Gametogenesis and double fertilization of flowering plants. Pollination and double fertilization of flowering plants Double fertilization of flowering plants

A unique feature of flowering plants is double fertilization.

Two sperm penetrate the ovary of angiosperms, one of them merges with the egg, giving rise to a diploid embryo. The other connects to the central diploid cell. A triploid cell is formed, from which the endosperm will arise - the nutrient material for the developing embryo (Fig. 77). This process, characteristic of all angiosperms, was discovered at the end of the last century by S.G. Navashin and was called double fertilization. The significance of double fertilization, apparently, lies in the fact that the active development of the nutrient tissue is ensured after fertilization. Therefore, the ovule in angiosperms does not store nutrients for the future and, consequently, develops much faster than in many other plants, such as gymnosperms.

Flowering plants have a number of features in the formation of germ cells and fertilization. Fertilization in them is preceded by the formation of a strongly reduced haploid generation - gametophytes. After fertilization, pollen germination of flowering plants begins with the swelling of the grain and the formation of pollen tube, which breaks through the sporoderm in its thinner place - the so-called aperture. The tip of the pollen tube secretes special substances that soften the tissues of the stigma and style into which the pollen tube is embedded. As the pollen tube grows, the nucleus of the vegetative cell and both sperm pass into it. In the vast majority of cases, the pollen tube penetrates the megasporangium (nucellus) through the micropyle of the ovule, rarely in another way. Having penetrated the embryo sac, the pollen tube ruptures and its contents pour out inside. One of the sperm fuses with the egg, and a diploid zygote is formed, which then gives rise to the embryo. The second sperm fuses with the secondary nucleus located in the center of the embryo sac, which leads to the formation of a triploid nucleus, which then develops into a triploid endosperm. This whole process is called double fertilization. It was first described in 1898 by the outstanding Russian cytologist and embryologist S.G. Navashin. Other cells of the embryo sac - antipodes and synergids do not participate in fertilization and are quickly destroyed.

The biological meaning of double fertilization is very great. Unlike gymnosperms, where a rather powerful haploid endosperm develops independently of the fertilization process, in angiosperms, triploid endosperm is formed only in the event of fertilization. Taking into account the gigantic number of generations, this achieves significant savings in energy resources. An increase in the ploidy level of the endosperm to 3n, apparently, contributes to the faster growth of this polyploid tissue compared to the diploid tissues of the sporophyte.

The interaction of the pollen tube of the gametophyte with the tissues of the sporophyte is a complex process regulated by chemicals. So, it turned out that if pollen is washed with distilled water, it loses its ability to germinate. If you concentrate the resulting solution and treat the pollen with a concentrate, it will again become full. After germination, the growth of the pollen tube is controlled by the tissues of the pistil. For example, in cotton, the growth of the tube to the egg takes 12-18 hours, but after 6 hours it is possible to establish which ovule the pollen tube is directed to: in this ovule, the destruction of a special cell begins - synergy. How the plant directs the growth of the tube in the right direction and how the synergid learns about its approach is not yet known.

In many cases, flowering plants have a "ban" on self-pollination: the sporophyte "recognizes" its male gametophyte and does not allow it to participate in fertilization. In some cases, however, its own pollen does not germinate on the stigma of the pistil. In most cases, the growth of the pollen tube begins, but then stops and it does not reach the egg. For example, in the spring primrose, Ch. Darwin discovered two forms of flowers - long-columnar (with a long style and short stamens) and short-column (short column, long stamen filaments). In short-columnar plants, the pollen is almost twice as large, and the cells of the papillae of the stigma are small. All of these traits are controlled by a group of closely linked genes.

Pollination is effective only when pollen is transferred from one form to another. Receptor molecules, which are complex complexes of proteins with carbohydrates, are responsible for recognizing their pollen. It has been shown that wild cabbage plants that do not produce receptor molecules in the stigma tissues can self-pollinate. In normal plants, the receptors appear on the stigma a day before the flower opens. If you open a bud and apply your own pollen on it two days before blooming, then fertilization will occur, but if one day before blooming, then no.

Interestingly, in some cases pollen self-incompatibility in plants is determined by a series of multiple alleles of the same gene, similar to incompatibility in tissue transplants in animals. These alleles are denoted by the letter S, and their number in a population can reach tens or even hundreds. If, for example, the genotype of an egg-producing plant is s1s2, and a pollen-producing plant is s2s3, then only 50% of the dust particles will germinate during cross-pollination - those that carry the s3 allele. In the presence of dozens of alleles, most of the cross-pollinated pollen germinates normally, and self-pollination is completely prevented.

Double fertilization in plants is of great biological importance. It was discovered by Navashin in 1898. Next, we will consider in more detail how double fertilization occurs in plants.

biological significance

The process of double fertilization promotes the active development of nutrient tissue. In this regard, the ovule does not store substances for future use. This, in turn, explains its rapid development.

Double fertilization scheme

Briefly, the phenomenon can be described as follows. Double fertilization in angiosperms consists in the penetration of two sperm into the ovary. One fuses with the egg. This contributes to the beginning of the development of a diploid embryo. The second sperm connects to the central cell. As a result, a triploid element is formed. From this cell comes the endosperm. It is a nutritious material for the developing embryo.

Pollen tube development

Double fertilization in angiosperms begins after the formation of a haploid highly reduced generation. It is represented by gametophytes. Double fertilization of flowering plants promotes pollen germination. It begins with the swelling of the grain and the subsequent formation of a pollen tube. It breaks through the sporoderm in its thinnest section. It's called aperture. Specific substances are released from the tip of the pollen tube. They soften the tissues of the style and stigma. Due to this, the pollen tube enters them. As it develops and grows, both sperm and the nucleus from the vegetative cell pass into it. In the vast majority of cases, the penetration of the pollen tube into the nucellus (megasporangium) occurs through the micropyle of the ovule. It is extremely rare that this is done in a different way. After penetration into the embryo sac, the pollen tube ruptures. As a result, all its contents are poured inward. Double fertilization of flowering plants continues with the formation of a diploid zygote. This is facilitated by the first sperm. The second element connects to the secondary nucleus, which is located in the central part of the embryo sac. The formed triploid nucleus subsequently transforms into the endosperm.

Cell Formation: General Information

The process of double fertilization of flowering plants is carried out by special sex cells. Their formation occurs in two stages. The first stage is called sporogenesis, the second - hematogenesis. In the case of the formation of male cells, these stages are called microsporogenesis and microhematogenesis. With the formation of female sexual elements, the prefix changes to "mega" (or "macro"). Sporogenesis is based on meiosis. This is the process of forming haploid elements. Meiosis, as well as in representatives of the fauna, is preceded by cell reproduction through mitotic divisions.

Sperm formation

The primary formation of male sexual elements is carried out in a special tissue of the anther. It is called archesporial. In it, as a result of mitoses, the formation of numerous elements occurs - the mother cells of pollen. They then enter meiosis. Due to two meiotic divisions, 4 haploid microspores are formed. For some time they lie side by side, forming tetrads. After that, they disintegrate into pollen grains - individual microspores. Each of the formed elements begins to be covered with two shells: outer (exine) and inner (intine). Then the next stage begins - microgametogenesis. It, in turn, consists of two mitotic successive divisions. After the first, two cells are formed: generative and vegetative. Subsequently, the first goes through another division. As a result, two male cells are formed - sperm.

Macrosporogenesis and megasporogenesis

In the tissues of the ovule, one or more archesporial elements begin to separate. They start to grow rapidly. As a result of this activity, they become much larger than the rest of the cells surrounding them in the ovule. Each archesporial element undergoes division by mitosis one, two or more times. In some cases, the cell can immediately transform into the mother cell. Inside it, meiosis occurs. As a result, 4 haploid cells are formed. As a rule, the largest of them begins to develop, turning into an embryo sac. The remaining three are gradually degenerating. At this stage, macrosporogenesis is completed, macrohematogenesis begins. During it, mitotic divisions occur (in most angiosperms there are three of them). Cytokinesis does not accompany mitosis. As a result of three divisions, an embryo sac with eight nuclei is formed. They subsequently separate into independent cells. These elements are distributed in a certain way throughout the embryo sac. One of the isolated cells, which, in fact, is an egg, together with the other two - synergids, occupies a place near the micropyle, into which the penetration of sperm occurs. In this process, synergies play a very significant role. They contain enzymes that help dissolve the membranes on the pollen tubes. On the opposite side of the embryo sac are the other three cells. They are called antipodes. With the help of these elements, nutrients are transferred from the ovule to the embryo sac. The remaining two cells are located in the central part. Often they merge. As a result of their connection, a diploid central cell is formed. After double fertilization occurs, and sperm enter the ovary, one of them, as mentioned above, will merge with the egg.

Pollen Tube Features

Double fertilization is accompanied by its interaction with the tissues of the sporophyte. It is quite specific. This process is regulated by the activity of chemical compounds. It has been established that if the pollen is washed in distilled water, it will lose its ability to germinate. If the resulting solution is concentrated and then processed, then it will again become complete. The development of the pollen tube after germination is controlled by the tissues of the pistil. For example, in cotton, its growth to the egg takes about 12-18 hours. However, already after 6 hours it is quite possible to determine to which particular ovule the pollen tube will be sent. This is understandable because the destruction of the synergy begins in it. At present, it has not been established how the plant can direct the development of the tube in the right direction and how the synergid learns about the approach.

"Ban" on self-pollination

It is often seen in flowering plants. This phenomenon has its own characteristics. The "ban" on self-pollination is manifested in the fact that the sporophyte "identifies" its own male hematophyte and does not allow it to participate in fertilization. At the same time, in some cases, the germination of its own pollen does not occur on the stigma of the pistil. However, as a rule, the growth of the tube still begins, but subsequently stops. As a result, pollen does not reach the egg and, as a result, double fertilization does not occur. Even Darwin noted this phenomenon. So, he found flowers of two forms in the spring primrose. Some of them were long-columnar with short stamens. Others are short-column. They had long filaments. Short-columnar plants are distinguished by large pollen (twice as much as others). In this case, the cells in the papillae of the stigma are small. These traits are controlled by a group of closely intertwined genes.

Receptors

Double fertilization is effective when pollen is transferred from one form to another. Special receptor molecules are responsible for the recognition of their own elements. They are complex compounds of carbohydrates with proteins. It has been established that forms of wild cabbage that do not produce these receptor molecules in the tissues of the stigma are capable of self-pollination. Normal plants are characterized by the appearance of carbohydrate-protein compounds the day before the opening of the flower. If you open a bud and treat it with your own pollen two days before it blooms, then double fertilization will occur. If this is done the day before the opening, then it will not be.

alleles

It is noteworthy that in a number of cases pollen "self-incompatibility" in plants is established by a series of multiple elements of one gene. This phenomenon is similar to incompatibility in tissue grafting in animals. Such alleles are denoted by the letter S. The number of these elements in the population can reach tens or even hundreds. For example, if the genotype of an egg-producing plant is s1s2 and a pollen-producing plant is s2s3, only 50% of the dust grains will germinate when cross-pollinated. These will be those that carry the s3 allele. If there are several tens of elements, then most of the pollen will germinate normally during cross-pollination, while self-pollination is completely prevented.

Finally

Unlike gymnosperms, which are characterized by the development of a sufficiently powerful haploid endosperm regardless of fertilization, in angiosperms tissue is formed only in this single case. Given the huge number of generations, significant energy savings are achieved in this way. An increase in the degree of ploidy of the endosperm, apparently, promotes faster tissue growth in comparison with the diploid layers of the sporophyte.

Let's remember the structure of a flower

In angiosperms, the process of formation of germ cells consists of two stages: sporogenesis and gametogenesis. Pollen is produced in the anthers of the stamens. Microsporogenesis is the process of formation of microspores in microsporangia (anther nests), where as a result mitoses Pollen mother cells are produced and undergo meiosis. After two meiotic divisions, 4 haploid microspores are formed - pollen grains. Pollen grain (mote) is covered with two shells. Then microgametogenesis occurs inside the pollen grain (the process of formation of a male gametophyte from a microspore) - two successive mitotic divisions. As a result of the first, vegetative and generative cells are formed, and after the second division, two sperm cells are formed from the generative cell.

Macrosporogenesis (the process of formation of megaspores) occurs in the ovules, which are located in the ovary of the pistil (covered). In the area of ​​the micropyle, one cell begins to grow - a megasporocyte, or the mother cell of megaspores. It happens in it meiosis, and 4 haploid cells are formed. One of these cells develops into the embryo sac, the other three are destroyed. Next, macrogametogenesis begins - the formation of the female gametophyte. After three mitotic divisions, an eight-nuclear embryo sac is formed. The nuclei subsequently separate into independent cells. One of these cells (ovum) with two synergic cells is located near the micropyle. Synergids contain enzymes that dissolve the shell of the pollen tube and, if necessary, can replace the egg. On the opposite side of the embryo sac are three antipodal cells (transmitters of nutrients from the ovule to the embryo sac). The two remaining cells fuse and form a large central diploid cell. The embryo sac has integuments.

The process of fertilization is preceded by the process of pollination. Once on the stigma, the pollen grain germinates into the pollen tube, which contains two immobile sperm. Synergids secrete enzymes that destroy the pollen tube, and its contents are poured out next to the egg. Fertilization does not depend on water. Double fertilization (S.G. Navashin, 1898): one sperm fuses with the egg (a zygote is formed), the second with a diploid central cell, forming a triploid endosperm. From the zygote, the embryo develops further, the integuments turn into the seed coat. The developing seed is protected by the walls of the ovary (pericarp). The fruit ripens in place of the flower.

double fertilization

the sexual process in angiosperms, in which both the egg and the central cell of the embryo sac are fertilized (See Embryo sac). Before. discovered by the Russian scientist S. G. Navashin in 1898 on 2 plant species - lilies (Lilium martagon) and hazel grouse (Fritillaria orientalis). In D. about. both sperm are involved, brought into the embryo sac by the pollen tube; the nucleus of one sperm (See sperm) merges with the nucleus of the egg, the nucleus of the second - with the polar nuclei or with the secondary nucleus of the embryo sac. A fertilized egg develops into an embryo , from the central cell - Endosperm. In embryo sacs with a three-celled egg apparatus, the contents of the pollen tube usually pour into one of the synergids (See Synergids) , which is destroyed in this case (there are visible remains of the nucleus of the synergy and the vegetative nucleus of the pollen tube); the second synergid subsequently dies off. Further, both spermatozoa, together with the altered cytoplasm of the pollen tube, move into the slit-like gap between the egg and the central cell. Then the sperm dissociate: one of them penetrates the egg and comes into contact with its nucleus, the other penetrates into the central cell, where it contacts the secondary nucleus or one, and sometimes both, polar nuclei. Sperm lose their cytoplasm while still in the pollen tube or when they penetrate the embryo sac; sometimes sperm in the form of unchanged cells are also observed in the embryo sac.

At D. about. the nuclei of the embryo sac are in interphase (See Interphase) and are usually much larger than sperm nuclei, the shape and condition of which may vary. In skerda and some other Compositae, the sperm nuclei have the form of a double twisted or twisted chromatin filament; in many plants they are elongated, sometimes twisted, more or less chromatized, and do not have nucleoli; usually spermatozoa are rounded interphase nuclei with nucleoli, sometimes not differing in structure from female nuclei.

According to the nature of the association of male and female nuclei, it was proposed (E. N. Gerasimova-Navashina) to distinguish two types of D. o.: premitotic - the sperm nucleus is immersed in the female nucleus, its chromosomes are despiralized; the unification of the chromosome sets of both nuclei occurs in the interphase (in the zygote); postmitotic - male and female nuclei, retaining their shells, enter prophase (See Prophase) , at the end of which their unification begins; interphase nuclei containing chromosome sets of both nuclei are formed only after the first mitotic division of the zygote. At D. about. 2 haploid nuclei fuse in the egg, so the nucleus of the zygote is diploid. The number of chromosomes in the nuclei of the endosperm depends on the number of polar nuclei in the central cell and on their ploidy (See Ploidy) ; Most angiosperms have 2 haploid polar nuclei and their endosperm is triploid. Consequence D. o. - Xenia - manifestation of dominant traits of the endosperm of the paternal plant in the endosperm of hybrid seeds. If several pollen tubes penetrate the embryo sac, the sperm of the first of them participate in D. o., the sperm of the rest degenerate. Cases of dyspermia, i.e. fertilization of the egg by two sperm, are very rare.

Lit.: Navashin S. G., Izbr. works, vol. 1, M.-L., 1951; Mageshwar and P., Angiosperm Embryology, trans. from English, M., 1954; Poddubnaya Arnoldi V. A., General embryology of angiosperms, M., 1964; Steffen, K., Fertilisation, in: Maheshwari P. (ed.). Recent advances in the embryology of angiosperms, Delhi, 1963.

I. D. Romanov.

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

See what "Double fertilization" is in other dictionaries:

Only characteristic of flowering plants. In double fertilization, one of the sperm fuses with the egg, and the second with the central cell of the embryo sac. An embryo develops from a fertilized egg, a secondary one develops from a central cell ... ... Big Encyclopedic Dictionary

A type of sexual process peculiar only to flowering plants. Discovered in 1898 by S. G. Navashin in lilies. Before. lies in the fact that during the formation of the seed, not only the egg is fertilized, but also the center, the nucleus of the embryonic sac. From the zygote ... ...

double fertilization- The type of sexual process characteristic of flowering plants: one of the sperm fertilizes the egg, and the other (from the same pollen tube) fertilizes the central nucleus of the embryo sac, as a result of the first process, a diploid is formed ... ... Technical Translator's Handbook

Only characteristic of flowering plants. In double fertilization, one of the sperm fuses with the egg, and the second with the central cell of the embryo sac. An embryo develops from a fertilized ovum, a secondary one develops from a central cell ... ... encyclopedic Dictionary

Double fertilization The type of sexual process characteristic of flowering plants: one of the sperm fertilizes the egg, and the other (from the same pollen tube ) fertilizes the central nucleus ... ... Molecular biology and genetics. Dictionary.

It is peculiar only to flowering rows. At D. about. one of the sperm merges with the egg, and the second with the center. embryo sac cell. From a fertilized egg, an embryo develops from the center. cells are the secondary endosperm of the seed, containing ... ... Natural science. encyclopedic Dictionary

double fertilization- the process of fertilization that occurs in angiosperms, in which both sperm are formed. One of them merges with the egg, the second - with the central diploid cell of the embryo sac. Opened by S. G. Navashin in ... ... Plant anatomy and morphology

DOUBLE FERTILIZATION- the sexual process in angiosperms, which consists in the fusion of one male gamete of the pollen tube (sperm) with the egg of the embryo sac, and the second male gamete with the secondary nucleus of the embryo sac ... Glossary of botanical terms

double fertilization by navashin- PLANT EMBRYOLOGY DOUBLE FERTILIZATION ACCORDING TO NAVASHIN - the fusion of the egg and sperm to form a zygote (2p) and the simultaneous fusion of another sperm and a double nucleus to form the primary endosperm nucleus (3p). A common feature of all… General Embryology: Terminological Dictionary

Syngamy, the fusion of a male reproductive cell (sperm, sperm) with a female (egg, egg), leading to the formation of a zygote, the edge gives rise to a new organism. Animal O. is preceded by insemination. In the process of O., eggs are activated, ... ... Biological encyclopedic dictionary

From the sperm merges with the egg, and the second - with the central cell of the embryo sac. The embryo develops from the fertilized egg, and the secondary endosperm of the seed containing nutrients develops from the central cell. Opened in 1898 by S. G. Navashin.

Big Encyclopedic Dictionary. 2000 .

See what "DOUBLE FERTILIZATION" is in other dictionaries:

A type of sexual process peculiar only to flowering plants. Discovered in 1898 by S. G. Navashin in lilies. Before. lies in the fact that during the formation of the seed, not only the egg is fertilized, but also the center, the nucleus of the embryonic sac. From the zygote ... ...

double fertilization- The type of sexual process characteristic of flowering plants: one of the sperm fertilizes the egg, and the other (from the same pollen tube) fertilizes the central nucleus of the embryo sac, as a result of the first process, a diploid is formed ... ... Technical Translator's Handbook

The sexual process in angiosperms in which both the egg and the central cell of the embryo sac are fertilized (See Embryo sac). Before. discovered by the Russian scientist S. G. Navashin in 1898 on 2 types of lily plants ... ... Great Soviet Encyclopedia

Only characteristic of flowering plants. In double fertilization, one of the sperm fuses with the egg, and the second with the central cell of the embryo sac. An embryo develops from a fertilized ovum, a secondary one develops from a central cell ... ... encyclopedic Dictionary

Double fertilization The type of sexual process characteristic of flowering plants: one of the sperm fertilizes the egg, and the other (from the same pollen tube ) fertilizes the central nucleus ... ... Molecular biology and genetics. Dictionary.

It is peculiar only to flowering rows. At D. about. one of the sperm merges with the egg, and the second with the center. embryo sac cell. From a fertilized egg, an embryo develops from the center. cells are the secondary endosperm of the seed, containing ... ... Natural science. encyclopedic Dictionary

double fertilization- the process of fertilization that occurs in angiosperms, in which both sperm are formed. One of them merges with the egg, the second - with the central diploid cell of the embryo sac. Opened by S. G. Navashin in ... ... Plant anatomy and morphology

DOUBLE FERTILIZATION- the sexual process in angiosperms, which consists in the fusion of one male gamete of the pollen tube (sperm) with the egg of the embryo sac, and the second male gamete with the secondary nucleus of the embryo sac ... Glossary of botanical terms

double fertilization by navashin- PLANT EMBRYOLOGY DOUBLE FERTILIZATION ACCORDING TO NAVASHIN - the fusion of the egg and sperm to form a zygote (2p) and the simultaneous fusion of another sperm and a double nucleus to form the primary endosperm nucleus (3p). A common feature of all… General Embryology: Terminological Dictionary

Syngamy, the fusion of a male reproductive cell (sperm, sperm) with a female (egg, egg), leading to the formation of a zygote, the edge gives rise to a new organism. Animal O. is preceded by insemination. In the process of O., eggs are activated, ... ... Biological encyclopedic dictionary