What is polyploidy and what is its role? What is polyploidy? What role does it play in selection and in nature? II. The role of polyploidy in speciation
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.
Detailed solution paragraph § 60 in biology for 10th grade students, authors Kamensky A.A., Kriksunov E.A., Pasechnik V.V. 2014
1. Define the species. What species criteria do you know?
Answer. A species is a collection of individuals that have similar genetic, morphological, physiological characteristics, are capable of interbreeding with the formation of fertile offspring, inhabit a certain area, have a common origin and similar behavior. A species is a basic systematic unit. It is reproductively isolated and has its own historical destiny. Species characteristics ensure the survival of both the individual and the species as a whole. At the same time, behavior that is beneficial for the species can even suppress the instinct of self-preservation (bees die defending the family).
Basic criteria of the type
1. Morphological criterion of the species. Based on the existence of morphological characters characteristic of one species, but absent in other species. For example: in the common viper, the nostril is located in the center of the nasal shield, and in all other vipers (nosed, Asia Minor, steppe, Caucasian, viper) the nostril is shifted to the edge of the nasal shield.
2. Geographical criterion. It is based on the fact that each species occupies a certain territory (or water area) - a geographic range. For example, in Europe, some species of malaria mosquito inhabit the Mediterranean, others - the mountains of Europe, Northern Europe, Southern Europe.
3. Ecological criterion. It is based on the fact that two species cannot occupy the same ecological niche. Consequently, each species is characterized by its own relationship with its environment.
Additional type criteria
4. Physiological-biochemical criterion. Based on the fact that different species may differ in the amino acid composition of proteins. Based on this criterion, for example, some types of gulls are distinguished (herring, black-billed, western, Californian).
At the same time, within a species there is variability in the structure of many enzymes (protein polymorphism), and different species may have similar proteins.
5. Genetic-karyotypic criterion. It is based on the fact that each species is characterized by a certain karyotype - the number and shape of metaphase chromosomes. For example, all durum wheat has 28 chromosomes in the diploid set, and all soft wheat has 42 chromosomes.
However, different species can have very similar karyotypes: for example, most species of the cat family have 2n=38. At the same time, chromosomal polymorphism can be observed within one species. For example, moose of Eurasian subspecies have 2n=68, and moose of North American species have 2n=70 (in the karyotype of North American moose there are 2 less metacentrics and 4 more acrocentrics). Some species have chromosomal races, for example, the black rat has 42 chromosomes (Asia, Mauritius), 40 chromosomes (Ceylon) and 38 chromosomes (Oceania).
6. Physiological and reproductive criterion. It is based on the fact that individuals of the same species can interbreed with each other to form fertile offspring similar to their parents, and individuals of different species living together do not interbreed, or their offspring are infertile.
However, it is known that interspecific hybridization is often common in nature: in many plants (for example, willow), a number of species of fish, amphibians, birds and mammals (for example, wolves and dogs). At the same time, within the same species there can be groups that are reproductively isolated from each other.
Some Pacific salmon (pink salmon, chum salmon, etc.) live for two years and spawn only before dying. Consequently, the descendants of individuals that spawned in 1990 will breed only in 1992, 1994, 1996 (“even” race), and the descendants of individuals that spawned in 1991 will breed only in 1993, 1995, 1997 (“ odd" race). An "even" race cannot interbreed with an "odd" race.
7. Ethological criterion. Associated with interspecific differences in behavior in animals. In birds, song analysis is widely used to recognize species. Depending on the nature of the sounds produced, different types of insects differ. Different species of North American fireflies vary in the frequency and color of their light flashes.
8. Historical criterion. Based on the study of the history of a species or group of species. This criterion is complex in nature, since it includes a comparative analysis of the modern ranges of species, analysis
None of the considered species criteria is the main or most important. To clearly separate species, it is necessary to carefully study them according to all criteria.
2. In what cases can differences between populations arising from changes in living conditions lead to the formation of new species?
Answer. This process can be divided into the following stages:
1. Spontaneous mutations and the beginning of divergence within one population.
2. Natural selection of the most adapted individuals, continuation of divergence.
3. The death of less adapted individuals as a result of the influence of environmental conditions is the continuation of natural selection and the formation of new populations and subspecies.
4. Isolation of subspecies, resulting in the emergence of new species due to reproductive separation.
Divergence, or divergence of characters, is the basis of the evolutionary process. Any species consists of a large number of populations that differ in a number of characteristics. But a population is never homogeneous: due to mutational variability, there are individuals in it that are more and less adapted to the conditions of existence. Recessive mutations that do not manifest themselves phenotypically constantly accumulate in populations. When conditions of existence change, divergence begins. It lies in the fact that individuals with extreme manifestations of any trait will preferentially survive or die out, leaving no offspring. The group of individuals that is best adapted to new conditions will actively reproduce, passing on useful hereditary traits from generation to generation. The least fit individuals will quickly die out, and individuals with an intermediate value of the trait will be gradually replaced by more fit ones. Thus, new subspecies and species arise. Not only species diverge, but also genera, families, and orders.
Divergence always has the character of group selection of individuals with useful traits due to natural selection. Since the material for natural selection, that is, hereditary changes, arise as a result of various mutations, it is mutational variability that leads to divergence. As a result of divergence, from one species of tits a whole genus arose, uniting 5 species that feed on different foods. More than 20 species of buttercups have the same ancestor. The reason for their divergence was geographic specialization: some species live in swamps, others in meadows, others in forests, etc.
This type of isolation is associated with the expansion of the species’ habitat area – its range. At the same time, new populations find themselves in different conditions compared to other populations: climatic, soil, etc. Hereditary changes are constantly accumulating in the population, natural selection acts - as a result, the gene pool of the population changes and a new subspecies arises. Free crossing of new populations or subspecies can be hindered by rivers, mountains, glaciers, etc. For example, based on geographical isolation factors, a whole series of species arose from one species of lily of the valley over several million years. Speciation along this path occurs slowly, over hundreds, thousands and millions of generations.
Temporary isolation. This type of isolation is due to the fact that if the timing of reproduction does not coincide, two close subspecies will not be able to interbreed, and further divergence will lead to the formation of two new species. In this way, new species of fish arise if the spawning periods of the subspecies do not coincide, or new plant species arise if the flowering periods of the subspecies do not coincide.
Reproductive isolation occurs when it is impossible to cross individuals of two subspecies due to a discrepancy in the structure of the genital organs, differences in behavior, and incompatibility of genetic material.
In any case, any isolation leads to reproductive separation - the impossibility of crossing of emerging species.
Questions after § 60
1. Name the main forms of speciation. Give examples of geographic speciation.
Answer. Depending on the result of what isolating mechanisms - spatial or other - a species arises, two forms of speciation are distinguished: 1) allopatric (geographic), when species arise from spatially separated populations; 2) sympatric, when species arise in a single territory. An example of geographic speciation is the emergence of different species of lily of the valley from an 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 involves physically separated populations diverging genetically, eventually becoming completely isolated and distinct from each other due to natural selection.
2. What is polyploidy? What role does it play in the formation of species?
Answer. Polyploidy is a type of mutational change in the body in which there is a multiple increase in the number of chromosomes. It is most characteristic of plants, but is also known among animals. Polyploidy is one of the possible ways of speciation, and in populations inhabiting the same geographical area and not separated by barriers.
3. Which species of plants and animals known to you arose as a result of chromosomal rearrangements?
Answer. The emergence of new species through chromosomal rearrangements can occur spontaneously, but more often 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. Among animals, polyploids are, for example, some species of fish (sturgeon, spined loach, etc.), grasshoppers, etc.
Discuss how different isolation mechanisms play a role in speciation. What form of selection plays a decisive role in the processes of speciation?
Answer. An important factor in evolution is isolation, which leads to divergence of characteristics within one species and prevents the crossing of individuals. Isolation can be geographical, ethological (behavioural) and environmental. The following methods of speciation are distinguished.
Geographic speciation - new forms of organisms arise as a result of range rupture and spatial isolation. In each isolated population, the gene pool changes due to genetic drift and selection. Next comes reproductive isolation, which leads to the formation of new species.
The reasons for the range gap may be mountain processes, glaciers, river formation and other geological processes. For example, different species of larches, pines, and Australian parrots were formed as a result of the rupture of their range.
Ecological speciation is a method of speciation in which new forms occupy different ecological niches (spatial) within the same habitat. Isolation occurs due to inconsistencies in the time and place of crossing, animal behavior, adaptation to different methods of pollination in plants, consumption of different foods, etc. For example, species of Sevan trout have different spawning sites, different types of Buttercup are adapted to life in different conditions.
Natural selection plays a decisive role in the processes of speciation.
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.
Question 1. Name the main forms of speciation. Give examples of geographic speciation.
Depending on whether a species arises as a result of any isolating mechanisms—spatial or other—two forms of speciation are distinguished: 1) allopatric (geographic), when species arise from spatially separated populations; 2) sympatric, when species arise in a single territory.
An example of geographic speciation is the emergence of different species of lily of the valley from the original species that lived millions of years ago in the deciduous forests of Europe. The invasion of the glacier tore apart the single habitat of the lily of the valley for several hours. 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.
Such speciation occurs slowly; to complete it, hundreds of thousands of generations must change in populations. This form of speciation involves physically separated populations diverging genetically, eventually becoming completely isolated and distinct from each other due to natural selection.
Question 2. What is polyploidy? What role does it play in the formation of species?
Polyploidy is a type of mutational change in the body, in which there is a multiple increase in the number of chromosomes. It is most characteristic of plants, but is also known among animals.
Polyploidy is one of the possible ways of speciation, and in populations inhabiting the same geographical area and not separated by barriers.
Question 3. Which species of plants and animals known to you arose as a result of chromosomal rearrangements?Material from the site
The emergence of new species through chromosomal rearrangements can occur spontaneously, but more often 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) with a subsequent doubling of 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.
Among animals, polyploids are, for example, some species of fish (sturgeon, spined loach, etc.), grasshoppers, etc.
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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 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.
