline of the USDA's Agricultural Research Service (ARS) and Herb Aldwinckle, a plant pathologist from Cornell, trekked to the forests of Kazakhstan, Kyrgyzstan, Russia, and China on multiple trips to collect seeds and grafts of M. sieversii. According to Thomas Chao, the apples, grapes, and tart cherries curator at USDA/ARS in Geneva, New York, the pair collected 130,000 seeds ofM. sieversii. More than 1,300 M. sieversii seedlings have since been grown in Geneva orchards and screened for disease resistance, drought, cold tolerance, and other traits.

The goal, says USDA plant physiologist Gayle Volk, is to "capture and conserve" the diversity not just of wild apple species in China and Central Asia but also of native species in the U.S. Volk, who describes herself as "very passionate about apples" is based at the ARS National Center for Genetic Resources Preservation in Fort Collins, Colorado, which houses what she describes as a "monstrous vault" storing hundreds of thousands of seeds of many different species. one of Volk's projects is sequencing and fingerprinting the DNA of wild apples to identify genes that may code for disease resistance, crunchiness, or flavor.

"Commercial crops are limited to about 15 different kinds of apples; these are what everyone knows and grows," she says. Yet commercial varieties are under threat not just from the "classic biggies"—fire blight, apple scab, leaf spot—but also from climate change. Apple growers across the U.S. have suffered crop losses in the past few years because of increasingly frequent warm spells in February that wake the trees from winter dormancy.

"The trees flower in March and lo and behold, another snow comes along, and they get clobbered by the snow, and they lose a lot of blossoms and a lot of fruit set, because the climate is not ready to accept baby apples yet," Volk says. one possible solution, she adds, would be to introduce genes from apples adapted to warmer climes, such as the Southeast Asian apple Malus doumeri, or ideally from species that remain dormant during brief warm spells.

Conserving both wild populations and their descendants is "absolutely crucial," adds horticulturalist Susan Brown of Cornell University, who is mining young M. sieversii trees in the Geneva orchards for genes promoting apple scab resistance and nutritional compounds. The Geneva collection, she says, is a "Noah's ark of apples" ferrying potentially valuable mutations or genetic variants into the future.

A bowl of fruit and nuts collected from the forests of Kyrgyzstan.

Protecting the Garden

Adrian Newton and his colleagues have spent the past eight years traveling back and forth to Kyrgyzstan to work with forest ecologists at Kyrgyz National Agrarian University in the capital, Bishkek, to better protect the fruit and nut forests. The challenges are considerable.

An uprising roiled the country in 2010, leading to the overthrow of then President Kurmanbek Bakiyev. The breakup of the Soviet Union ended coal subsidies and deliveries, leading to greater dependence on fuel-wood harvested from the forests. Rural residents often lease forest plots from the government, grazing cattle, horses, sheep, and goats in the woodlands.

Some fruit species, including wild apricot, are imperiled by overcollection of seeds by national and international plant-breeding companies. Pretty flowers can also become a liability: The pink blooms of one endangered species of wild almond from Kazakhstan, for example, are "particularly in demand for International Women's Day," according to Newton's report.

Some of the most recent news has been bright, however. In a recently completed field survey in Kyrgyzstan, Newton and Bournemouth colleague Elena Cantarello discovered that seedlings, saplings, and adult trees of M. sieversii, the ancestral apple, were "not as threatened as was originally thought," says Newton, but "still very restricted in extent."

And in one just-published model of species diversity in the Sary-Chelek Biosphere Reserve in the Tien Shan mountains of Kyrgyzstan, Newton and his colleagues found that moderate livestock grazing—not overgrazing—may be beneficial to walnut trees, as it opens sunlit spots for the shade—intolerant trees and patches of bare ground on which new tree seedlings can establish themselves.

Local conservation programs are helping, according to Liesje Birchenough, Eurasia program manager at Fauna and Flora International. FFI, based in Cambridge, U.K., has worked for the past six years with the forest services of both Kyrgyzstan and Tajikistan to protect the forests. The organization has funded nurseries that are cultivating pear and apple species for reforestation, paying for fencing, irrigation, and seeds. It also organizes surveys of rare trees and supports school programs in which teachers take children to the forests to collect seeds and then plant them.

"All of the apples that we're eating today and cultivating originate from this area," Newton says. "So if we want to add genetic variation to our crops to cope with new pests or climate change, then the genetic resource is these forests. It's true for apples, apricots, peaches, walnuts, pears. In terms of a wild genetic resource for cultivated fruit trees, there's nothing like it on the planet."

http://news.nationalgeographic.com/news/apples-of-eden-saving-the-wild-ancestor-of-modern-apples/