Sexual reproduction is divided. Asexual and sexual reproduction in plants. Summary: Asexual and sexual reproduction

The textbook corresponds to the basic level of the Federal component state standard general education in biology and is recommended by the Ministry of Education and Science of the Russian Federation.

The textbook is addressed to students in grades 10-11 and completes the line of N. I. Sonin. However, the features of the presentation of the material make it possible to use it at the final stage of studying biology after the textbooks of all existing lines.

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Remember!

What are the two main types of reproduction in nature?

What is vegetative propagation?

What set of chromosomes is called haploid? Diploid?

Tens of thousands of organisms die every second on Earth. Some from old age, others because of illnesses, others are eaten by predators ... We pick a flower in the garden, accidentally step on an ant, kill a mosquito that has bitten us and catch a pike on the lake. Every organism is mortal, so every species must take care that its numbers do not decrease. The mortality of some individuals is compensated by the birth of others.

The ability to reproduce is one of the main properties of living matter. reproduction, that is, the reproduction of their own kind ensures the continuity and continuity of life. In the process of reproduction, exact reproduction and transmission of genetic information from the parent generation to the next, daughter generation occurs, which ensures the existence of the species for a long time, despite the death of individual individuals. Reproduction is based on the ability of a cell to divide, and the transfer of genetic information ensures the material continuity of generations of any kind. In order for an individual to be able to reproduce its own kind, that is, to become capable of reproduction, it must grow up and reach a certain stage of development. Not all organisms survive to the reproductive stage and not all leave offspring, therefore, in order to maintain the existence of the species, each generation must produce more offspring than there were parents. The properties of living organisms - growth, development and reproduction - are inextricably linked with each other.

All types of organisms are capable of reproduction. Even viruses - a non-cellular form of life - albeit not independently, but also multiply in the cells of the host organism. In the process of evolution in nature, several methods of reproduction have arisen, each of which has its own advantages and disadvantages. All the various forms of reproduction can be grouped into two main types - asexual And sexual.

Asexual reproduction. This type of reproduction occurs without the formation of specialized germ cells (gametes), and only one organism is needed for its implementation. A new individual develops from one or more somatic (non-sex) cells of the parent organism and is its absolute copy. Genetically homogeneous offspring from the same parent is called clone.

Rice. 54. Amoeba division

Asexual reproduction is the most ancient form of reproduction, therefore it is especially widespread in unicellular organisms, but it is also found among multicellular organisms.

There are several ways of asexual reproduction.

Division. Prokaryotic organisms (bacteria and blue-green algae) reproduce by simple division preceded by the duplication of a single circular DNA molecule.

By mitotic division into two or more cells, protozoa (amoeba, ciliates, flagellates) (Fig. 54) and unicellular green algae reproduce.

Some protozoa (malarial plasmodium) have a special method of asexual reproduction, the so-called schizogony. The nucleus of the mother individual divides several times in a row without dividing the cytoplasm, and then the formed multinuclear cell breaks up into many single-nuclear cells.

Sporulation. This method of reproduction is typical mainly for fungi and plants. Specialized cells - spores - can be formed in special organs - sporangia (as occurs in plants) or openly, on the surface of the body (as, for example, in some molds).

The spores are produced in large numbers and are very light in weight, making them easy to spread by wind and also by animals, mostly insects. In one grain of wheat, affected by hard smut, from 8 to 20 million spores are formed, and in the whole ear - up to 200 million. In some types of fungi, the number of spores produced per day reaches 30 billion! The losses of spores are very high, only an insignificant part of them falls into favorable conditions for germination. However, those disputes that are “unfortunate” can wait a long time in the wings. So, for example, spores of smut fungi remain viable for 25 years.

vegetative reproduction. The method of asexual reproduction, in which the daughter organism develops from a group of parent cells, is called vegetative reproduction.

Such reproduction is widespread in plants. Under natural conditions, it usually occurs using specialized parts of the plant body. A tulip bulb, a gladiolus corm, a horizontally growing underground stalk (rhizome) of an iris, a creeping blackberry stalk creeping along the surface of the soil, strawberry whiskers, potato tubers and dahlia root tubers are all organs vegetative propagation plants.

Especially common various forms vegetative propagation among plants living in harsh climatic conditions. Unexpected frosts on a summer day can destroy the flowers or unripe fruits of tundra plants. Vegetative reproduction allows them not to depend on such surprises. Some saxifrages are able to form brood buds that spread like seeds, bluegrass form small daughter plants in place of flowers that can fall off and take root, and the meadow core reproduces exclusively by modified leaf segments.

Vegetative reproduction in animals is carried out in two main ways: fragmentation and budding.

Fragmentation- this is the division of the body into two or more parts, each of which gives rise to a new full-fledged individual. This process is based on the ability to regenerate. In this way, annelids and flatworms, echinoderms and coelenterates can reproduce.

Fragmentation also occurs in the plant kingdom. The green algae spirogyra reproduces by scraps of its filaments, and lower mosses by pieces of thallus.

budding- this is the formation on the body of the maternal individual of a group of cells - the kidney, from which a new individual develops. For some time, the daughter individual develops as part of the mother's organism, and then either separates from it and passes on to independent existence (freshwater hydra polyp), or, continuing to grow, forms its own buds, forming a colony (coral polyps). Budding also occurs in unicellular yeast fungi (Fig. 55).

Rice. 55. Yeast budding

Sexual reproduction. Sexual reproduction is the process of formation of a daughter organism with the participation of germ cells - gametes. In most cases, a new generation arises from the fusion of two specialized germ cells. various organisms. Gametes that give rise to a daughter organism have a half (haploid) set of chromosomes of a given species and are formed as a result of a special process - meiosis(§ ). As a rule, gametes are of two types - male and female, and they are formed in special organs - the gonads.

A new organism resulting from the fusion of gametes receives hereditary information from both parents: 50% from the mother and 50% from the father. Although similar to them, he nevertheless has his own unique combination of genetic material, which can be very good for surviving in changing environmental conditions.

Species that have both males and females are called dioecious; most of the animals are among them. Species in which the same individual is able to form both male and female gametes are called bisexual or hermaphroditic. Such organisms include most angiosperms, coelenterates, flatworms and many annelids, some crustaceans and molluscs, and even certain species of fish and reptiles. Hermaphroditism implies the possibility of self-fertilization, which is very important for organisms leading a solitary lifestyle (for example, pork tapeworm in the human body). True, it should be noted that, if possible, hermaphrodites prefer to exchange germ cells with each other, carrying out cross-fertilization.

Rice. 56. Sexual dimorphism

In most species of angiosperms, the flower contains both stamens, which form male germ cells - sperm, and pistils, containing eggs.

However, in about a quarter of the species, male (staminate) and female (pistillate) flowers develop independently, i.e., same-sex flowers are formed. Examples of unisexual plants in which male and female flowers are formed on different individuals are sea buckthorn, willow, poplar. In some plants, such as oak, birch, hazel, both male and female flowers develop on the same individual.

The dioeciousness that arose in the process of evolution had clear advantages. It became possible to combine the genetic information of different individuals, forming new combinations and increasing the genetic diversity of the species, which contributed to its adaptation to changing habitat conditions. In addition, this made it possible to distribute functions between individuals of different sexes. Most organisms have sexual dimorphism- external differences between male and female individuals (Fig. 56).

Importance of asexual and sexual reproduction. Both asexual and sexual reproduction have a number of advantages. Sexual reproduction often involves wasting time and energy looking for a mate, or losing a huge amount of gametes, as occurs with cross-fertilization in plants (how much pollen is wasted!). With asexual reproduction, the continuation of the genus is easier and the number of individuals increases much faster, but all daughter individuals are the same and are a copy of the mother's organism. This can be an advantage if the species lives in constant environmental conditions. But for many species whose environments are changeable and fickle, asexual reproduction will not ensure survival. The amoeba reproduces only asexually, and, for example, mammals only reproduce sexually, and each is “suited” by its form of reproduction. What is good in one situation may be inappropriate in another situation, so many species have an alternation different forms reproduction, which allows them to optimally solve the problem of reproducing their own kind in various conditions a habitat.

Review questions and assignments

1. Prove that reproduction is one of the most important properties of wildlife.

2. What are the main types of reproduction you know?

3. What is asexual reproduction? What is the process behind it?

4. List the ways of asexual reproduction; give examples.

5. Is it possible for genetically heterogeneous offspring to appear during asexual reproduction?

6. How does sexual reproduction differ from asexual reproduction? Define the definition of sexual reproduction.

7. Think about the importance for the evolution of life on Earth was the emergence of sexual reproduction.

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reproduction- the property of living organisms to reproduce their own kind. There are two main breeding method- asexual and sexual.

Asexual reproduction is carried out with the participation of only one parent and occurs without the formation of gametes. The daughter generation in some species arises from one or a group of cells of the parent organism, in other species - in specialized organs. There are the following methods of asexual reproduction: fission, budding, fragmentation, polyembryony, spore formation, vegetative reproduction.

Division- a method of asexual reproduction, characteristic of unicellular organisms, in which the mother individual is divided into two or more daughter cells. We can distinguish: a) simple binary fission (prokaryotes), b) mitotic binary fission (protozoa, unicellular algae), c) multiple fission, or schizogony (malarial plasmodium, trypanosomes). During the division of paramecium (1), the micronucleus is divided by mitosis, the macronucleus by amitosis. During schizogony (2), the nucleus is first repeatedly divided by mitosis, then each of the daughter nuclei is surrounded by cytoplasm, and several independent organisms are formed.

budding- a method of asexual reproduction, in which new individuals are formed in the form of outgrowths on the body of the parent individual (3). Daughter individuals can separate from the mother and move on to an independent lifestyle (hydra, yeast), they can remain attached to it, in this case forming colonies (coral polyps).

Fragmentation(4) - a method of asexual reproduction, in which new individuals are formed from fragments (parts) into which the parent individual breaks up (annelids, sea ​​stars, spirogyra, elodea). Fragmentation is based on the ability of organisms to regenerate.

Polyembryony- a method of asexual reproduction, in which new individuals are formed from fragments (parts) into which the embryo breaks up (monozygous twins).

Vegetative reproduction- a method of asexual reproduction, in which new individuals are formed either from parts of the vegetative body of the mother individual, or from special structures (rhizome, tuber, etc.) specially designed for this form of reproduction. Vegetative propagation is characteristic of many groups of plants, it is used in horticulture, horticulture, plant breeding (artificial vegetative propagation).

Vegetative organ	Method of vegetative propagation	Examples
Root	root cuttings	Rosehip, raspberry, aspen, willow, dandelion
	Root offspring	Cherry, plum, thistle, thistle, lilac
Aerial parts of shoots	The division of the bushes	Phlox, daisy, primrose, rhubarb
	stem cuttings	Grapes, currants, gooseberries
	layering	Gooseberries, grapes, bird cherry
Underground parts of shoots	Rhizome	Asparagus, bamboo, iris, lily of the valley
	Tuber	Potato, weekday, Jerusalem artichoke
	Bulb	Onion, garlic, tulip, hyacinth
	Corm	Gladiolus, crocus
Sheet	leaf cuttings	Begonia, Gloxinia, Coleus

sporulation(6) - reproduction through spores. controversy- specialized cells, in most species are formed in special organs - sporangia. In higher plants, spore formation is preceded by meiosis.

Cloning- a set of methods used by humans to obtain genetically identical copies of cells or individuals. Clone- a set of cells or individuals descended from a common ancestor through asexual reproduction. Cloning is based on mitosis (in bacteria, simple division).

Sexual reproduction is carried out with the participation of two parent individuals (male and female), in which specialized cells are formed in special organs - gametes. The process of formation of gametes is called gametogenesis, the main stage of gametogenesis is meiosis. The daughter generation develops from zygotes- a cell formed as a result of the fusion of male and female gametes. The process of fusion of male and female gametes is called fertilization. An obligatory consequence of sexual reproduction is the recombination of genetic material in the daughter generation.

Depending on the structural features of gametes, the following can be distinguished forms of sexual reproduction: isogamy, heterogamy and ovogamy.

isogamy(1) - a form of sexual reproduction in which gametes (conditionally female and conditionally male) are mobile and have the same morphology and size.

Heterogamy(2) - a form of sexual reproduction in which female and male gametes are mobile, but female are larger than male and less mobile.

Ovogamy(3) - a form of sexual reproduction in which the female gametes are immobile and larger than the male gametes. In this case, the female gametes are called eggs, male gametes, if they have flagella, - spermatozoa if they don't have - sperm.

Ovogamy is characteristic of most animal and plant species. Isogamy and heterogamy are found in some primitive organisms (algae). In addition to the above, some algae and fungi have forms of reproduction in which germ cells are not formed: chologamy and conjugation. At chologamy unicellular haploid organisms merge with each other, which in this case act as gametes. The resulting diploid zygote then divides by meiosis to form four haploid organisms. At conjugations(4) the contents of individual haploid cells of the filamentous thalli are fused. Through specially formed channels, the contents of one cell flows into another, a diploid zygote is formed, which usually also divides by meiosis after a dormant period.

Go to lectures №13"Methods of division of eukaryotic cells: mitosis, meiosis, amitosis"

Go to lectures №15"Sexual reproduction in angiosperms"

Plant propagation. One of the obligatory properties of living organisms is the reproduction of offspring (reproduction). Reproduction is associated with the subsequent resettlement of plants. According to V. I. Vernadsky, reproduction and resettlement, that is, the spread of life, is the most important biological factor of our planet. During reproduction, the number of individuals of this species increases. The term "reproduction" reflects the qualitative aspect. The number of individuals as a result of reproduction can sometimes be reduced (diatoms).

Reproduction as a property of living matter, i.e. the ability of one individual to give rise to its own kind existed in the early stages of its development. The evolution of life went parallel to the evolution of the ways of reproduction.

Forms of plant reproduction can be divided into two types: asexual and sexual.

Actually asexual reproduction carried out by specialized cells dispute. They are formed in the organs of asexual reproduction - sporangia as a result of mitotic division. The spore, during its germination, reproduces a new individual, similar to the mother, with the exception of spores of seed plants, in which the spore has lost the function of reproduction and settlement.

asexual reproduction carried out without the participation of germ cells, with the help of spores that are formed in specialized organs - sporangia win zoosporangia. Inside the sporangium, a reduction division occurs and unicellular spores, or zoospores (with flagella), spill out. Most of the lower plants reproduce by spores (algae), of the higher spores - bryophytes, lycopsids, horsetails, ferns.

Reproduction of plants with the help of vegetative organs (part of a shoot, leaf, root) or division of unicellular algae in half, etc. called vegetative(Fig. 134). It is widely used in agriculture, especially when propagating varietal material, where it is necessary to preserve the maternal characteristics of the variety. So, many cultures reproduce well with the help of lignified and green cuttings (sea buckthorn, lemongrass, actinidia, blackcurrant, etc.), other fruit trees (apple, pear, cherry, apricot, etc.) - by grafting varietal cuttings into the crown of wild seedlings. bulbous plants propagated by bulbs (tulips, hyacinths, gladioli, etc.); many perennial herbaceous plants bred with rhizomes (lily of the valley, kupena, perennial lupine, asparagus, etc.), root tubers (dahlias, Jerusalem artichoke, etc.). Some plants reproduce with the help of shoots (chokeberry,

sea ​​buckthorn, common raspberry, etc.) or layering (garden strawberries, gooseberries, etc.).

sexual reproduction carried out by special sex cells - gametes. Gametes are formed as a result of meiosis, they are male and female. As a result of their merger, zygote from which a new organism develops. Plants differ in the types of gametes. In some unicellular organisms, for a certain period, it functions as a gamete. Diverse organisms (gametes) merge. This sexual process is called chologamia. If male and female gametes are morphologically similar, mobile, these are isogametes, and the sexual process is called isogamous(see fig. 160, B, 2). If the female gamete is somewhat larger and less mobile than the male gamete, then these are heterogametes, and the sexual process is called heterogamy(Fig. 160, B, 3). More perfect in terms of evolution oogamy(Fig. 160, B, 5), in which female gametes are rather large and immobile, while male gametes are small and mobile. The female gamete is called ovum, and the gametangy, in which the egg is formed, in the lower

plants (algae) is called oogonium, and for higher archegonium. Male gametes - spermatozoa- have flagella.

In most seed plants, the male gametes have lost their flagella and are called sperm. The gametangia in which spermatozoa are produced are called antheridia.

Most plants have all methods of reproduction, however, for many algae, higher spore and seed plants, alternation of asexual and sexual types of reproduction is characteristic. On the asexual generation sporophyte, or diplobionte(2l), as a result of maturation of spores, and then reduction division, disputes (n), and on the sexual generation - gametophyte- female and male gametes (n), which, when combined, form zygote(2l). A sporophyte (2l) will again grow out of it, i.e., the alternation of generations occurs with a change in nuclear phases.

Alternation of development phases. The alternation of development phases in different systematic groups of plants has been established. It was possible to find out the general pattern: the sporophyte develops better and becomes independent; the gametophase, on the contrary, is increasingly reduced and completely loses its independence and depends on the sporophyte (gymnosperms and angiosperms). In the evolution of sexual reproduction, the reduction of the gametophyte had a progressive significance, which led to the formation of new rudiments of reproduction and distribution - seeds and fruits.

The most primitive cycle of development in mosses. Only in them among the higher plants can one see a well-developed independent gametophyte (see Fig. 169).

In club mosses, horsetails, ferns, the sporophyte prevails in life expectancy, and the gametophyte is represented by a thallus (growth).

In these plants, the sexual process and the gametophase serve to reproduce the sporophase, and the sporophase, although not for long, is still dependent on the gametophase.

Greater adaptability to the conditions of terrestrial existence is associated with the life cycle of gymnosperms and angiosperms. Specificity life cycle gymnosperms is expressed in the structure of the ovule and its transformation into a seed. The megaspore of these plants has completely lost the function of the germ of reproduction and distribution. The male gametophyte (pollen) in the absence of an aquatic environment acquires a new meaning: with the help of a pollen tube, it delivers gametes to the egg. Male gametes - spermatozoa - are immobile. Thus, the change of generations of the sporophyte and gametophyte in gymnosperms differs significantly from the previous groups of plants, since the sexual generation - the male gametophyte (pollen grain) and the female gametophyte (primary endosperm) - in a significantly reduced state is enclosed in the tissues of the sporophyte and is completely dependent on it. . And

The life cycle of angiosperms differs significantly from the life cycle of previous plant groups. The female gametophyte of angiosperms is more strongly reduced than the gametophyte of gymnosperms. This is the embryo sac. Archegonia are absent. Fertilization is double (one sperm fertilizes the egg, the other - the secondary nucleus of the embryo sac). The endosperm is trishyoid.

Thus, in angiosperms, although there is a change of generations - sporophyte and gametophyte, however, male and female gametophytes are reduced even more - to a few cells located in the tissues of the sporophyte flower. The sporophyte, on the other hand, is ordinary trees, shrubs and herbs that are well known to us (Fig. 135).

Reproduction is a universal property of living things at the cellular level. The ability of organisms to produce their own kind is one of the basic properties of all living beings. The ability to reproduce is often evaluated as a hallmark of the living.

Somatic cells reproduce by mitosis, amitosis, and endomitosis.

Sex cells reproduce by meiosis.

Reproduction comes in two forms:

asexual;

Sexual.

Asexual reproduction is a form of reproduction that is not associated with the exchange of genetic information between individuals - the sexual process.

Asexual reproduction is the oldest and most in a simple way reproduction and is widespread in unicellular organisms (bacteria, blue-green bacteria, chlorella, amoeba, ciliates). This method has its advantages: there is no need to search for a partner, and beneficial hereditary changes remain almost forever. However, with this method of reproduction, the variability necessary for natural selection is achieved only through random mutations and therefore is carried out very slowly. However, it should be noted that the ability of a species to reproduce only asexually does not exclude the ability to have a sexual process, but only when these events are separated in time.

The most common way of reproduction of unicellular organisms is the division into two parts, with the formation of two separate individuals.

Among multicellular organisms, almost all plants and fungi have the ability to reproduce asexually - an exception is, for example, velvichia. Asexual reproduction of these organisms occurs vegetatively or by spores.

Among animals, the ability to reproduce asexually is more common in lower forms, but absent in more advanced ones. The only method of asexual reproduction in animals is vegetative.

It is a widely held misconception that individuals resulting from asexual reproduction are always genetically identical to the parent organism (except for mutations). The most striking counterexample is reproduction by spores in plants, since during sporulation, reduction cell division occurs, as a result of which spores contain only half of the genetic information available in sporophyte cells.

Sexual reproduction is associated with the sexual process (cell fusion), as well as the fact of the existence of two complementary sexual categories (male organisms and female organisms).

During sexual reproduction, the formation of gametes, or germ cells, occurs. Unlike ordinary cells, these cells have a haploid (single) set of chromosomes. According to the similarity-difference of the resulting gametes, several types of gamete formation are distinguished among themselves:

Isogamy is a sexual process in which merging gametes do not differ morphologically, that is, they are identical in structure. (algae, lower fungi);

Anisogamy - gametes different size, but of a similar structure, with flagella;

Oogamy - gametes of various sizes and structures. The small, flagellar male gametes are called spermatozoa, and the large, flagellaless female gametes are called eggs.

When two gametes merge (in the case of oogamy, a fusion of different types of gametes is necessary), a zygote is formed, which now has a diploid (double) set of chromosomes. A zygote develops into a daughter organism, the cells of which contain genetic information from both parent individuals.

An animal that has both male and female gonads is called a hermaphrodite. Hermaphroditism is widespread among lower animals and to a lesser extent among higher ones. A similar feature in plants is called monoeciousness (in contrast to dioeciousness) and is associated with the overall evolutionary advancement of the species to a lesser extent than in animals.

Types of asexual reproduction.

In unicellular organisms:

Division (mitosis in amoeba, bacteria);

Shizogony - multiple division (malarial plasmodium);

Sporulation (unicellular algae);

Budding (yeasts and yeast-like fungi).

In multicellular organisms:

Sporulation (mushrooms, mosses, ferns);

Vegetative reproduction, characteristic of plants, occurs due to vegetative organs (tubers, mustaches, leaf cuttings, stem cuttings, layering, etc.);

Fragmentation (jellyfish);

Budding (hydra);

Polyembryony (armadillos, monozygotic twins).

Division is characteristic primarily of unicellular organisms. As a rule, it is carried out by a simple cell division in two. Some protozoa (for example, foraminifera) divide into a larger number of cells. In all cases, the resulting cells are completely identical to the original. The extreme simplicity of this method of reproduction, associated with the relative simplicity of the organization of unicellular organisms, makes it possible to multiply very quickly. An asexually reproducing organism is capable of endlessly reproducing itself until a spontaneous change in the genetic material occurs - a mutation. If this mutation is favorable, it will be preserved in the progeny of the mutated cell, which will be a new cell clone.

Often asexual reproduction of bacteria is preceded by the formation of spores. Bacterial spores are dormant cells with reduced metabolism, surrounded by a multilayered membrane, resistant to desiccation and other adverse conditions that cause the death of ordinary cells. Sporulation serves both to survive such conditions and to spread bacteria: once in a suitable environment, the spore germinates, turning into a vegetative (dividing) cell.

Asexual reproduction with the help of unicellular spores is also characteristic of various fungi and algae. Spores in this case are formed by mitosis (mitospores), and sometimes (especially in fungi) in large quantities; when germinating, they reproduce the mother organism. In many organisms, as well as in all higher plants, spores of a different kind are formed, namely meiospores, which are formed by meiosis. They contain a haploid set of chromosomes and give rise to a generation that usually does not look like the mother and reproduces sexually. Thus, the formation of meiospores is associated with the alternation of generations - asexual (giving spores) and sexual.

Another variant of asexual reproduction is carried out by separating from the body of its part, consisting of a larger or smaller number of cells. They develop into adults. An example is budding in sponges and coelenterates or propagation of plants by shoots, cuttings, bulbs or tubers. This form of asexual reproduction is commonly referred to as vegetative reproduction. Basically, it is similar to the process of regeneration.

Asexual reproduction, reproducing individuals identical to the original organism, does not contribute to the emergence of organisms with new variants of traits, and thereby limits the ability of species to adapt to new environmental conditions. The means of overcoming this limitation was the transition to sexual reproduction.

The effectiveness of combining genetic material in offspring obtained as a result of sexual reproduction is facilitated by:

Chance meeting of two gametes;

Random arrangement and divergence to the poles of the division of homologous chromosomes during meiosis;

Crossing over between chromatids.

Such a form of sexual reproduction as parthenogenesis does not involve the fusion of gametes. But since the organism develops from a germ cell (oocyte), parthenogenesis is still considered sexual reproduction.

In many groups of eukaryotes, the secondary extinction of sexual reproduction has occurred, or it occurs very rarely. In particular, the department of deuteromycetes (fungi) combines an extensive group of phylogenetic ascomycetes and basidiomycetes that have lost their sexual process. Until 1888, it was assumed that among the terrestrial higher plants, sexual reproduction was completely lost in sugar cane. The loss of sexual reproduction in any group of metazoans has not been described. However, many species are known (lower crustaceans - daphnia, some types of worms) that, under favorable conditions, can reproduce parthenogenetically for tens and hundreds of generations. For example, some species of rotifers reproduce only parthenogenetically for millions of years, even forming new species in the process.

In a number of polyploid organisms with an odd number of sets of chromosomes, sexual reproduction plays a small role in maintaining genetic variability in the population due to the formation of unbalanced sets of chromosomes in gametes and in offspring.

The ability to combine genetic material during sexual reproduction has great importance for selection of model and economically important organisms.

The biological significance of asexual reproduction: provides rapid reproduction, most effective under constant environmental conditions.

The biological significance of sexual reproduction: increases the genetic diversity of individuals within a species, increases the evolutionary potential of the species, increases the likelihood of survival under changing environmental conditions.

During sexual reproduction, two parent individuals produce female and male gametes (), which merge to form a new organism.

The development of the female gamete in some species of animals can occur without fertilization (see) - parthenogenesis. The phenomenon of parthenogenesis is observed in invertebrates (bees, wasps, etc.) and vertebrates (amphibians). Each type of breeding has its own advantages and special meaning in the living world. With asexual reproduction, the heredity (see) of subsequent generations does not change, but the total number of individuals increases (which in some cases is very important). During sexual reproduction, on the contrary, the probability of the appearance of hereditary variability increases (see), since during fertilization, the germ cells of organisms with different heredity merge.

Reproduction (synonym: self-reproduction, generative process) is the process of reproduction by organisms of their own kind. Reproduction is based on cell division, accompanied by self-reproduction of cell organelles, each of which arises only from its own kind and, if lost, cannot be restored. These organoids include chromosomes (see), algal chromatophores, and also, probably, mitochondria and plastids. Chromatophores, plastids and mitochondria reproduce by division. When chromosomes self-reproduce on the maternal chromosome, as on a matrix, a daughter one is built, in the DNA of which the same regular alternation of nucleotides is preserved as in the maternal DNA strand (see Genetics). Thus, during reproduction, the material continuity of generations is preserved, which consists in the preservation of the molecular structures characteristic of the mother's organism.

asexual reproduction is the growth of the daughter body outside the parent body. It can be carried out both by individual cells (agamic cytogony) and by multicellular formations (vegetative reproduction). Agamous cytogony occurs either by cell division into two equal parts, as in many unicellular organisms, or by the formation of a smaller daughter cell by the mother cell (budding, for example, in yeast), or the mother body separates special cells or breaks up into cells that serve for reproduction (spores) . There may be special organs of vegetative reproduction - bulbs and tubers.

sexual reproduction consists in the fact that a number of cell divisions are interrupted by the sexual process. When this occurs, the fusion of two cells or two nuclei, followed sooner or later by meiosis (see). The product of the sexual process - the zygote - contains twice as many chromosomes as gametes, or germ cells. In haplobionts (many algae and fungi), meiosis occurs during the germination of zygotes and all body cells have one set of chromosomes. In diplobionts (all animals, some algae) body cells have two sets of chromosomes and meiosis occurs when germ cells mature. In haplo-diplobionts (most plants), generations alternate between diploid, producing spores as a result of meiosis, and haploid, sexually reproducing. In ciliates, the sexual process is reduced to the exchange of nuclei that have previously undergone meiosis; after the fusion of these nuclei during the conjugation of ciliates, a double set of chromosomes is restored.

In the process of homogony, or gametogenesis, i.e., cell division leading to the formation of gametes, the latter arise from mother cells called gametocytes. Isogametes - germ cells, the same in both sexes; anisogametes differ in size and behavior. The sexual process often occurs between organisms of different origin (amphimixis), sometimes between organisms originating from the same cell (automixis), between two sister cells (pedogamy), and even between two nuclei within the same cell (autogamy). Female reproductive cells can sometimes give an organism even without fertilization (see) (parthenogenesis, or virgin reproduction). In many insects, some vertebrates, such as lizards and turkeys, in many flowering plants, parthenogenesis is a natural phenomenon; in some species it is the rule, in others it is the exception.

Artificial parthenogenesis can be induced by a number of influences, such as the prick of a needle.

In some cases, sexual and asexual reproduction occur at the same time. In other cases, they naturally alternate. In haplo-diplobionts, the alternation of generations is called antithetic. With it, the asexual and sexual generations differ in the number of chromosomes. Such a difference in there is no number of chromosomes when alternating vegetative and sexual reproduction - metagenesis (for example, in hydromedusa), as well as when alternating sexual reproduction and parthenogenetic, called heterogony (for example, in aphids).