What is polyploidy? What role does it play in breeding and in nature

After reading this article, you will learn what polyploidy is. We will consider what role it plays. You will also find out what kinds of polyploidy there are.

Formation of polyploids

First of all, let's talk about what is meant by this mysterious word. Cells or individuals with more than two sets of chromosomes are called polyploids. Polyploid cells with a small frequency arise as a result of "errors" in mitosis. This occurs when the chromosomes divide, and cytokinesis does not occur. Thus, cells with a doubled number of chromosomes (diploids) can form. If they, after passing through the interphase, share, they can give birth (sexual or asexual) to new individuals whose cells will have twice as many chromosomes as their parents. Accordingly, the process of their formation is what polyploidy is. Polyploid plants can be obtained artificially with the help of colchicine-alkaloid, which suppresses the formation of the mitotic spindle as a result of microtubule formation disorders.

Properties of polyploids

In these plants, variability is often significantly narrower than that of related diploids, since each gene is present at least in doubled numbers. When splitting in progeny, homozygous for some recessive gene, the individuals will be only 1/16 instead of 1/4 for diploids. (In both cases, the frequency of the recessive alleles is assumed to be 0.50.) The polyploids are characterized by self-pollination, which further reduces their variability, in spite of the fact that the related diploids preferentially cross-pollinate.

Where polyploids meet

So, we answered the question, what is polyploidy. And where do such plants occur?

Some polyploids are better adapted to dry places or lower temperatures than the original diploid forms, while others are better suited to specific types of soils. Because of this, they can populate places with extreme conditions of existence, in which their diploid ancestors would most likely have died. With a small frequency, they occur in many natural populations. They are lighter than the corresponding diploids, enter into unrelated crossings. In this case, prolific hybrids can immediately be obtained. Less often, polyploids of hybrid origin are formed by doubling the number of chromosomes in sterile diploid hybrids. This is one of the ways to restore fertility.

The first documented case of polyploidy

In this way, less common, polyploid hybrids between radish and cabbage were formed. This was the first well-documented case of polyploidy. Both genus belong to the family of cruciferous and are closely related. In somatic cells of both types there are 18 chromosomes, and 9 pairs of chromosomes are always found in the first metaphase of meiosis. With some difficulty, a hybrid was obtained between these plants. In meiosis he had 18 unpaired chromosomes (9 from radish and 9 from cabbage) and was completely sterile. Among these hybrid plants spontaneously formed a polyploid, in which there were 36 chromosomes in somatic cells and 18 pairs were regularly formed during meiosis. In other words, the polyploid hybrid had all 18 chromosomes, both radishes and cabbages, and they functioned normally. This hybrid was quite prolific.

Polyploidy-weeds

Some polyploids arose as weeds in places associated with human activities, and sometimes they achieved amazing prosperity. One of the well-known examples is the inhabitants of salt marshes of the genus Spartina. One species, S. maritima (pictured below), occurs in marshes along the shores of Europe and Africa. Another species, S. alterniflora, was introduced to the United Kingdom from the east of North America around 1800 and subsequently spread widely to form large local colonies.

Wheat

One of the most important polyploid groups of plants can be considered the genus Triticum wheat (pictured below). The most widespread bread culture in the world - soft wheat (T. aestivum) - has 2n = 42. Soft wheat arose at least 8000 years ago, probably in Central Europe, as a result of natural hybridization of cultivated wheat having 2n = 28, with wild grass Of the same genus, having 2n = 14. Wild grass probably grew like a weed among wheat crops. Hybridization, which gave rise to soft wheat, could occur between polyploids that appeared from time to time in populations of both parental species.

It is likely that as soon as the 42-chromosome wheat with its useful traits appeared on the margins of the first farmers, they immediately noticed it and selected it for further cultivation. One of its parental forms, 28-chromosome cultivated wheat, occurred as a result of hybridization of two wild 14-chromosome species from the Middle East. Wheat species having 2n = 28, and now continue to be cultivated along with 42-chromosome. Such 28-chromosome wheat is the main source of grain for macaroni production due to the high stickiness of their protein. This is the role polyploidy plays.

Triticosecale

Recent studies have shown that new lines obtained by hybridization can improve agricultural production. Polyploidy in breeding is used very widely. Especially promising is Triticosecale, a group of man-made hybrids between wheat (Triticum) and rye (Secale). Some of them, combining the productivity of wheat with the unpretentiousness of rye, are most resistant to linear rust - a disease that causes great damage to agriculture. These properties are especially important in the highland areas of the tropics and subtropics, where rust is the main factor that limits the cultivation of wheat. Triticosecale is now grown on a large scale and has gained wide popularity in France and other countries. The 42-chromosome line of this grain culture is most famous. It was obtained by doubling the number of chromosomes after hybridization of 28-chromosome wheat with 14-chromosome rye.

The variety of polyploids

In nature, they are selected under the influence of external conditions, and not because of human activities. Their emergence is one of the most important evolutionary mechanisms. In our time, many polyploids are represented in the world's flora (more than half of all plant species). Among them, many of the most important crops - not only wheat, but also cotton, sugar cane, banana, potatoes and sunflower. To this list you can add most beautiful garden flowers - chrysanthemums, pansies, dahlias.

Now you know what polyploidy is. Its role in agriculture, as you can see, is very great.