What is parthenogenesis? Definition, classification and features. Is the virgin birth possible, or what is parthenogenesis? Who experiences parthenogenesis?

In biology, parthenogenesis is the so-called “virgin reproduction”, that is, a form of sexual reproduction of organisms characterized by the fact that female reproductive cells develop into an adult organism without fertilization. And even despite the fact that during parthenogenesis there is no fusion of male and female gametes, such reproduction is still considered sexual, because the organism develops from a germ cell.

Biological significance of parthenogenesis

The main significance of parthenogenesis is that thanks to it, those species whose individuals are represented predominantly by females (for example,) can rapidly reproduce without the participation of males. It also often happens that females emerge from fertilized eggs, and males from unfertilized eggs, and thus, with the help of parthenogenesis, the numerical sex ratio is regulated.

Types of parthenogenesis

In science, there are several ways to classify this amazing biological phenomenon:

According to the method of reproduction: natural (occurring in natural conditions) and artificial (reproduced in the laboratory).
According to the completeness of the course: rudimentary - when unfertilized cells begin to divide, but embryonic development stops at an early stage; and complete, when this very embryonic development reaches the formation of an adult individual.
Depending on the sex of the organism, gynogenesis (parthenogenesis of females) and androgenesis (parthenogenesis of males) differ.

Parthenogenesis in animals: examples

In the animal world, the phenomenon of parthenogenesis occurs in:

ants
some plants

And often parthenogenesis coexists with conventional sexual reproduction, used in cases where rapid population growth is necessary.

Parthenogenesis in bees

In bees, during parthenogenesis, males, or drones, are born from unfertilized eggs, and only females are born from fertilized eggs, which in turn are divided into a reproducing queen (the queen of the hive) and a sterile worker bee.

Parthenogenesis in ants

In the ant kingdom, the phenomenon of parthenogenesis is present in eight species of ants and can be divided into three types:

the females produce worker ants and other females through it, with the male workers being sterile.
worker ants produce females through parthenogenesis.
females produce other females through parthenogenesis, and male worker ants through normal sexual intercourse.

Parthenogenesis in plants

In plants, the process of parthenogenesis has its own excellent academic term - apomixis. It represents vegetative propagation or propagation by seeds that appeared without fertilization: either in the case of a variety or from diploid cells of the ovule. In many plants, double fertilization occurs, and in some, as a consequence, the phenomenon of pseudogamy is possible, when plant seeds are obtained with an embryo formed from an unfertilized egg.

Parthenogenesis in lizards

There are only a few species of lizards that reproduce in such an unusual way, among them, for example, Komodo dragons, which have a double copy of DNA eggs and a special substance - polocyte, which can act as sperm, fertilizing the egg, turning it into an embryo.

Parthenogenesis in humans

At the moment, cases of parthenogenesis in humans look like pure fantasy, albeit scientific. But it is quite possible that in the future something similar will be possible, the only question is why?

Parthenogenesis, video

And in conclusion, interesting thoughts about the possibility of parthenogenesis in humans, about what it would be like to be born from oneself.

Parthenogenesis ( Parthenogenesis- from Greek parthenos- girl, virgin + genesis-Generation) is a form of sexual reproduction in which the development of an organism occurs from a female reproductive cell (egg) without fertilization by a male one (sperm).

In cases where parthenogenetic species are represented (always or periodically) only by females, one of the main biological advantages of parthenogenesis is to accelerate the rate of reproduction of the species, since all individuals of such species are capable of leaving offspring. In cases where females develop from fertilized eggs, and males from unfertilized eggs, parthenogenesis helps regulate numerical sex ratios (for example, in bees).

Parthenogenesis should be distinguished from asexual reproduction, which is always carried out with the help of somatic organs and cells (reproduction by division, budding, etc.).

There are parthenogenesis natural- a normal way of reproduction of some organisms in nature and artificial, caused experimentally by the action of various stimuli on an unfertilized egg, which normally requires fertilization.

Parthenogenesis in animals

The initial form of parthenogenesis - rudimentary, or rudimentary parthenogenesis - is characteristic of many animal species in cases where their eggs remain unfertilized. As a rule, embryonic parthenogenesis is limited to the initial stages of embryonic development; however, sometimes development reaches its final stages.

At androgenesis the nucleus of the female germ cell (egg) does not participate in development, and a new organism develops from two fused nuclei of male germ cells (sperm). Natural androgenesis occurs in nature, for example, in hymenopteran insects. Artificial androgenesis is used to produce offspring in silkworms: with androgenesis, only males are produced in the offspring, and the cocoons of males contain significantly more silk than the cocoons of females.

In case gynogenesis the sperm nucleus does not fuse with the nucleus of the egg, but only stimulates its development (false fertilization). Gynogenesis is characteristic of roundworms, bony fish and amphibians. In this case, the offspring produced are only females.

U person There are known cases when, under the influence of stressful situations of high temperatures and in other extreme situations, a female egg can begin to divide, even if it is not fertilized, but in 99.9% of cases it soon dies (according to some sources, 16 cases of immaculate conception occurred in history are known in Africa and European countries).

The material was prepared based on information from open sources

Parthenogenesis (from the Greek parthenos - virgin and ... genesis (See ... genesis))

virgin reproduction, one of the forms of sexual reproduction of organisms in which female reproductive cells (eggs (See Ovum)) develop without fertilization (See Fertilization). P. - sexual, but unisexual reproduction - arose in the process of the evolution of organisms in dioecious forms. In cases where parthenogenetic species are represented (always or periodically) only by females, one of the main biological advantages of P. is to accelerate the rate of reproduction of the species, since all individuals of such species are capable of leaving offspring. In cases where females develop from fertilized eggs, and males from unfertilized eggs, fertilization helps regulate numerical sex ratios (for example, in bees). Often parthenogenetic species and races are polyploid and arise as a result of distant hybridization, displaying heterosis and high viability in this regard. P. should be distinguished from asexual reproduction (See Asexual reproduction), which is always carried out with the help of somatic organs and cells (reproduction by division, budding, etc.). A distinction is made between natural fertilization, the normal method of reproduction of some organisms in nature, and artificial fertilization, which is caused experimentally by the action of various stimuli on an unfertilized egg that normally requires fertilization.

Parthenogenesis in animals. The initial form of P. is rudimentary, or rudimentary, P., characteristic of many species of animals in cases where their eggs remain unfertilized. As a rule, rudimentary P. is limited to the initial stages of embryonic development; however, sometimes development reaches final stages (random, or accidental, P.). Complete natural transformation—the emergence of a fully developed organism from an unfertilized egg—occurs in all types of invertebrates. It is common in arthropods (especially insects). P. is also discovered in vertebrates - fish, amphibians, and is especially common in reptiles (at least 20 races and species of lizards, geckos, etc. reproduce in this way). In birds, a greater tendency to P., enhanced by artificial selection to the ability to produce sexually mature individuals (always males), has been found in some breeds of turkeys. In mammals, only cases of rudimentary P. are known; isolated cases of complete development were observed in rabbits with artificial P.

A distinction is made between obligate P., in which eggs are capable only of parthenogenetic development, and facultative P., in which eggs can develop both through P. and as a result of fertilization [in many hymenopteran insects, such as bees, males (drones) develop from unfertilized eggs ), of the fertilized ones - females (queens and worker bees)]. Often reproduction through P. alternates with bisexual reproduction - the so-called cyclic P. Parthenogenetic and sexual generations with cyclic P. are externally different. Thus, successive generations of aphids of the genus Chermes differ sharply in morphology (winged and wingless forms) and ecology (associated with different food plants); In some gallstones, individuals of parthenogenetic and bisexual generations are so different that they were mistaken for different species and even genera. Usually (in many aphids, daphnia, rotifers, etc.) summer parthenogenetic generations consist of only females, and in the fall generations of males and females appear, which leave fertilized eggs for the winter. Many species of animals that do not have males are capable of long-term reproduction through reproduction - the so-called constant reproduction. In some species, along with the parthenogenetic female race, there is a bisexual race (the original species), which sometimes occupies another area - the so-called geographic reproduction (butterflies case-bearing beetles, many beetles, centipedes, mollusks, rotifers, daphnia, vertebrates - lizards, etc.).

Based on their ability to produce males or females through fertilization, they are distinguished: arrhenotoky, in which only males develop from unfertilized eggs (bees and other hymenoptera, scale insects, mites, and from vertebrates - parthenogenetic lines of turkeys); thelytoky, in which only females develop (the most common case); deuterotokia, in which individuals of both sexes develop (for example, with random P. in butterflies; in a bisexual generation with cyclic P. in daphnia, rotifers, and aphids).

The cytogenetic mechanism of maturation of an unfertilized egg is very important. It is precisely because of whether the egg undergoes meiosis and a reduction in the number of chromosomes by half - reduction (meiotic P.) or does not undergo (ameiotic P.), whether the number of chromosomes characteristic of the species is preserved due to the loss of meiosis (zygotic P.) or whether this number is restored after reduction by the fusion of the nucleus of the egg with the nucleus of the guiding body or in some other way (automictic P.), the hereditary structure (Genotype) of the parthenogenetic embryo and all its most important hereditary features - sex, preservation or loss of heterosis, acquisition of homozygosity, etc. - ultimately depend.

P. is also divided into generative, or haploid, and somatic (it can be diploid and polyploid). In generative P., a haploid number of chromosomes (n) is observed in the dividing cells of the body; this case is relatively rare and is combined with arrhenotoky (haploid males are drones of bees). In somatic P., in the dividing cells of the body, the initial diploid (2n) or polyploid (Zn, 4n, 5 n, rarely even 6 n and 8 n) number of chromosomes. Often within one species there are several races characterized by multiple numbers of chromosomes - the so-called polyploid series. Due to the very high frequency of polyploidy (See Polyploidy), parthenogenetic species of animals present a sharp contrast with bisexuals, in which polyploidy, on the contrary, is very rare. Polyploid dioecious animal species apparently arose through P. and distant hybridization.

Artificial P. in animals was first obtained by the Russian zoologist A. A. Tikhomirov. He showed (1886) that unfertilized silkworm eggs can be stimulated to develop by solutions of strong acids, friction, and other physicochemical stimuli. Subsequently, artificial parasites were obtained by J. Loeb and other scientists from many animals, mainly from marine invertebrates (sea urchins and stars, worms, mollusks), as well as from some amphibians (frogs) and even mammals (rabbits). At the end of the 19th - beginning of the 20th centuries. experiments on artificial fertilization attracted the special attention of biologists, giving hope, with the help of this physicochemical model of egg activation (See Egg Activation), to penetrate into the essence of fertilization processes. Artificial P. is caused by the action of hypertonic or hypotonic solutions on eggs (the so-called osmotic P.), pricking the egg with a needle moistened with hemolymph (the so-called traumatic P. of amphibians), sudden cooling and especially heating (the so-called temperature P.), as well as the action acids, alkalis, etc. With the help of artificial P. it is usually possible to obtain only the initial stages of the development of the organism; complete P. is rarely achieved, although cases of complete P. are known even in vertebrates (frog, rabbit). The method of mass production of complete P., developed (1936) for the silkworm by B. L. Astaurov, is based on precisely dosed short-term heating (up to 46 ° C for 18 min) unfertilized eggs extracted from a female. This method makes it possible to obtain from silkworms only female individuals that are hereditarily identical with the original female and with each other. The resulting di-, tri-, and tetraploid clones can be propagated through P. indefinitely. At the same time, they retain their original heterozygosity and “hybrid vigor.” Selection has resulted in clones that reproduce through P. as easily as bisexual breeds through fertilization (more than 90% hatching of activated eggs and up to 98% viability). P. is of diverse interest for the practice of sericulture.

Parthenogenesis in plants. P., common among seed and spore plants, is usually of the constant type; facultative P. was found in isolated cases (in some species of hawkweed and in the cornflower Thalictrum purpurascens). As a rule, the sex of parthenogenetically reproducing plants is female: in dioecious plants, P. is associated with the absence or extreme rarity of male plants, in monoecious plants - with the degeneration of male flowers, the absence or abortiveness of pollen. As with P. in animals, a distinction is made between generative, or haploid, P. and somatic, which can be diploid or polyploid. Generative P. is found in algae (cutleria, spirogyra, ectocarpus) and fungi (saprolegnia, mucor, endomyces). In flowering plants it is observed only under experimental conditions (tobacco, grass, cotton, cereals, and many others). Somatic P. is found in algae (Chara, Cocconeis), in ferns (Marselia Drummond), and in higher flowering plants (chondrilla, mantle, hawkweed, cat's paw, dandelion, etc.). Polyploid P. is very common in plants; however, polyploidy is not a feature of parthenogenetic species here, since it is also widespread in bisexual plants. Other methods of reproduction that are close to P. in plants are apogamy, in which the embryo develops not from an egg, but from other gametophyte cells, and Apomixis. Artificial parasites in plants have been obtained from some algae and fungi by the action of hypertonic solutions, as well as by short-term heating of female germ cells. The Austrian scientist E. Cermak (1935-48) developed artificial pollen in flowering plants (cereals, legumes, and many others), causing it by irritating the stigma with dead or foreign pollen or powdery substances (talc, flour, chalk, etc.). The Soviet scientist E. M. Vermel obtained (1972) diploid P. in currants, tomatoes, and cucumbers by the action of dimethyl sulfoxide.

P. also includes peculiar methods of development of animals and plants - Gynogenesis and Androgenesis, in which the egg is activated for development by penetrating sperm of its own or a similar species, but the nuclei of the egg and sperm do not merge, fertilization turns out to be false, and the embryo develops only with the female (gynogenesis) or only with a male (androgenesis) nucleus.

Lit.: Astaurov B.L., Artificial parthenogenesis in the silkworm (Experimental study), M. - L., 1940; by him, Cytogenetics of silkworm development and its experimental control, M., 1968; Tyler A., Artificial parthenogenesis, trans. from English, in the book: Some problems of modern embryophysiology, M., 1951; Astaurov B.L., Demin Yu.S., Parthenogenesis in birds, “Ontogenesis”, 1972, vol. 3, no. 2; Rostand J., La parthenogenèse animale. P., 1950.

B. L. Astaurov.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

Synonyms:

See what “Parthenogenesis” is in other dictionaries:

Parthenogenesis... Spelling dictionary-reference book

- (from the Greek parthenos virgin and ... genesis) (virgin reproduction), a form of sexual reproduction, the development of an egg without fertilization. Characteristic of many invertebrate animals (daphnia, aphids, bees, etc.) and many plants. Called... ... Modern encyclopedia

PARTHENOGENESIS, one of the forms of sexual reproduction of organisms, in which the female reproductive cell or GAMETE develops without FERTILIZATION. Since the male gamete is not involved in this process, offspring are produced that are genetically identical... Scientific and technical encyclopedic dictionary

- (from the Greek parthenos virgin and ... genesis), virgin reproduction, one of the forms of sexual reproduction of organisms, with a swarm of wives. germ cells (ovules, eggs) develop without fertilization. Thus, P. sexual, but same-sex reproduction, ... ... Biological encyclopedic dictionary

- (from the Greek parthenos virgin and...genesis), the development of an embryo from an egg without fertilization. Found in many plant and animal organisms. Parthenogenesis is the main possibility of reproduction of organisms during rare contacts of different sexes... ... Ecological dictionary

- (from the Greek parthenos virgin and ... genesis) (virgin reproduction) a form of sexual reproduction, the development of an egg without fertilization. Characteristic of many invertebrate animals (daphnia, rotifers, aphids, bees, etc.) and many seed and... ... Big Encyclopedic Dictionary Dictionary of synonyms

Parthenogenesis. See virgin reproduction. (

Parthenogenesis is one of the modifications of sexual reproduction in which the female gamete develops into a new individual without fertilization by the male gamete. Parthenogenetic reproduction occurs in both the animal and plant kingdoms and has the advantage of increasing the rate of reproduction in some cases.

In aphids, diploid parthenogenesis occurs, in which the female oocytes undergo a special form of meiosis without chromosome segregation - all chromosomes pass into the egg, and the polar bodies do not receive a single chromosome. The eggs develop in the mother's body, so that young females are born fully formed, rather than hatching from eggs. This process is called viviparity. It can continue for several generations, especially in the summer, until almost complete nondisjunction occurs in one of the cells, resulting in a cell containing all pairs of autosomes and one X chromosome. From this cell the male develops parthenogenetically. These autumn males and parthenogenetic females produce haploid gametes through meiosis that participate in sexual reproduction. Fertilized females lay diploid eggs, which overwinter, and in the spring they hatch into females that reproduce parthenogenetically and give birth to living offspring. Several parthenogenetic generations are followed by a generation resulting from normal sexual reproduction, which introduces genetic diversity into the population through recombination. The main advantage that parthenogenesis gives to aphids is the rapid growth of the population, since all its mature members are capable of laying eggs. This is especially important during periods when environmental conditions are favorable for the existence of a large population, i.e. during the summer months.

Parthenogenesis is widespread in plants, where it takes various forms. One of them, apomixis, is parthenogenesis, simulating sexual reproduction. Apomixis is observed in some flowering plants in which the diploid ovule cell, either a nucellus cell or a megaspore, develops into a functional embryo without the participation of a male gamete. The rest of the ovule forms the seed, and the ovary develops into the fruit. In other cases, the presence of a pollen grain is required, which stimulates parthenogenesis, although it does not germinate; the pollen grain induces hormonal changes necessary for the development of the embryo, and in practice such cases are difficult to distinguish from true sexual reproduction.

The beginning of individual development is preceded by the emergence of germ cells, i.e. gametogenesis, which can be considered progenesis during individual development.

The process of development of female germ cells is called ovogenesis (oogenesis). Unlike spermatogenesis, it has some features. The course of oogenesis and its differences from the development of male gametes are shown in Fig. 3.

There are 3 periods in oogenesis: reproduction, growth and maturation. Undifferentiated female germ cells - oogonia - reproduce in the same way as spermatogonia, through normal mitosis. After division, they become first-order oocytes and enter the growth period.

The growth of oocytes lasts a very long time - weeks, months and even years. During the growth period, two stages are distinguished: small, or slow growth, when new substances are assimilated and the cytoplasm is enriched with them, and large, or fast growth, when nutrient yolk accumulates in the cell. The nucleus also undergoes profound changes during the growth period; it swells greatly, its contents seem to blur. Cell sizes increase enormously (for example, perch eggs increase almost a million times).

Then the first order oocyte enters the period of maturation, or meiosis. Here, too, reduction and equational divisions take place. The division processes in the nucleus proceed in the same way as during meiosis of spermatocytes, but the fate of the cytoplasm is completely different. During reduction division, one nucleus carries with it most of the cytoplasm, and only a small part of it remains to the share of the other. Therefore, only one full-fledged cell is formed - an oocyte of the second order, and a second tiny one - a directional, or reduction, body, which can be divided into two reduction bodies.

During the second, equational division, the asymmetrical distribution of the cytoplasm is repeated and again one large cell is formed - the ovotide and the third polar body. The ovotide, in terms of its nuclear composition and functionality, is a completely mature germ cell.

The formation period, unlike spermatogenesis, is absent in oogenesis. Thus, in oogenesis, only one mature egg arises from one oogonia. Polar bodies remain underdeveloped and soon die and are phagocytosed by other cells. Mature female gametes are called ova or eggs, and those deposited in water are called caviar.

Features of oogenesis in humans are presented in Fig. 5. The development of female germ cells occurs in the ovaries. The period of reproduction begins in oogonia while still in the embryo and stops by the time the girl is born. The period of growth during oogenesis is longer, because In addition to preparation for meiosis, a supply of nutrients is accumulated, which will be necessary in the future for the first divisions of the zygote. During the small growth phase, a large number of different types of RNA are formed. Rapid accumulation of RNA occurs due to a special mechanism - gene amplification (multiple copying of individual DNA sections encoding ribosomal RNA). The rapid increase in mRNA occurs due to the formation of “lampbrush” chromosomes. As a result, more than a thousand additional nucleoli are formed, which are a necessary structure for the synthesis of rRNA, from which ribosomes involved in protein synthesis are subsequently formed. During the same period, meiotic chromosome transformations occur in the oocyte, characteristic of the prophase of the first division.

During the period of great growth, the follicular cells of the ovary form several layers around the first-order oocyte, which facilitates the transfer of nutrients synthesized elsewhere into the cytoplasm of the oocyte.

In humans, the growth period of oocytes can be 12–50 years. After completion of the growth period, the first order oocyte enters the maturation period.

During the period of oocyte maturation (as well as during spermatogenesis), meiotic cell division occurs. During the first reduction division, from an oocyte of the first order, one oocyte of the second order (1n2C) and one polar body (1n2C) are formed. During the second equational division, a mature egg cell (1n1C) is formed from a second-order oocyte, which has retained almost all the accumulated substances in the cytoplasm, and a second polar body of small size (1n1C). At the same time, division of the first polar body occurs, giving rise to two second polar bodies (1n1C).

As a result, during oogenesis, 4 cells are obtained, of which only one will later become an egg, and the remaining 3 (polar bodies) are reduced. The biological significance of this stage of oogenesis is to preserve all the accumulated substances of the cytoplasm around one haploid nucleus to ensure normal nutrition and development of the fertilized egg.

During oogenesis in women, at the stage of the second metaphase, a block is formed, which is removed during fertilization, and the maturation phase ends only after the sperm penetrates the egg.

The process of oogenesis in women is a cyclical process, repeating approximately every 28 days (from the period of growth until just after fertilization). This cycle is called menstrual.

The distinctive features of spermatogenesis and oogenesis in humans are presented in the table. The most obvious distinguishing feature of the egg is its large size. A typical egg cell has a spherical or oval shape, and its diameter in humans is about 100 microns (the size of a typical somatic cell is about 20 microns). The size of the nucleus can be just as impressive; in anticipation of the rapid divisions immediately following fertilization, reserves of proteins are deposited in the nucleus.

The cell's need for nutrients is satisfied mainly by the yolk, a protoplasmic material rich in lipids and proteins. It is usually found in discrete structures called yolk granules. Another important specific structure of the egg is the outer egg membrane - a covering of a special non-cellular substance consisting mainly of glycoprotein molecules, some of which are secreted by the egg itself, and the other part by surrounding cells. In many species, the membrane has an inner layer directly adjacent to the plasma membrane of the egg and is called the zona pellucida in mammals and the vitelline layer in other animals. This layer protects the egg from mechanical damage, and in some eggs it also acts as a species-specific barrier for sperm, allowing only sperm of the same species or very closely related species to penetrate.

Many eggs (including mammals) contain specialized secretory vesicles located under the plasma membrane in the outer, or cortical, layer of the cytoplasm. When the egg is activated by sperm, these cortical granules release the contents by exocytosis, as a result of which the properties of the egg membrane change in such a way that other sperm cannot penetrate through it. The process of formation of male germ cells is spermatogenesis. As a result, sperm are formed.

Somatic cells, having reached a certain mature physiological state, divide mitotically (sometimes by amitosis), while germ cells in their development undergo special phases of transformation until they mature and become capable of fertilization. This difference has a deep biological meaning. Somatic cells must retain the entire amount of hereditary information during divisions so that the daughter cells remain the same as the mother cells. The transfer of information is ensured during mitosis by the precise distribution of chromosomes between dividing cells: the number of chromosomes, their biological structure, DNA content and, consequently, the hereditary information contained in it are preserved in a number of cell generations, ensuring the constancy of the structure of the individual and species.

During fertilization, the nuclei of male and female germ cells unite into a common nucleus, and if there were as many chromosomes in each as in somatic cells, then in the zygote it would double, and such a double number would pass into all cells of the developing embryo. In the future, during the development of the germ cells of the next generations of young organisms, there will be a sequential accumulation of chromosomes in the cells, and the species could not preserve its hereditary characteristics unchanged. In addition, the nuclear-plasma coefficient in favor of the nucleus would gradually be disrupted, and after several generations a moment would come when the addition of chromosomes to the nucleus would lead to the inevitable death of the cell. As a result, fertilization would serve not to preserve, but to destroy organisms. However, this does not happen, since the process of gametogenesis includes two special divisions, during which the number of chromosomes in the nuclei of both the male and female germ cells is halved. Intracellular processes associated with a decrease in the number of chromosomes constitute the essence of maturation of germ cells - the essence of meiosis. During fertilization, half the number of chromosomes in the nuclei of the father's cells and half the number of chromosomes in the nuclei of the mother's cells are combined, and the set of chromosomes characteristic of this species is restored in the zygote.

There are 4 periods in spermatogenesis: reproduction, growth, maturation (meiosis) and formation (Fig. 3).

During the reproduction period, the original undifferentiated germ cells - spermatogonia, or gonia - divide through normal mitosis. After making several such divisions, they enter a period of growth. At this stage, they are called first-order spermatocytes (or I spermatocytes). They intensively assimilate nutrients, enlarge, undergo a deep physicochemical restructuring, as a result of which they prepare for the third period - maturation, or meiosis.

In meiosis, spermatocytes I undergo two processes of cell division. In the first division (reduction), the number of chromosomes decreases (reduction). As a result, from one cyte I, two equal-sized cells arise - spermatocytes of the second order, or cytes II. Then comes the second division of maturation. It proceeds like ordinary somatic mitosis, but with a haploid number of chromosomes. Such a division is called equational (“equatio” - equality), since two identical divisions are formed, i.e. completely equivalent cells called spermatids.

In the fourth period - formation - the rounded spermatid takes on the shape of a mature male reproductive cell: a flagellum grows, the nucleus becomes denser, and a shell is formed. As a result of the entire process of spermatogenesis, from each initial undifferentiated spermatogonia, 4 mature germ cells are obtained, each containing a haploid set of chromosomes.

In Fig. Figure 4 shows a diagram of the processes of spermatogenesis and spermiogenesis in humans. Spermatogenesis occurs in the convoluted seminiferous tubules of the testes. The development of sperm begins during the period of prenatal development during the laying of generative tissues, then resumes during the onset of puberty and continues until old age.

During the breeding season, a series of successive mitoses occur, resulting in an increase in the number of cells called spermatogonia. Some spermatogonia enter a period of growth and are called first-order spermatocytes.

The period of growth corresponds to the period of interphase of the cell cycle, in which the hereditary material of first order spermatocytes (2n4C) is doubled, and then they enter prophase I of meiotic division. During prophase I, conjugation of homologous chromosomes and exchange between homologous chromatids (crossing over) occurs. Crossing over has important genetic significance because it results in genetic differences between individuals.

Rice. 3. Scheme of gametogenesis:

1st – reproduction period: cells divide mitotically, the set of chromosomes in them is 2n; 2nd – growth period: accumulation of nutrients in cells, a set of chromosomes 2n; 3rd – period of maturation – meiosis: a) 1st, or reduction, division, formation from diploid cells with a set of chromosomes equal to 2n, cells with a haploid set equal to n; b) 2nd division of meiosis, proceeds as mitosis, but in cells with a haploid set of chromosomes; 4th – period of formation – occurs only in spermatogenesis

The maturation period occurs in two stages, which corresponds to the I meiotic (reduction) and II meiotic (equational) divisions. In this case, from one first-order spermatocyte, two second-order spermatocytes (1n2C) are first obtained, then 4 spermatids (1n1C). Spermatids differ from each other in the set of chromosomes: they all contain 22 autosomes, but half of the cells contain an X chromosome, and the other half a Y chromosome. Autosomes differ from each other and from their parents in a different combination of alleles, since an exchange occurred during crossing over.

During the formation period, the number of cells and the number of chromosomes in them does not change, because During this period, 4 spermatids are formed from 4 spermatozoa, in which morphological reorganization of cellular structures occurs and a tail is formed. In humans, this phase lasts 14 days.

Male germ cells do not develop singly; they grow in clones and are interconnected by cytoplasmic bridges. Cytoplasmic bridges exist between spermatogonia, spermatocytes and spermatids. At the end of the formation phase, spermatozoa are freed from cytoplasmic bridges.

In humans, the maximum daily sperm productivity is 10 8, the duration of sperm existence in the vagina is up to 2.5 hours, and in the cervix up to 48 hours.

The sperm is an elongated, motile cell. The main spermatozoa are the nucleus, which occupies the main volume of the head, and the organ of movement – the flagellum, which makes up the tail. The spermatozoon is a motile nucleus. The structure of the sperm is determined primarily by its functions.

The sperm has very little cytoplasm, but there are several supporting structures:

1) mitochondria, which provide it with energy

2) acrosome, an organelle similar to a lysosome and containing enzymes necessary for the penetration of sperm into the egg

3) centriole - the beginning of the flagellum, during fertilization it is used during the first division of the zygote

The acrosome is located in front of the nucleus in the head, and the centriole and mitochondria are in the middle part of the cell. The nucleus contains a haploid set of chromosomes (see also CELL), it is dense, condensed; the long flagellum is similar in structure to the flagella of protozoa and the cilia of the ciliated epithelium of multicellular animals.

Spermatozoa are very tenacious cells and under appropriate conditions (in the uterus) they remain viable for up to five days.

DIFFERENCES IN SPERMATOGENESIS FROM OVOGENESIS IN HUMANS

In sexual reproduction, offspring are produced by the fusion of genetic material from haploid nuclei. Usually these nuclei are contained in specialized germ cells - gametes; During fertilization, the gametes fuse to form a diploid zygote, which during development produces a mature organism. Gametes are haploid - they contain one set of chromosomes resulting from meiosis; they serve as a link between this generation and the next (during sexual reproduction of flowering plants, not cells, but nuclei, merge, but usually these nuclei are also called gametes).

Meiosis is an important stage in life cycles involving sexual reproduction, as it leads to a reduction in the amount of genetic material by half. Thanks to this, in a series of generations that reproduce sexually, this number remains constant, although during fertilization it doubles each time. During meiosis, as a result of random divergence of chromosomes (independent distribution) and the exchange of genetic material between homologous chromosomes (crossing over), new combinations of genes appear in one gamete, and such shuffling increases genetic diversity. The fusion of haploid nuclei contained in gametes is called fertilization or syngamy; it leads to the formation of a diploid zygote, that is, a cell containing one chromosome set from each parent. This combination of two sets of chromosomes in the zygote (genetic recombination) represents the genetic basis of intraspecific variation. The zygote grows and develops into a mature organism of the next generation. Thus, during sexual reproduction in the life cycle, an alternation of diploid and haploid phases occurs, and in different organisms these phases take different forms.

Gametes usually come in two types, male and female, but some primitive organisms produce only one type of gamete. In organisms that produce two types of gametes, they can be produced by male and female parents, respectively, or it may be that the same individual has both male and female reproductive organs. Species in which there are separate male and female individuals are called dioecious; such are most animals and humans. Among flowering plants there are also dioecious species; If in monoecious species male and female flowers are formed on the same plant, as, for example, in cucumber and hazel, then in dioecious species some plants bear only male, and others only female flowers, as in holly or yew.

Parthenogenesis

A distinction is made between natural parthenogenesis, the normal method of reproduction of some organisms in nature, and artificial parthenogenesis, caused experimentally by the action of various stimuli on an unfertilized egg, which normally requires fertilization. Classification of parthenogenesis:

Obligate - when it is the only way of reproduction

Cyclic - parthenogenesis naturally alternates with other methods of reproduction in the life cycle (for example, in daphnia and rotifers).

Facultative - occurring as an exception or as a backup method of reproduction in forms that are normally bisexual.

There are two types of parthenogenesis - haploid and diploid, depending on the number of chromosomes in the female gamete. In many insects, including ants, bees and wasps, various castes of organisms arise within a given community as a result of haploid parthenogenesis. In these species, meiosis occurs and haploid gametes are formed. Some eggs are fertilized and develop into diploid females, while unfertilized eggs develop into fertile haploid males. For example, in the honey bee, the queen lays fertilized eggs (2n = 32), which develop into females (queens or workers), and unfertilized eggs (n = 16), which produce males (drones) that produce sperm by mitosis, and not meiosis. This mechanism of reproduction in social insects has adaptive significance, since it makes it possible to regulate the number of descendants of each type. In aphids, diploid parthenogenesis occurs, in which the female oocytes undergo a special form of meiosis without chromosome segregation - all chromosomes pass into the egg, and the polar bodies do not receive a single chromosome. The eggs develop in the mother's body, so that young females are born fully formed, rather than hatching from eggs. This process is called viviparity. It can continue for several generations, especially in the summer, until almost complete non-divergence occurs in one of the cells, resulting in a cell containing all pairs of autosomes and one X chromosome. From this cell the male develops parthenogenetically. These autumn males and parthenogenetic females produce haploid gametes through meiosis that participate in sexual reproduction. Fertilized females lay diploid eggs that overwinter, and in the spring they hatch into females that reproduce parthenogenetically and give birth to live offspring. Several parthenogenetic generations are followed by a generation resulting from normal sexual reproduction, which introduces genetic diversity into the population through recombination. The main advantage that parthenogenesis gives to aphids is the rapid growth of the population, since all its mature members are capable of laying eggs. This is especially important during periods when environmental conditions are favorable for the existence of a large population, i.e. during the summer months.

Parthenogenesis is widespread in plants, where it takes various forms. One of them, apomixis, is parthenogenesis, simulating sexual reproduction. Apomixis is observed in some flowering plants in which the diploid ovule cell, or nucellus cell, or megaspore develops into a functional embryo without the participation of a male gamete. The rest of the ovule forms the seed, and the ovary develops into the fruit. In other cases, the presence of a pollen grain is required, which stimulates parthenogenesis, although it does not germinate; the pollen grain induces hormonal changes necessary for the development of the embryo, and in practice such cases are difficult to distinguish from true sexual reproduction.

Fertilization occurs in flowering plants in a unique way. After fertilization, the ovule produces a seed containing an embryo and a supply of nutrients. How is the supply of nutrients formed in the seed?

In flowering plants, double fertilization occurs. During pollination, the pollen grain lands on the stigma of the pistil and germinates, forming a pollen tube. It is formed from a vegetative cell and grows quickly, reaching the ovary. At the end of the pollen tube there are two sperm cells.

Unlike the motile sperm of lower plants, the sperm of flowering plants are immobile and can penetrate to the egg only through the pollen tube.

The pollen tube grows into the ovule, its tip ruptures, and the sperm enter the embryo sac. One of them fuses with the egg. A diploid cell is formed - a zygote. The second sperm fuses with the diploid secondary nucleus of the embryo sac. As a result, a cell is formed with a triple set of chromosomes, from which endosperm is formed through repeated mitoses - tissue containing a supply of nutrients.

Hermaphroditism

Conjugation

Conjugation (Latin “conjugatio” - connection) is a form of sexual process without the participation of gametes. Characteristic of Escherichia coli (division Bacteria), slipper ciliates (type Protozoa), in which two single-celled individuals come together and exchange genetic material through a cytoplasmic bridge.

Fig.4

As a result of conjugation, bacteria do not increase the number of individuals. In the green alga Spirogyra, conjugation occurs differently: two multicellular filaments stand parallel to each other, form opposing cytoplasmic bridges, along which the protoplast of a physiologically male individual flows into the female filament. As a result, many zygotes are formed.

Copulation

Some single-celled organisms experience a type of sexual process called copulation. Copulation (from the Latin “copulatio” - connection) is the process of fusion of two sex cells.

During copulation (in protozoa), the formation of sexual elements and their pairwise fusion occur. In this case, two individuals acquire sexual differences and completely merge, forming a zygote. Combination and recombination of hereditary material occurs, so individuals are genetically different from their parents.