The role of polyploidy. Formation of species
The role of polyploidy in speciation. In plants, new species can be formed quite easily using polyploidy chromosome doubling mutations. The new form thus arising will be reproductively isolated from the parent species, but through self-fertilization it will be able to leave offspring.
For animals, this method of speciation is not feasible, since they are not capable of self-fertilization. Among plants there are many examples of closely related species that differ from each other in a multiple number of chromosomes, which indicates their origin by polyploidy. So, in potatoes, there are species with a number of chromosomes equal to 12, 24, 48 and 72; wheat with 14, 28 and 42 chromosomes. Polyploids are usually resistant to adverse influences, and under extreme conditions natural selection will favor their emergence.
Thus, on Spitsbergen and Novaya Zemlya, about 80 species of higher plants are represented by polyploid forms. Plum fruits Cherry plum fruits Sloe fruits Another, more rare method of chromosomal speciation occurs in plants by hybridization followed by polyploidy. Closely related species often differ in their chromosome sets, and hybrids between them are infertile due to disruption of the maturation process of germ cells. Hybrid plants, however, can exist for quite a long time, reproducing vegetatively.
The polyploidy mutation restores the ability of hybrids to reproduce sexually. It was in this way that the cultivated plum arose through the hybridization of sloe and cherry plum with subsequent polyploidy, see Fig. III. The importance of polyploidy in plant breeding Many cultivated plants are polyploid, that is, they contain more than two haploid sets of chromosomes. Among the polyploids are many of the main food crops: wheat, potatoes, and onece. Since some polyploids have great resistance to unfavorable factors and good yield, their use and selection are justified.
There are methods that make it possible to experimentally obtain polyploid plants. In recent years, with their help, polyploid varieties of rye, buckwheat, and sugar beets have been created. For the first time, the domestic geneticist G.D. Karpechenko in 1924, on the basis of polyploidy, overcame infertility and created a cabbage-radish hybrid. Cabbage and radish in the diploid set each have 18 chromosomes 2n 18. Accordingly, their gametes carry 9 chromosomes each, a haploid set.
A hybrid of cabbage and radish has 18 chromosomes. The chromosome set consists of 9 cabbage and 9 rare chromosomes. This hybrid is sterile, since the chromosomes of cabbage and radish do not conjugate, so the process of gamete formation cannot proceed normally. As a result of doubling the number of chromosomes, the sterile hybrid contained two complete diploid sets of chromosomes of radish and cabbage 36. As a result, normal conditions arose for meiosis of the cabbage chromosome and radishes respectively conjugated with each other.
Each gamete carried one haploid set of radish and cabbage 9 9 18. The zygote again had 36 chromosomes, the hybrid became fertile. Bread wheat is a natural polyploid, consisting of six haploid sets of chromosomes of related cereal species. In the process of its emergence, distant hybridization and polyploidy played an important role. Using the polyploidization method, domestic breeders created a rye-wheat form of triticale that had not previously been found in nature.
The creation of triticale, a new type of grain with outstanding qualities, is one of the largest achievements of breeding. It was developed by combining chromosomal complexes from two different genera of wheat and rye. Triticale is superior to both parents in yield, nutritional value and other qualities. In terms of resistance to unfavorable soil and climatic conditions and the most dangerous diseases, it is superior to wheat, not inferior to rye. This work undoubtedly ranks among the brilliant achievements of modern biology.
Currently, geneticists and breeders are creating new forms of cereals, fruits and other crops using polyploidy.
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Polyploidy
The first ones soon died, and cells with two nuclei successfully divided. When counting the chromosomes, it turned out that there were twice as many of them as in ordinary cells. So.. Each mother cell, when divided into two daughter cells, strictly distributes.. So, the gamete contains a haploid set of chromosomes - i.e. one from each homologous pair. All somatic cells...
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The polyploidy method is widely used by breeders to create new plant varieties. The essence of this process is to increase the number of sets of chromosomes in the cells of the body's tissues, a multiple of a single (haploid) set of chromosomes. As a result, there is an increase in the size of the cells themselves and the entire organism as a whole. This is a phenotypic manifestation of polyploidy.
Those organisms whose cells have more than two sets of chromosomes are called polyploids. Thus, triploids contain three sets, tetraploids - four, pentaploids - five, etc. Polyploids that have an odd set of chromosomes are sterile due to the fact that their germ cells with an incomplete set of chromosomes, not a multiple of the haploid one, do not divide. They do not produce offspring. It has been proven that an increase in the number of chromosomes increases the resistance of plants to pathogenic microorganisms and some other unfavorable environmental factors, in particular to radiation. This is explained by the fact that if one or two homologous chromosomes are damaged, the rest remain intact. Thus, polyploid organisms are more viable than diploid ones.
The emergence of polyploidy
The cause is non-disjunction of chromosomes during meiosis. In this case, the germ cell has a complete set of somatic cells. If such a gamete merges with a normal one, a triploid zygote is obtained, giving rise to a triploid. Provided that two gametes contain a diploid set, their fusion leads to the formation of a tetraploid.
Also, polyploid organisms can appear during unfinished mitosis. So, if after cell doubling there is no cell division, then the result is a tetraploid. Tetraploid zygotes are the precursors of tetraploid shoots, and diploid gametes will be formed in flowers instead of haploid ones. With self-pollination, a tetraploid can be formed, and with normal pollination by a gamete, a triploid. If the plant reproduces vegetatively, the original ploidy is maintained. 
In the wild, polyploidy is widespread, but is unevenly represented among different communities of plant and animal organisms. This type of mutation plays an important role in the evolutionary transformations of wild and cultivated angiosperms, among which about 50% of species are polyploids.
Since polyploid plants are characterized by valuable economic properties, artificial polyploidization is used in plant growing to obtain breeding material. For this purpose, special mutagens are used in selection, for example, colchicine, which disrupts chromosome segregation in meiosis and mitosis.
Approximately 80% of currently existing varieties of different types of cultivated plants are polyploids. These include vegetable and fruit crops, cereals, citrus fruits, industrial, ornamental and medicinal plants. A striking example of the result of polyploidy is triploid sugar beet, which, unlike ordinary sugar beets, has a higher yield of vegetative mass and larger sizes of root crops, combined with their increased sugar content and resistance to various diseases. But triploid plants do not produce offspring. Therefore, breeders can obtain hybrid seeds only by crossing tetraploid and diploid forms. Due to the proven sterility of triploid hybrids, seedless fruits of watermelon, grapes, and bananas were obtained, which are in great demand.
There are the following types of polyploidy: autopolyploidy and allopolyploidy. The first type is described above. In allopolyploidy, scientists combined the method of artificial polyploidy with remote hydridization. Thus, fertile hybrids of plants were obtained, for example, radish and cabbage, wheat and rye, wheat and wheatgrass. These hybrids have high yields, cold resistance, unpretentiousness, and disease resistance.
After reading this article, you will learn what polyploidy is. We'll look at what role it plays. You will also learn what types of polyploidy there are.
Polyploid formation
First of all, let's talk about what is meant by this mysterious word. Cells or individuals that have more than two sets of chromosomes are called polyploids. Polyploid cells arise with low frequency as a result of mitotic “errors.” This occurs when chromosomes divide and cytokinesis does not occur. In this way, cells with double the number of chromosomes (diploids) can be formed. If, after going through interphase, they divide, they will be able to give rise (sexually or asexually) to new individuals, the cells of which will have twice as many chromosomes as those of their parents. Accordingly, the process of their formation is what polyploidy is. Polyploid plants can be obtained artificially using colchicine, an alkaloid that suppresses the formation of the mitotic spindle as a result of disruption of microtubule formation.
Properties of polyploids
In these plants the variability is often much narrower than in related diploids, since each gene is represented in them at least twice as many. When splitting in the offspring, individuals homozygous for some recessive gene will make up only 1/16 instead of 1/4 in diploids. (In both cases, the frequency of recessive alleles is assumed to be 0.50.) Polyploids are characterized by self-pollination, which further reduces their variability, despite the fact that related diploids are predominantly cross-pollinated.
Where are polyploids found?
So, we have answered the question of what polyploidy is. Where are such plants found?
Some polyploids are better adapted to dry areas or cooler temperatures than the original diploid forms, while others are better adapted to specific types of soil. Thanks to this, they can inhabit places with extreme living conditions in which their diploid ancestors would most likely die. They occur with low frequency in many natural populations. They enter into unrelated crosses more easily than their corresponding diploids. In this case, fertile hybrids can be obtained immediately. Less commonly, 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.
First documented case of polyploidy
It was in this less usual way that polyploid hybrids between radish and cabbage were formed. This was the first well-documented case of polyploidy. Both genera belong to the cruciferous family and are closely related. In somatic cells of both species there are 18 chromosomes, and in the first metaphase of meiosis 9 pairs of chromosomes are always found. 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, a polyploid spontaneously formed, in which there were 36 chromosomes in somatic cells and 18 pairs were regularly formed during the process of meiosis. In other words, the polyploid hybrid had all 18 chromosomes of both radish and cabbage, and they functioned normally. This hybrid was quite prolific.
Polyploid weeds
Some polyploids originated as weeds in human-affected areas, and sometimes flourished astonishingly. One well-known example is the salt marsh inhabitants of the genus Spartina. One species, S. maritima (pictured below), is found in marshes along the coasts of Europe and Africa. Another species, S. alterniflora, was introduced into Britain from eastern North America around 1800 and subsequently spread widely, forming large local colonies.
Wheat
One of the most important polyploid groups of plants can be considered the genus Triticum of wheat (pictured below). The world's most common grain crop, bread wheat (T. aestivum), has 2n = 42. Bread wheat arose at least 8,000 years ago, probably in Central Europe, as a result of the natural hybridization of cultivated wheat, which has 2n = 28, with a wild grain of the same genus, having 2n = 14. The wild cereal probably grew as a weed among wheat crops. The hybridization that gave rise to bread wheat may have occurred between polyploids that appeared from time to time in populations of both parent species.
It is likely that as soon as 42-chromosomal wheat with its beneficial traits appeared in the fields of the first farmers, they immediately noticed it and selected it for further cultivation. One of its parent forms, 28-chromosomal cultivated wheat, resulted from the hybridization of two wild 14-chromosomal species from the Middle East. Wheat species with 2n = 28 continue to be cultivated along with those with 42 chromosomes. These 28-chromosomal wheats are a major source of grain for pasta production due to the high stickiness of their protein. This is the role polyploidy plays.
Triticosecale
Research in recent years has shown that new lines obtained through hybridization can improve agricultural production. Polyploidy is used very widely in breeding. Particularly promising is Triticosecale, a group of man-made hybrids between wheat (Triticum) and rye (Secale). Some of them, combining the yield of wheat with the unpretentiousness of rye, are the most resistant to line rust, a disease that causes great damage to agriculture. These properties are especially important in the highlands of the tropics and subtropics, where rust is the main factor limiting the cultivation of wheat. Triticosecale is now grown on a large scale and has gained widespread popularity in France and other countries. The most famous is the 42-chromosomal line of this grain crop. It was obtained by doubling the number of chromosomes after hybridizing 28-chromosome wheat with 14-chromosomal rye.
Diversity of polyploids
In nature, they are selected under the influence of external conditions, and not due to human activity. Their emergence is one of the most important evolutionary mechanisms. Nowadays, many polyploids are represented in the world flora (more than half of all plant species). Among them are many of the most important crops - not only wheat, but also cotton, sugar cane, banana, potato 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.
>> Speciation
1. Define the species.
2. What species criteria do you know? What is a species?
With the advent of population genetics, the species category was defined more precisely. Modern scientists define a species as a group of actually or potentially interbreeding populations, which are reproductively isolated from other such groups.
Reproductive isolation is a key concept in modern interpretation of the species. Individuals of the same species can interbreed with each other, but never with organisms of another species. For example, rose and cherry - both species of the Rosaceae family - never interbreed. Reproductive isolation thus provides an accurate standard for determining whether given organisms belong to the same species.
The emergence of new species can occur in various ways. The most important role in this process is played by isolating mechanisms, and the process of speciation itself is called micro evolution.
Geographic speciation.
A new species may appear as a result of the dismemberment of the range of a population or group of populations by barriers. This process can occur at the border of the distribution area of the original species, where living conditions are somewhat different from usual and where natural selection processes are actively occurring. Such speciation, associated with spatial separation of populations, is often called geographic. The process of geographic speciation is shown schematically in drawing 78.
Suppose that a population of some species is separated by a barrier. This may be a physical or geographical barrier - a river, canal, quarry, etc. The presence of a barrier prevents the free crossing of individuals, and therefore gene exchange. As a result of natural selection, more and more genetic differences accumulate in populations. Over time, these differences become so significant that certain mechanisms of reproductive isolation are activated.
An example of such a process could be the emergence of certain species of fish, whose ancestors lived in the sea, but during the Ice Age they were able to colonize first brackish water bodies that arose during the melting of glaciers on the borders of the sea and the continent, and then fresh water in the territory of modern Europe and Asia. As the glacier retreated, fresh water bodies became completely isolated. Under the influence of new conditions, some fish, having undergone significant changes, formed new species. These include, for example, burbot - a close relative of the typical marine species of cod
Another example is the emergence of different types of lily of the valley from the original species that lived millions of years ago in the broad-leaved forests of Europe. The invasion of the glacier tore the single habitat of the lily of the valley into several parts. It has been preserved in forest areas that escaped glaciation: in the Far East, southern Europe, and Transcaucasia. When the glacier retreated, lily of the valley again spread across Europe, forming a new species - a larger plant with a wide corolla and, in the Far East, a species with red petioles and a waxy coating on the leaves.
This speciation occurs slowly; for it to be completed, populations must undergo hundreds of thousands of generations. This form of speciation assumes that: physically separated populations diverge genetically; over time they become completely isolated and distinct from each other due to natural selection.
Polyploidization.
Research shows that genetic differences between populations can accumulate not only as a result of long-term natural selection genotypes, carrying characteristics useful for given conditions, but also in a different, faster way. In plants, for example, isolating mechanisms can arise during the life of a single generation through a sudden increase in the number of chromosomes, or polyploidy - A multiple increase in the number of chromosomes within one species can occur spontaneously; but sometimes chromosome multiplication occurs as a result of crossing closely related organisms. For example, a cultivated plum with 2n = 48 arose by crossing sloe (n = 16) with cherry plum (n = 8), followed by doubling the number of chromosomes.
Many economically valuable plants are polyploids, for example potatoes, tobacco, cotton, sugar cane, coffee, etc. In plants such as tobacco, potatoes, the initial number of chromosomes is 12, but there are species with 24, 48, 72 chromosomes.
The chromosome sets of animals can also quickly: change. For example, some species of fish (sturgeon, spined loach, etc.), grasshoppers, and other animals are polyploids. It is believed that the giant panda evolved from the bear as a result of sudden chromosomal changes. The panda has 42 chromosomes, the bear has 74, the chromosomes of the panda and the bear differ in shape (Fig. 79). The panda is very different from the bear both in external structure and in its way of life: it eats bamboo and eats almost no meat.
The formation of new species as a result of chromosomal rearrangements can occur in populations inhabiting the same geographical area and not separated by barriers.
Thus, we can conclude that species can arise in various ways - both over thousands of years and very quickly.
Microevolution. Geographic speciation. Barriers. Polyploidy.
1. Name the main forms of speciation. Give examples of geographic speciation.
2. What is polyploidy? What role does it play in the formation of species?
3. Which species of plants and animals known to you arose as a result of chromosomal rearrangements?
Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology 9th grade
Submitted by readers from the website
Polyploidy is an extremely valuable source of variation for plant breeding. The increase in the number of sets of chromosomes in cultivated plants played an outstanding role in the evolution of species and selection. Folk selection, not knowing the phenomenon of polyploidy itself, has long used it as a source of variability in the creation of a number of such valuable crops as wheat, oats, cotton, potatoes, as well as in floriculture. The study of polyploidy has made it possible to master this source of plant variability. Advances in theoretical research immediately affected the production of artificial polyploids in agricultural crops. The number of artificially created polyploids is progressively increasing every year. Currently, several dozen tetraploids have been obtained from rye alone. The use of colchicine accelerated the production of polyploids. In this case, success depends on the method of tissue processing, the type of plant, and the stage of exposure. A solution of colchicine of varying concentrations is used to treat the seeds, the seedlings of the growth point of an adult plant, and also act through the root system.
Somatic cells of plants and animals, as a rule, contain a double (diploid) number of chromosomes (2 n); one of each pair of homologous chromosomes comes from the maternal and the other from the paternal organisms. Unlike somatic cells, germ cells have a reduced initial (haploid) number of chromosomes ( n). In haploid cells, each chromosome is single and does not have a homologous pair. The haploid number of chromosomes in the cells of organisms of the same species is called the main, or basic, and the set of genes contained in such a haploid set is called the genome. The haploid number of chromosomes in germ cells arises due to the reduction (halving) of the number of chromosomes in meiosis, and the diploid number is restored during fertilization. (Quite often, plants in a diploid cell have so-called B chromosomes, additional to one of the chromosomes. Their role has been little studied, although corn, for example, always has such chromosomes.) The number of chromosomes in different plant species is very diverse . Thus, one of the species of fern (Ophioglosum reticulata) has 1260 chromosomes in the diploid set, and in the most phylogenetically developed family of Asteraceae, the species Haplopappus gracilis has only 2 chromosomes in the haploid set.
With P., deviations from the diploid number of chromosomes in somatic cells and from the haploid number in reproductive cells are observed. With P., cells may appear in which each chromosome is represented three times (3 n) - triploid, four times (4 n) - tetraploid, five times (5 n) - pentaploid, etc. Organisms with a corresponding multiple increase in sets of chromosomes - ploidy - in cells are called triploids, tetraploids, pentaploids, etc. or in general - polyploids.
A multiple increase in the number of chromosomes in cells can occur under the influence of high or low temperature, ionizing radiation, chemicals, as well as as a result of changes in the physiological state of the cell. The mechanism of action of these factors is reduced to disruption of chromosome segregation in mitosis or meiosis and the formation of cells with a multiple increase in the number of chromosomes compared to the original cell. Of the chemical agents that cause disruption of the correct segregation of chromosomes, the most effective is the alkaloid colchicine, which prevents the formation of the spindle threads of cell division. (By treating seeds and buds with a dilute solution of colchicine, experimental polyploids in plants are easily obtained.) P. can also occur as a result of endomitosis - the doubling of chromosomes without dividing the cell nucleus. In the case of nondisjunction of chromosomes in mitosis (mitotic p.), polyploid somatic cells are formed; in case of nondisjunction of chromosomes in meiosis (meiotic p.), germ cells with an altered, often diploid, number of chromosomes are formed (so-called unreduced gametes). The fusion of such gametes gives a polyploid zygote: tetraploid (4 n) - upon the fusion of two diploid gametes, triploid (3 n) - when an unreduced gamete merges with a normal haploid one, etc.