Engineering The Apple

Natural History, Oct, 2001 by Sue Hubbell

Eating apples were first harvested millennia ago in central Asia. Humans have been tinkering with the fruit ever since.

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 starters.

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 ancestral lines.

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.