笔记|葡萄树的繁殖和育种

在人类出现之前，大自然就使用最简单、最常见的方式，即「有性生殖」的方式来繁殖出数量惊人的品种或品系。

野生葡萄树是雌雄异株的，即单株植物要么是雄性要么是雌性，并且后代可能表现出亲本性状的不规律结果（可能雄可能雌）。由于亲本植物对后代植物有雄性和雌性的贡献，因此需要跟踪两者的性状，以便预测后代的可能性状，这是遗传学家或育种家的工作。早期，遗传学家意识到有两种主要的特征：要么都明确表达，要么跳过一代，他们将这两种特征中明确表达的称为显性遗传特征，将跳过一代的特征称为隐性遗传特征。然而，栽培葡萄比野生葡萄更容易繁殖，它们是雄雌同株：它们可以与自己交配并仍然产生可生殖的后代。

而且这样一代一代地繁殖过程往往会使后代的基因构成趋于同质化。

使葡萄树繁殖并产生理想品种的另一种方法是将两种葡萄树杂交，不同品种葡萄树提供不同的所需性状。植物可以做到这一点，因为卵子和花粉结合期间存在的染色体数量对于育种来说并不重要。在这方面，植物与动物不同，当精子和卵子结合时，动物必须具有匹配的染色体。具体来说，动物受精卵要求每个受精卵具有相同数量的染色体，如果数量不匹配会出现基因排异现象。因此，不同植物物种之间的杂交比动物之间的杂交容易得多。如果杂交按照葡萄栽培者的设想进行，新的杂交后代葡萄树将表现出其亲本所需的不同特征，随后可以广泛繁殖。

有多种方法可以严格保持特定葡萄品种所需的特性。

第一个方法是绕过生殖器官类似于克隆的方式，这就像从藤蔓上切下藤蔓并将它们种植在地里一样简单，或者将葡萄藤进行空中压条繁殖air-layer，这是一种古老的繁殖方式，通过将植物的一部分包裹在湿润的介质中，使其产生新的根系，然后将其从母株上剪下，形成新的植物。其基础是诱导藤蔓茎上形成新根，而不将其与植物分离。

但在实际繁殖育种中存在一些复杂情况：在早期发育过程中，动物的干细胞一开始并不知道自己想成为什么，但最终分化成许多不同类型的细胞诸如神经元、皮肤细胞等。干细胞具有成为任何类型细胞的能力，使其具有多能性。但是，一旦动物干细胞决定了其在生命中的作用，它就会被固定并失去其多能性。相比之下，即使分化成特殊的细胞类型，植物细胞仍保留发育成其他种类组织的潜力，这就是能使用植物的一个部分来再生一个整体的原因。

这种方式通常葡萄栽培师会采取插条（对于葡萄藤，首选藤条）并将其浸泡在植物激素中，以诱导根部生长。这些处理激活了生根基因。然后种植生根的插条，由于克隆不涉及有性生殖，因此产生的藤蔓是亲本的基因复制品。通过这种方式，葡萄种植者可以确保任何与克隆过程配合的葡萄树的精确繁殖，从而确保葡萄园中葡萄树和葡萄之间的高度一致性。如果所有的葡萄藤都是彼此的克隆并且它们的环境相似，那么它们应该生产出完全相同的品种。

如果葡萄栽培者选择压条繁殖，则将母株的一根藤蔓向下拉伸到地面（类似于脐带）并埋起来，只留下藤蔓的尖端及其芽在地面上。这种延伸的茎通常需要一两年的时间才能达到比母茎“脐带”更大的树干直径，但当它达到时，母茎会被剪断，子代植物就会自行生长了。

种植者用来确保葡萄树遗传一致性的另一种方法是砧木育种。使用这种方法的是具有健康根系的已经发育良好的植物，可以是整株植物，也可以是更常见的树桩。插条（接穗）是由具有理想特性的藤蔓制成的，并嫁接到树桩上。接穗通常与砧木很好地融合，随着时间的推移，完全融合，使得两个部分表现为单一植物。但砧木有一组基因：通常被选择来增强根部生长或对病原体的抵抗力；而接穗有另一组基因，通常控制果实的特性。

值得一提的是，葡萄树这种可嫁接的特性在十九世纪拯救葡萄酒业免受根瘤蚜虫的破坏里起到至关重要的作用。

Plant breeding has a rich history, and has resulted in a staggering number of grape varieties (or strains) that have been produced through the many different ways plant reproduction and growth can be manipulated. The simplest and most common approach—and indeed the method that nature used before humans came along—employs sexual reproduction.

Wild grapevines have what is called a dioecious sex life: individual plants are strictly male or strictly female, and offspring may show capricious combinations of parental traits. Because there is a male contribution and a female contribution to offspring plants, the traits of both need to be tracked in order to predict probable traits in the offspring. This is the job of the geneticist or breeder. Early on, geneticists realized that there were two major kinds of traits—those that were clearly expressed in every generation and those that skipped a generation. They began to call the regularly expressed traits dominant and the traits that skipped a generation recessive. Cultivated grapes, however, have been bred to be more manipulable than wild grapes. Their flowers possess both male and female parts: they can mate with themselves and still produce fertile offspring.

The term for this kind of sex is monoecious, and over breedings the process tends to homogenize the genetic makeup of offspring.

Yet another way to make grapevines reproduce and give a desirable product is to hybridize two kinds of grapevine, each with a different desired trait. Plants can do this because the number of chromosomes present during the union of the ova and pollen is immaterial to the breeding. In this respect plants differ from animals, which must have matching chromosomes when the sperm and egg unite. Specifically, animal zygotes (the union of an egg and sperm) require there to be the same number of chromosomes in each. Hybridization between divergent plant species is thus much easier than in animals. If the hybridization goes as planned by the viticulturist, the new hybrid offspring grapevine will exhibit the desired divergent traits of its parents, and subsequently can be widely propagated.

At the same time, there are several ways of strictly maintaining the characteristics desired of a particular kind of grape. The first is to clone the grapevine, bypassing the reproductive organs. In principle, this is as simple as cutting canes from a vine and planting them in the ground, or air-layering the grapevine, an ancient system based on inducing roots to form on a vine stem without detaching it from the plant. But in practice there are complications. There is something weird about plant cells. In early development, animals have stem cells that start out not knowing what they want to be but eventually differentiate into many different kinds of cells—neurons, skin cells, and so forth. The stem cells’ ability to become any kind of cell makes them pluripotent.

But once an animal stem cell has decided on its role in life it becomes fixed and loses its pluripotency. In contrast, even when differentiated into specialized cell types,

plant cells retain the potential to develop into other kinds of tissue. It is a simple procedure to use one plant part to regenerate the whole.

Usually when cloning a plant the viticulturist takes cuttings (for grape-vines, canes are preferred) and soaks them in plant hormones that will in-duce roots to grow. These treatments activate the root-generating genes. The rooted cutting is then planted, and since the cloning does not in-volve sex, the resulting vines are genetic replicas of the parent. In this way, grape growers can ensure the exact reproduction of any vine that will cooperate with the cloning process, and this in turn ensures great consistency among the vines—and therefore the grapes—in a vineyard. If all the vines are clones of one another and their environment is similar, they ought to produce an effectively identical product.

If the viticulturist opts for layering, a cane from one plant (called the mother) is stretched down to the ground—rather resembling an umbilical cord—and buried, leaving only the tip of the cane and its buds aboveground. This extended cane usually takes a year or two to reach a trunk diameter bigger than the “umbilical cord” of the mother cane, but when it does the mother cane is snipped and the daughter plant allowed to grow on its own.

Another method growers use to ensure genetic uniformity among their grapevines is rootstock breeding. This technique involves an already well- established plant with a healthy root system—either a whole plant or, more commonly, a stump. A cutting—the scion—is made from a vine with desirable characteristics and grafted onto the stump. The scion usually fuses nicely with the rootstock, and over time fusion becomes complete so that the two parts behave as a single plant. But the rootstock has one set of genes—which are usually chosen to enhance root growth or resistance to pathogens—and the scion has another set, which usually controls the characteristics of the fruit. This particular property of grapevines—that they can be grafted—has also been exploited for purposes that go far be- yond breeding: grafting was, for example, critical to rescuing the wine in- dustry in the nineteenth century from the depredations of the phylloxera insect.