Fertilization in plants, animals and humans is the fusion of male and female reproductive cells - gametes, as a result of which the first cell of a new organism is formed - a zygote. Fertilization is associated with sexual reproduction and the transfer of hereditary information from parents to offspring.

Fertilization is common to most plants. It is usually preceded by the formation of gametangia (genital organs), in which gametes develop. If a plant undergoes a sexual process during its development cycle, then meiosis also occurs, i.e., a change in nuclear phases is detected (see Alternation of generations).

The types of sexual process in lower plants are varied. Let's name only the main ones. The fusion of gametes with flagella, the shape and size of which are the same, is called isogamy, and the gametes are called isogametes. Thus, many unicellular algae are isogamous, for example some Chlamydomonas; being single-celled, they themselves become gametangia, forming gametes. In the multicellular alga Ulothrix, some cells that are no different from others become gametangia. In some isogamous brown algae, the gametangia are distinct from the rest of the plant cells.

In many isogamous algae, not every pair of gametes can form a zygote, since the gametes are physiologically different. Outwardly identical gametes cannot be called either male or female; Physiological differences are indicated in isogamy by the signs + and -. Only gametes of different signs, formed by physiologically different (+ and -) individuals of the algae, can merge.

The fusion of gametes with flagella that differ physiologically and in size is called heterogamy, and the gametes are female (larger) and male (smaller). For example, some Chlamydomonas are heterogamous. The fusion of a flagellated large female gamete (egg) with a small, male gamete, usually having a flagellum or flagella (sperm), is called oogamy. The female gametangia of most oogamous lower plants are called oogonia, and the male gametangia are called antheridia. For example, many green and brown algae, as well as red algae, are oogamous.

In iso-, hetero-, and many oogamous lower plants, gametes emerge from the gametangia into the water, where fertilization occurs. In some (for example, in the green alga Volvox), the egg remains in the oogonia, where the sperm released into the water penetrate and where the fusion of gametes occurs.

All higher plants are oogamous. Their typical gametangia - antheridia (male) and archegonia (female) - are multicellular. The archegonium produces one egg, and the antheridium produces many sperm. In bryophytes and pteridophytes, spermatozoa released from the antheridia swim in the water to the opened archegonia and merge with the eggs inside the archegonia. In pteridophytes and seed plants, fertilization occurs on (or in) shoots (gametophytes), which develop independently in the former, and on sporophytes in the latter (see Alternation of generations). Thalli of homosporous fern-like ferns are bisexual, while heterosporous and all seed ferns are dioecious (see Disputes). The strong reduction of the male prothallus of seed plants has led to the fact that antheridia are not formed in it (i.e., in the pollen grain) in either holosperms or angiosperms. In the female prothallus (primary endosperm) of almost all gymnosperms, archegonia are still developing, but in the female prothallus - the embryo sac - of angiosperms they are no longer present.

In seed plants, fertilization is preceded by pollination - the transfer of pollen grains from microsporangia, where they began to develop from microspores, into the pollen chamber of the ovule (in gymnosperms) or on the stigma of the pistil (in angiosperms). Only a few gymnosperms (cycads, ginkgo) produce multiflagellar spermatozoa in the male prothalla, while in the rest, for example, in conifers and all angiosperms, male gametes - sperm - do not have flagella.

Spermatozoa reach the archegonia, moving in the liquid produced by the plant itself. In seed plants that have sperm, the latter go to the eggs through pollen tubes formed by male prothellae. In angiosperms, after pollination, the pollen grain forms a pollen tube, which, lengthening, grows between the cells of the stigma and style, enters the cavity of the ovary and, having passed through the pollen passage of the ovule, grows with its end into the embryo sac. Here, sperm cells emerge from the opened pollen tube (see figure). One sperm fuses with the egg to form a diploid zygote, giving rise to an embryo. The second fuses with the central cell of the embryo sac, which in most angiosperms has two haploid nuclei or one diploid (if the nuclei are fused). After the fusion of the central cell with the sperm, its nucleus becomes triploid. This peculiar process, characteristic only of angiosperms, was first described by the Russian scientist S. G. Navashin (1898) and called double fertilization. From the triploid cell, multicellular storage tissue develops - the secondary endosperm, the nutrients of which are used by the embryo in the early stages of its development.

Fertilization, independent of the presence of free water, is one of the most important adaptations of seed plants to existence on land.

Fertilization in multicellular animals involves the fusion of two gametes of different sexes - a sperm and an egg. The sperm introduces into the egg the hereditary material contained in its nucleus. The location of sperm penetration into the egg can determine the location of parts of the future organism. For example, in amphibians, the part of the egg into which the sperm entered will, during development, turn into the anterior end of the body.

Until the moment when one of the sperm touches the surface of the egg, the latter influences their behavior by releasing certain substances. They make sperm move faster or, conversely, glue and immobilize them (this is necessary if there are too many sperm). Particularly active interactions begin as soon as the sperm touches the surface of the egg. Within a few seconds, the front part of the sperm turns into a tube, the tip of which sticks to the surface of the egg. Through this tube, the contents of the sperm, including its nucleus with hereditary material, are pressed into the egg.

Violent changes immediately begin in the egg, which are externally manifested in the formation of a membrane on its surface that prevents the penetration of other sperm. In addition, rapid rearrangements of the cytoplasmic structures responsible for protein synthesis occur in the egg: the synthesis processes are immediately and many times accelerated. Only after this the hereditary material of the sperm that has entered the egg is combined with the hereditary material of the egg nucleus. Maternal and paternal chromosomes (carriers of hereditary material) are distributed equally throughout all cells of the embryo formed from the zygote - a fertilized egg.

Fertilization is the process of combining male and female gametes, which leads to the formation of a zygote and the subsequent development of a new organism. During the process of fertilization, a diploid set of chromosomes is established in the zygote, which determines the outstanding biological significance of this process.

Depending on the species of organisms in animals that reproduce sexually, external and internal fertilization are distinguished.

External fertilization occurs in the environment into which male and female reproductive cells enter. For example, fertilization in fish is external. The male (milk) and female (caviar) reproductive cells secreted by them enter the water, where they “meet” and unite. Data on fertilization in sea urchins indicate that within 2 seconds after contact of sperm and egg, changes occur in the electrical properties of the plasma membrane of the egg. The fusion of the contents of the gametes occurs after 7 seconds.

Internal Fertilization is ensured by the transfer of sperm from the male body to the female body as a result of sexual intercourse. Such fertilization occurs in mammals, and the central point here is the outcome of the meeting between the germ cells. It is believed that the nuclear contents of only one sperm penetrate into the egg of these animals. As for the cytoplasm of the sperm, in some animals it enters the egg in small quantities, in others it does not enter the egg at all.

In humans, fertilization occurs in the upper part of the fallopian tube, and in fertilization, as in other mammals, only one sperm participates, the nuclear contents of which enter the egg. Sometimes there may be not one, but two or more eggs in the fallopian tube, as a result of which the birth of twins, triplets, etc. is possible. For example, in the 18th century. A case was registered in Russia of the birth of 16 twins, 7 triplets and 4 quadruplets (69 children in total) by one mother (the wife of the peasant Fyodor Vasilyev).

As a result of fertilization, the diploid set of chromosomes is restored in the fertilized egg. Eggs are capable of fertilization within about 24 hours after ovulation, while the fertilizing ability of sperm lasts up to 48 hours.

Much remains unclear about the mechanisms of fertilization. It is assumed that the penetration of nuclear material into the egg by only one of many sperm is associated with changes in the electrical properties of the plasma membrane of the egg. There are two hypotheses regarding the reasons for sperm activation of egg metabolism. Some researchers believe that the binding of sperm to external receptors on the surface of cells is a signal that enters the egg through the membrane and activates inositol triphosphate and calcium ions there. Others believe that sperm contain a special initiating factor.

A fertilized egg gives rise to a zygote; the development of organisms through the formation of zygotes is called zygogenesis. Experimental developments in recent years have shown that fertilization of eggs from mammals, including humans, is possible in vitro, after which the embryos developed in vitro can be implanted into a woman's uterus, where they can develop further. To date, numerous cases of the birth of “test tube” children are known (see section VI). It has also been established that not only spermatozoa, but also spermatids are capable of fertilizing a human egg. Finally, it is possible to fertilize eggs (artificially deprived of nuclei) of mammals with the nuclei of their somatic cells (see § 35).

In contrast to zygogenesis, many animal organisms are capable of reproduction in natural conditions through parthenogenesis (from the Greek parthenos - virgin and genesis - birth). There are obligate and facultative parthenogenesis. Obligate parthenogenesis is the reproduction of organisms from an unfertilized egg. This parthenogenesis serves as a means of reproduction for more than 90 species of animals, including some vertebrates. An example of obligate parthenogenesis is the reproduction of the Caucasian rock lizard, represented only by female individuals. On the contrary, facultative parthenogenesis means that eggs are able to develop both without fertilization and after fertilization. Facultative parthenogenesis, in turn, is female and male. Female parthenogenesis is common in bees, ants, and rotifers, in which males develop from unfertilized eggs. Male parthenogenesis occurs in some isogamous algae.

In plants, there are also cases where an embryo develops from an unfertilized egg. As noted above, this phenomenon is called apomix. It is very widely found in many angiosperms, including cultivated ones, such as beets, cotton, flax, tobacco and others.

Along with natural parthenogenesis, artificial (induced) parthenogenesis is distinguished, which can be caused by irritation of eggs using physical or chemical factors, which leads to activation of eggs and, as a consequence, to the development of unfertilized eggs. Artificial parthenogenesis has been observed in animals belonging to many systematic groups - echinoderms, worms, mollusks and even some mammals.

There is a known form of parthenogenesis, called androgenesis (from the Greek andros - man, genesis - birth). If the nucleus in an egg is inactivated and then several sperm penetrate into it, then a male organism develops from such an egg as a result of the fusion of male (sperm) nuclei. The experiments of V.L. Astaurov (1904-1974), who showed androgenesis on the silkworm, are widely known. These experiments consisted of the following. In the eggs of one species of silkworm (Bombyx mandarina), the nuclei were inactivated using high temperature, and then such eggs were fertilized with sperm of a silkworm of another species (B. mori). Having penetrated the eggs, the latter merged with each other, which gave rise to new organisms, which in their properties turned out to be paternal organisms (B. mori). Crosses of these organisms with B. mori females produced offspring belonging to B. mori.

The role of parthenogenesis and its forms in nature is small, since it does not provide broad adaptive capabilities of organisms. However, its use has practical significance. In particular, B.L. Astaurov developed a method for obtaining parthenogenetic offspring from the silkworm, which is widely used in the industrial production of silk.

In contrast to zygogenesis and parthenogenesis, there is gynogenesis (from the Greek gyne - woman), which is pseudogamy, which consists in the fact that the sperm meets the egg and activates it, but the sperm nucleus does not merge with the nucleus of the egg. In this case, the enabling offspring consists only of females. In certain species of roundworms, fish and amphibians, gynogenesis serves as a normal form of reproduction, producing offspring consisting only of females. Gynogenesis can also be induced artificially with the help of factors that can destroy cell nuclei (radiation, temperature, etc.). In particular, cases of artificial gynogenesis have been described in the silkworm and in some species of fish and amphibians. Obtaining such forms may have some practical significance in the case of economically useful species.

As noted above, fertilization in flowering plants (angiosperms) has a significant distinctive feature in the form of double fertilization (S. G. Navashin, 1896), which boils down to the fact that in the embryo sac a haploid egg and a diploid central cell are fertilized by sperm, in as a result of which a diploid embryo and a triploid cell are formed, developing into endosperm cells (see Chapter II).

Parthenogenesis, androgenesis and gynogenesis are forms of disorders of sexual reproduction. It is assumed that these forms arose during evolution as a result of particular evolutionary adaptations.

Fertilization is the process of fusion of a sperm with an egg, resulting in a diploid zygote; Each pair of chromosomes in it is represented by one paternal and another maternal. The essence of fertilization is to restore the diploid set of chromosomes and to combine the hereditary material of both parents, as a result of which the offspring, which combines the beneficial characteristics of the father and mother, are more viable.

Impaired fertilization and its consequences.

Fertilization is one of the links in the biological existence of a species. This is preceded by a long and complex preparation of two individuals, during which they are exposed to a variety of environmental influences, which negatively affect the fertilization process.

The egg and sperm have a limited lifespan and an even shorter duration of ability to fertilize. Thus, in mammals, and in humans in particular, an egg released from the ovary retains the ability to fertilize for 24 hours. Violation of this time period will inevitably lead to loss of the ability to fertilize.

A man's sperm in a woman's genital tract remains mobile for more than 4 days, but they lose their fertilizing ability after 1 - 2 days. With increasing time, unprotected cells experience the negative influence of various factors.

The latter can cause disturbances in the ascending state of the gamete gene pool, which will inevitably lead to unprogrammed deviations in the development of the zygote with corresponding consequences for the species as a whole.

The speed of sperm movement under normal conditions is 1.5-3 mm/min. Various deviations from such forward movement cause loss of the ability to fertilize. This is also caused by changes in the pH of the vaginal environment, inflammation, etc. The ejaculate of a man on average contains 350 million sperm capable of fertilization. If the number of sperm is less than 150 million (or less than 60 million in 1 ml), then the probability of fertilization is sharply reduced. So, the excessive concentration of sperm in the ejaculate is of exceptional importance in the mechanism of fertilization.

Fertilization disorders occur when there are pathological changes in the morphology of sperm. The biological usefulness of gametes is significantly influenced by the length of their stay in the woman’s genital tract. Thus, overripeness of sperm and eggs in the female reproductive tract for various reasons causes an increase in the frequency of chromosomal aberrations in aborted fetuses.

Irregular types of sexual reproduction.

Classification of irregular types of sexual reproduction.
Irregular types of sexual reproduction include parthenogenetic, gynogenetic and androgenetic reproduction of animals and plants (Fig. 27).
Parthenogenesis is the development of an embryo from an unfertilized egg. The phenomenon of natural parthenogenesis is characteristic of lower crustaceans, rotifers, hymenoptera (bees, wasps), etc. It is also known in birds (turkeys). Parthenogenesis can be stimulated artificially by causing activation of unfertilized eggs through exposure to various agents.
A distinction is made between somatic, or diploid, and generative, or haploid, parthenogenesis. During somatic parthenogenesis, the egg does not undergo reduction division, or if it does, then two haploid nuclei, merging together, restore the diploid set of chromosomes (autokaryogamy); Thus, the diploid set of chromosomes is preserved in the tissue cells of the embryo.
In generative parthenogenesis, the embryo develops from a haploid egg. For example, in the honey bee (Apis mellifera), drones develop from unfertilized haploid eggs through parthenogenesis.

Parthenogenesis in plants is often called apomixis. Since apomixis is widespread in the plant world and is of great importance in the study of inheritance, let us consider its features.
The most common type of apomictic reproduction is the type of parthenogenetic formation of an embryo from an egg. In this case, diploid apomixis (without meiosis) is more common.
Hereditary information both during the formation of the endosperm and during the formation of the embryo is obtained only from
Different types of sexual reproduction:
1 - normal fertilization; 2 - parthenogenesis: 3 - gynogenesis; 4 - androgiesis.
mother. In some apomicts, the formation of full-fledged seeds requires pseudogamy - activation of the embryo sac by the pollen tube. In this case, one sperm from the tube, reaching the embryo sac, is destroyed, and the other merges with the central nucleus and participates only in the formation of endosperm tissue (species from the genera Potentilla, Rubus, etc.). Inheritance here occurs somewhat differently from the previous case. The embryo inherits characteristics only from the maternal line, and the endosperm inherits both maternal and paternal characteristics.
Gynogenesis. Gynogenetic reproduction is very similar to parthenogenesis. Unlike parthenogenesis, gynogenesis involves sperm as stimulators of egg development (pseudogamy), but fertilization (karyogamy) does not occur in this case; the development of the embryo is carried out exclusively at the expense of the female nucleus (Fig. 27, 3). Gynogenesis has been found in roundworms, the viviparous fish Molliensia formosa, in silver crucian carp (Platypoecilus) and in some plants - buttercup (Ranunculus auricomus), bluegrass (genus Poa pratensis), etc.
Gynogenetic development can be induced artificially if sperm or pollen are irradiated with X-rays, treated with chemicals or exposed to high temperatures before fertilization. In this case, the nucleus of the male gamete is destroyed and the ability for karyogamy is lost, but the ability to activate the egg is retained.

Fertilization is the process of fusion of two cells, resulting in the formation of a new cell, giving rise to another organism of the same genus or species. What is it in flowering plants and how it happens, read in this article.

The essence of fertilization

It occurs as a result of the fusion of two cells, female and male, and the formation of a diploid zygote. Each pair of chromosomes contains one father and one mother cell. The essence of the fertilization process is to restore and combine the hereditary material of the parents. Their offspring will be more viable, as they will combine the most useful qualities from their father and mother.

Fertilization - what is it?

This is the process of inducing the egg to develop as a result of the union of nuclei. Fertilization - what is it? This is an irreversible process that occurs as a result of the fusion of different-sex gametes and the union of their nuclei. does not undergo this procedure a second time.

But there are plants that reproduce a new generation only with the help of the female gamete without fertilization. This type of reproduction is called virgin reproduction. It is noteworthy that these two methods of reproduction can alternate in one plant species.

Double fertilization of flowering plants

Both origins are called gametes. Moreover, the female cells are the eggs, and the male cells are the sperm cells, which are immobile in seed plants and mobile in spore plants. Fertilization - what is it? This is the appearance of a special cell - a zygote, containing the hereditary characteristics of the sperm and egg.

They have complex fertilization, which is called double, because in addition to the egg, another special cell is fertilized. The formation of sperm occurs in pollen grains, and their maturation occurs in the stamens, more precisely in their anthers. The place of formation of eggs is the ovules located in the ovary of the pistil. When the egg is fertilized by sperm, seeds begin to develop from the ovule.

For fertilization to occur in flowering plants, the plant must first be pollinated, that is, grains of pollen must fall on the stigma of the pistil. Once on the stigma, they begin to grow inside the ovary, resulting in the formation of a pollen tube. At the same time, two sperm cells are formed in the dust particle. They do not stand still, but begin to move towards the pollen tube, which penetrates the ovule. Here, as a result of the division and elongation of one cell, the formation of the embryo sac occurs.

It is needed to house an egg and another cell in which a double set of hereditary information is concentrated. After this, the pollen tube grows into the embryo sac and one sperm merges with the egg, resulting in the formation of a zygote, and the other with a special cell. The development of the embryo occurs from the zygote. The second fusion forms the nutrient tissue, or endosperm, necessary to nourish the embryo during growth.

What does each plant species need to exist?

• First of all, it is necessary to restore the diploid set of chromosomes, and within it, their pairing.
• Ensure material continuity between successive generations.
• Combine the hereditary properties of two parents in one species or genus.

All this is done at the genetic level. In order for fertilization to take place, the maturation of maternal and paternal gametes must occur simultaneously.

Fertilization in angiosperms

This process was first characterized by the German scientist Strassburger in the second half of the nineteenth century. Fertilization of angiosperms occurs as a result of the fusion of two nuclei of different gametes: with a male and a female principle. Their cytoplasm is not involved in fertilization. Fertilization itself occurs when the sperm fuses with the nucleus of the egg.

The site of sperm production is the pollen grain or pollen tube. The grain begins to germinate after it hits the stigma. The time at which this process begins is different for each plant, as is the time of fertilization. For example, beet pollen grains germinate in two hours, and corn pollen grains germinate instantly. The first sign of grain germination is its increase in volume. Usually one pollen grain forms one tube. But some plants do not obey this rule and form several tubes, of which only one reaches its development.

The pollen tube with the sperm moving along it grows and eventually ruptures. All its contents end up inside the embryo sac. One of the sperm that penetrates here penetrates the egg and fuses with its haploid nucleus. Fertilization - what is it? This is the fusion of two nuclei: sperm and egg. The fertilized egg begins to divide, producing two new cells. They are divided by four and so on. Thus, repeated division occurs, as a result of which the plant embryo develops.

Angiosperms, after the fertilization process, have the ability to develop an additional organ called endosperm. This is nothing more than the nutrient medium of the embryo. When the second sperm and the diploid nucleus merge, a certain set of chromosomes is formed, of which two are of maternal origin and one is of paternal origin. Thus, double fertilization of organisms of plant origin occurs when one sperm fuses with the egg, and the other with the nucleus of the cell located in the center.

Distinctive features of angiosperms

• Great adaptability to growing in different conditions.
• Double fertilization, which allows you to have a supply of substances necessary for normal seed germination.
• Presence of triploid endosperm.
• The formation of ovules inside the ovary, in which the walls of the pistil protect them from damage.
• Development of angiosperm fruit from the ovary.
• The presence of the seed inside the fruit, the walls of which are its protection.
• The presence of a flower provides an opportunity for insects.

Thanks to these characteristics, they occupy a dominant position in the world.

Features of fertilization of angiosperms

It follows from the fact that these plants have double fertilization. A unique feature is represented by a phenomenon called xenia. Its meaning is that pollen directly affects the properties and characteristics of the endosperm. Let's take corn as an example.

It comes with yellow and white seeds. Their color depends on the shade of the endosperm. When female flowers of white kernel corn are pollinated with pollen from a yellow kernel variety, the color will still be yellow, although endosperm development occurs on the plant with white kernels.

What role do flowering plants play?

These plants number 13,000 genera and 250,000 species. They have become widespread throughout the world. Flowering plants are key components of the biosphere, producing organic substances that bind carbon dioxide and release oxygen. Pasture food chains begin with them. Many varieties of flowering plants are used by humans for food. Dwellings are built from them and various household materials are made.

Medicine cannot do without them. Certain species of angiosperms are dominant on the planet; they play a decisive role in the formation of vegetation cover and the creation of the main part of terrestrial phytomass. Ultimately, it is these plants that determine the possibility of the very existence of man on earth as a biological species.

PhaseI I – the acrosomal reaction occurs in it. The outer membrane of the sperm ruptures, releasing proteolytic enzymes and dissolving the membrane of the egg. Plasma membranes fuse, cytoplasms are connected. The nucleus and centriole of the sperm pass into the cytoplasm of the egg. The tail part is resorbed. Then the egg is activated, its potential changes and its vitelline membrane peels off and the fertilization membrane is formed (cortical reaction). Activation ends with the beginning of protein synthesis.

PhaseIII– syngamy. It distinguishes:

Stage 2 X pronuclei - the male nucleus swells, takes on a prophase appearance, during this time the DNA doubles and the male pronucleus receives a haploid set of reduplicated chromosomes (n2c). When the egg meets the sperm, it is in the stage of meiosis, blocked by a special factor. After meeting the sperm, the egg is activated and the block is removed. The nucleus of the egg, having completed meiosis, turns into the female pronucleus, also acquiring a set of n2c chromosomes.

The synkaryon stage is the fusion of nuclear material and the formation of a zygote.

The first mitotic division of the zygote leads to the formation of two embryonic cells (blastomeres) with a set of chromosomes 2n2c in each.

Parthenogenesis

the daughter organism sometimes develops from an unfertilized egg. This phenomenon is called virgin development or parthenogenesis. In this case, the source of hereditary material for the development of the descendant is usually the DNA of the egg - gynogenesis. Less commonly observed androgenesis- development of a descendant from a cell with the cytoplasm of the oocyte and the nucleus of the sperm. In the case of androgenesis, the nucleus of the female gamete dies.