New Fruit from Old
By “Awake!” correspondent in the British Isles
AN AMAZING variety of fruits is available today. And what a difference in flavor! There are crab apples that can be described as being ‘so sour as to take the edge off a knife,’ words used some two thousand years ago by naturalist Pliny. Many other varieties of apples, however, are a delight to the taste, and one’s choice is not restricted to just a few. Why, over a hundred years ago a fruit book published in the United States listed 1,823 different varieties! Yet all of these, with their distinctive features, have descended from common ancestors. New kinds of fruit have indeed come from old varieties. How has this been accomplished?
As men gained experience in agriculture, they doubtless were more selective in the seeds that they preserved for future planting, choosing those from the biggest clusters of grapes, the sweetest apples, the largest olives and the like. Gradually this produced kinds that were more and more unlike the wild varieties.
Deliberate crossing to combine desirable characteristics from different parents is a more recent development. Producing new kinds of fruit from old in this way is not always easy, as Professor L.H. Bailey discovered late in the nineteenth century.
Bailey made a cross with a Bergen fall squash and a gourd, recrossing the offspring with the squash. In 1891, the fruits from this second generation were Bergen-like with a thin paperlike shell over thick attractive yellow flesh. The shell protected the fruit from rough handling and frost. The flesh cooked well. “But the flavour,” Bailey bewailed, “dregs of quinine, gall and boneset! The gourd was still there.”
Since then man has learned much. At least five ways exist to produce improved varieties of fruits.
Sports
One way is by means of “sports.” To a plant breeder, sports are individual plants that in some respect differ markedly and in a new way from the parent type. How do these come into existence?
Perhaps, once in two hundred thousand times the mechanism by which genes reproduce goes slightly awry. This may be due to the effect of radiation, of heat or of a chemical. The resultant gene alteration is a “mutation.” Most mutations are recessive and therefore do not show up immediately. But in time a recessive mutation may manifest itself in the form of a new feature. This new characteristic may be eye-catching enough to attract attention (as, for example, double flowers) and be considered deserving of preservation despite its lower reproductive potential. Plants with such an altered trait are the “sports” of the plant breeder’s world. When new characteristics appear only on one bud or one branch, they are called bud-sports. The seeds from plants with gene alterations are also affected and, therefore, the new trait can be preserved.
Selective Breeding of Hybrids
Another way to improve varieties of fruit is by selective breeding of hybrids. This method was used to develop an apple tree that could survive the cold winters in northwestern Canada. In 1887, Dr. Saunders began raising seedlings of a small Siberian crab apple (Malus baccata) that could endure a temperature of −30 degrees Fahrenheit (−34 degrees Celsius). Seven years later these seedlings flowered, and he crossed them with sweet, cultivated varieties. The most promising of eight hundred seedlings were exposed to the rigors of the Canadian northwest and survived. In time they too flowered and Dr. Saunders, undaunted by the small size of the fruit, used them to make more crosses with cultivated kinds. Among the offspring were some with the sweet, large fruit of their cultivated parents and with the hardiness of their Siberian ancestor.
Crossing Inbred Lines
Another important way of producing new fruit varieties from old is by crossing inbred lines to bring about hybrid vigor. Many mutated genes may have accumulated in some food plants over centuries of cultivation and weakened these plants. This is so because inbreeding increases the probability that recessive traits carried by the mutated genes will manifest themselves in the offspring of the plants, leading to progressive loss of vigor and, hence, of yield. Combining two such inbred lines restores vigor to a startling degree; this is still more so if four unrelated inbred lines are combined in two generations. Only in certain plants can this kind of improvement be obtained, but in corn, or maize, the effect is phenomenal.
Doubling the Chromosome Number
A fourth way of making new kinds of fruit from old is by doubling the chromosome number. The normal contribution of chromosomes by each parent sex cell is a half-set (the haploid number). After fertilization, these combine to form the diploid number of chromosomes in each cell. For nearly all animals and most plants this is the normal number of chromosomes. But plants may have more chromosomes and still be healthy specimens. Those with three, four, five, six, seven or eight half-sets of chromosomes are triploids, tetraploids, pentaploids, hexaploids, heptaploids or octoploids respectively. Plants such as these are frequently bigger and more vigorous but less fertile than diploids. For instance, a cultivated red pepper (tetraploid) has leaves an average of four times as heavy as its diploid relatives. The fruit is up to five hundred times as heavy!
To form haploid sex cells from diploid body cells the chromosomes pair up like two rows of partners in a dance. Then four cells form from one original cell, each with only the haploid number of chromosomes. When there are more than two cells made up of similar chromosomes, usually one or more of the chromosomes fails to find a partner (inevitably, if the parent cell is triploid, pentaploid or heptaploid). Even in tetraploid cells, the chromosomes may separate by three and one rather than by two and two. This imbalance tends to sterility. However, even if 5 percent of the female sex cells are fertilized, this is sufficient to produce fruit.
By applying the substance colchicine to rapidly growing shoots, chromosome doubling nowadays can almost be done to order. Also, X rays may be used to kill off selectively the remaining diploid cells.
A Combination of Hybridization and Chromosome Doubling
The fifth way of making new fruits from old combines hybridization of species with chromosome doubling. When less closely related species of plants are crossed, the offspring are frequently vigorous but sterile. Chromosome doubling by means of colchicine restores fertility, often in full measure.
Not New in the Absolute Sense
We should not think, however, that plant breeders have only to select suitable parents and make pollinations and, then, presto, we have a new succulent fruit. At times, years of hard work, thousands of seedlings later, may, at best, only lead to a new variety, one no better than the old. Many are the disappointments. Moreover, not all new varieties are as nutritious as older ones. For example, white-hearted cabbage has less vitamin A and vitamin C than the green-hearted variety; curly-leaf kale is coarser, harder and less digestible.
Our modern understanding of genetics leads us to the knowledge that, in the final analysis, man has not really made anything new through his breeding efforts. Still less has natural selection or haphazard blind chance in some theoretical evolution done so. All credit must go to the Creator who put within plants the potential to survive in the wild for generations and to be developed by man into a wonderful variety. In producing new kinds of fruit from old, therefore, man has simply made use of the potential that has existed from the beginning of the plant creation.