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Sexual reproduction

The ability to reproduce or self-reproduce is one of the mandatory and the most important properties living organisms. Reproduction supports the long-term existence of the species and ensures continuity between parents and their descendants over many generations. It leads to an increase in the number of individuals of the species and contributes to its dispersal.

There are two types of reproduction: asexual and sexual

Asexual reproduction involves only one parent, which divides, buds, or produces spores. With asexual reproduction, the organism arises from somatic cells and random mutations can be the source of variability.

In the case of sexual reproduction, individuals of a new generation appear with the participation of two organisms: maternal and paternal. A new organism arises from specialized germ cells or individuals that perform these functions.

The advantage of sexual reproduction (evolutionarily it appeared later than asexual reproduction) is the recombination of hereditary characteristics of both parents, which is a source of variability. The offspring are more viable and adapted to living conditions. Evolution happens faster.

The classification of forms of reproduction is based on the type of cell division:
Asexual - due to mitotic division, in plants rarely due to meiotic division.
Sexual - due to meiotic division.

Scheme of forms of reproduction of organisms:

Asexual reproduction in multicellular organisms

1. Vegetative– based on the ability of organisms to restore (regenerate) missing parts. In plants, a variety of this form of reproduction is observed: it occurs through the formation of new buds on stems, roots, leaves, from which new plants grow. They can exist independently, without connection with the mother's body. For example, in multicellular algae, fungi, and lichens, reproduction is carried out by fragments of filaments, hyphae, and fragments of thalli. Angiosperms can reproduce: parts of the stem (cacti, elodea), leaves (violet, begonia, lily), roots (raspberries, gooseberries, dandelions), modified shoots: tubers (potatoes), bulbs (onion, garlic, tulip, daffodil), rhizomes (wheatgrass, horsetail, fireweed), mustache (strawberry), etc. In animals, due to the high specialization of body cells, vegetative reproduction is less common. Ciliated and annelid worms are divided by constrictions into several parts, in each of them the missing organs are restored and, thus, several individuals appear at once. In coelenterates, polyps begin to grow rapidly, transverse constrictions are formed, as a result of which daughter individuals are formed, and this method is called strobilation. At this point, the polyp resembles a stack of plates. The resulting individuals - jellyfish - break away and begin an independent life. In some species of mammals (armadillo) and insects (ichneumon wasps), vegetative reproduction of embryos occurs when early stages embryonic development, the dividing germ disk gives rise to several individuals (from 4 to 8). A similar thing can be observed in humans when blastomeres separate from which monozygotic twins will develop (such an increase in the number of individuals is called polyembryony.)
2. Budding characteristic of coelenterates (hydra). The bud (protrusion) contains ecto- and endoderm cells. The bud enlarges, tentacles form on it, and this bud is separated from the mother.
3. Reproduction by fragments - (fragmentation) occurs when an individual splits into two or larger number parts, each of which grows and forms a new individual. Regeneration is associated with fragmentation, i.e. the ability to restore the whole organism. Fragmentation has been described for flatworms, nemerteans and starfish.
4. Sporulation found in fungi, algae, mosses, mosses, horsetails and ferns. Spores are formed by meiosis in ordinary vegetative cells of the mother's body or special organs - sporangia and are microscopic single-celled formations. With any form of asexual reproduction - by body parts or spores - there is an increase in the number of individuals of a given species without increasing their genetic diversity: all individuals are an exact copy of the maternal organism. A set of individuals descended from one ancestor through asexual reproduction is called clones (Greek clon - branch, offspring).

Sexual reproduction

Sexual reproduction has very great evolutionary advantages compared to asexual reproduction. This is due to the fact that the genotype of the offspring arises from a combination of genes belonging to both parents. As a result, the ability of organisms to adapt to conditions increases environment. The sexual process consists of the fusion of two cells - gametes. Gamete formation is preceded by special shape fission - meiosis, which leads to a reduction in the number of chromosomes by half.

Sexual reproduction in unicellular animals

1. Conjugation– when special reproductive cells (sexual individuals) are not formed. For example: a) in ciliates - two individuals come together in pairs, a protoplasmic bridge is formed between them, along which micronuclei are exchanged. Then the individuals disperse and remain independent, but thanks to new hereditary information, new characteristics appear; b) in bacteria - individuals with different physiological signs come closer together, and parts of DNA pass from one individual to another. This leads to combinative variability; c) in filamentous algae (spirogyra) - two threads come closer together, a bridge is formed along which hereditary information is exchanged.
2. Copulation is a sexual process in unicellular organisms, in which two individuals acquire sexual differences, i.e. turn into gametes and completely fuse to form a zygote. In the process of evolution, a mechanism for differences in the structure of gametes is formed. At the first stage of sexual reproduction, gametes are not yet morphologically different - isogamy (gr. Isos - equal, gamos - marriage), i.e. both gametes are small and both are mobile. For example, such reproduction occurs in rhizomes, flagellates, and algae (Chlamydomonas). In further evolution, gametes differentiate into small (male) and large (female), but both still retain mobility, i.e. anisogamy (Greek anisos – unequal, gamos – marriage). For example, such reproduction occurs in the colonial flagellated organism Pandorina. The final path of evolution is Oogamy - when a large (female) cell loses its mobility, and a small (male) cell loses its mobility. For example, a Volvox colony from the class Flagellates.

Sexual reproduction in multicellular animals

Sexual dimorphism

These are differences between males and females in body structure, coloring, instincts and a number of other characteristics. Sexual dimorphism appears already at the early stages of evolution.
For example, at roundworms- females are larger, the male has a curved end of the body, in arthropods females and males differ in size and color, in fish - in size and body structure. In newts, males during the mating season have a brightly colored abdomen and a crest on their back. In birds, males are brightly colored during the mating season.

In humans, women and men differ in such characteristics as: height, massiveness of skeletal bones, massiveness of muscles, size of the skull (larger in men) and the ratio of the facial and brain parts, width of the pelvis and shoulders, facial hair, low timbre of voice protruding forward thyroid cartilage of the larynx (Adam's apple), development mammary glands, development of subcutaneous fat tissue.

The content of the article

REPRODUCTION, or reproduction, the function inherent in all living beings of reproducing their own kind. Unlike all other vital important functions organism, reproduction is not aimed at maintaining the life of an individual, but at preserving its genes in the offspring and procreation - thereby preserving the gene pool of a population, species, family, etc. During the course of evolution different groups organisms have evolved - in many cases independently - different ways and strategies of reproduction, and the fact that these groups have survived and exist proves the effectiveness of different ways of carrying out this process.

All the variety methods of reproduction can be divided into two main types: asexual (its version is vegetative) reproduction and sexual reproduction.

Asexual Reproduction

Asexual reproduction occurs by simply dividing a cell in two. It is characteristic primarily of unicellular organisms. In some protozoa (for example, foraminifera), division occurs into a larger number of cells. In all cases, the resulting cells are completely identical to the original one. The extreme simplicity of this method of reproduction, associated with the relative simplicity of the organization of single-celled organisms, allows reproduction very quickly. Thus, under favorable conditions, the number of bacteria can double every 30–60 minutes. An asexually reproducing organism is capable of endlessly reproducing itself until a spontaneous change in the genetic material occurs—mutation. If this mutation is favorable, it will persist in the progeny of the mutated cell, which will represent a new cell clone.

Asexual reproduction of bacteria is often preceded by the formation of spores. Bacterial spores are resting cells with reduced metabolism, surrounded by a multilayered membrane, resistant to desiccation and other unfavorable 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. In this case, spores are formed by mitosis (mitospores), and sometimes (especially in fungi) in huge quantities; upon germination, they reproduce the mother's organism. Some fungi, such as the harmful plant pest Phytophthora, form motile spores equipped with flagella, called zoospores or wanderers. After floating in droplets of moisture for some time, such a wanderer “calms down”, loses its flagella, becomes covered with a dense shell and then, under favorable conditions, germinates. In addition to mitospores, many of these organisms, as well as all higher plants, form spores of another kind, namely meiospores, formed by meiosis. They contain a haploid set of chromosomes and give rise to a generation that is usually not similar to the maternal one and reproduces sexually. Thus, the formation of meiospores is associated with the alternation of generations - asexual (producing spores) and sexual. MUSHROOMS.

Another option for asexual reproduction is carried out by separating from the body a part of it, consisting of a larger or smaller number of cells. From them the adult organism develops. An example is budding in sponges and coelenterates or the propagation of plants by shoots, bulbs or tubers. This form of asexual reproduction is usually called vegetative reproduction. It is fundamentally similar to the regeneration process.

Vegetative propagation plays important role in plant growing practice. Thus, it may happen that a sown plant (for example, an apple tree) has some successful combination of characteristics. In the seeds of a given plant, this successful combination will almost certainly be disrupted, since seeds are formed as a result of sexual reproduction, and this is associated with gene recombination. Therefore, when growing apple trees, vegetative propagation is usually used - by layering, cuttings or grafting buds onto other trees.

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

SEXUAL REPRODUCTION

The fundamental difference between sexual and asexual reproduction is that it usually involves two parent organisms, the characteristics of which are recombined in the offspring. Sexual reproduction is characteristic of all eukaryotes, but it predominates in animals and higher plants.

The transition to this type of reproduction was of great importance for the evolution of life on Earth. Sexual reproduction creates an infinite variety of individuals, including those that successfully adapt to changing external conditions, “conquer the world”, spreading to new habitats, and leave offspring, passing on their hereditary material. The offspring of two successful parents may turn out to have an even more successful combination of hereditary traits, and accordingly they will develop the success of their parents. Individuals with an unsuccessful combination of traits will be eliminated by natural selection. Thus, sexual reproduction creates rich material for natural selection and evolution. Another thing is curious: the very emergence of an individual as an individual, an indivisible and mortal being, is the result of the transition to sexual reproduction. During asexual reproduction, the cell divides endlessly, repeating itself: it is potentially immortal, but it can only be called an individual conditionally, since it is indistinguishable from an indefinite set of daughter cells. With sexual reproduction, on the contrary, all descendants differ from each other and differ from their parents, and they die over time, taking with them their unique characteristics. The American zoologist R. Hegner, discussing protozoa, expressed it this way: “They acquired another innovation - sex; the price of this acquisition is inevitable natural death... Isn’t this price too high?” Let us emphasize, however, that at the same time opportunities for development and improvement opened up, and they led to the emergence of various living forms that are not comparable in level of organization with those organisms that settled on asexual reproduction.

SEXUAL PROCESS

Many organisms that reproduce asexually have nevertheless developed a number of ways in which they occasionally exchange genetic material between two cells of the same species. This exchange is called the sexual process. In most forms it is carried out by conjugation (connection). A classic example of conjugation is demonstrated by ciliates. Their two individuals are temporarily connected by their mouthparts, and a cytoplasmic bridge is formed between them, through which nuclear material is exchanged. This exchange is preceded by meiotic division of the nucleus (micronucleus). Once the exchange is complete, the cells disperse and then multiply by fission (mitosis).

In some bacteria, during conjugation, a unidirectional transfer of a linear sequence of chromosome genes occurs from a “male” cell (donor) to a “female” cell (recipient), and the size of the transferred fragment usually depends on the time of contact of the cells.

Thus, the sexual process does not come down to reproduction, but to the creation of new combinations of genes in the cell; Reproduction itself occurs asexually.

SEXUAL REPRODUCTION OF ANIMALS

The transition to sexual reproduction is associated with the appearance of specialized germ cells - male and female gametes, as a result of the fusion of which (fertilization) a zygote is formed - a cell from which a new organism develops, possessing new combination original genetic characteristics.

Sexual reproduction first appeared in protozoa, but the transition to it was not associated with an immediate loss of the ability to reproduce asexually: a number of animals retained it, usually alternating asexual reproduction with sexual reproduction. This alternation of generations is observed in some protozoa, coelenterates and tunicates.

Gametes and gonads.

The basis for the formation of gametes (gametogenesis) is meiosis - cell division with a halving of the number of chromosomes, as a result of which gametes, unlike all other cells of the body, are haploid EMBRYOLOGY). The fusion of gametes restores the number of chromosomes in the zygote to diploid. Subsequent division of the zygote occurs through mitosis. Note that in all multicellular organisms, the division of all body cells, except sex cells, occurs through mitosis. Consequently, asexual reproduction of cells through fission in two has been preserved in evolution as the main mechanism of growth and development of the organism, but not its reproduction.

In many protozoa, sexual reproduction occurs with the participation of morphologically identical male and female gametes (in foraminifera, for example, they are represented by very small cells formed in the haploid parent cell in a cycle of alternating generations). This phenomenon is called isogamy. It is characteristic only of unicellular organisms.

However, already in some protozoa, for example, sporozoans, and in all multicellular organisms, differentiation of gametes occurred: they began to differ in form and function - heterogamy arose, i.e. division of germ cells into eggs (female gametes) and sperm (male gametes).

Most animals are characterized by the so-called oogamy: a large immobile ovum (egg) and a small motile sperm, due to active movements which comes into contact with the egg, leading to fertilization.

In sponges and some ciliated worms, sex cells are scattered in the body and are excreted through breaks in the body wall or through the mouth, but many flatworms (and in rudimentary form, hydra) have gonads - special glands, producing gametes. Male gonads are testes, female gonads are ovaries. True, in such hermaphroditic animals as gastropods, male and female reproductive cells mature in the same gonad, but usually in different time, so the gonad functions either as a testis or as an ovary, and self-fertilization does not occur. In other hermaphroditic animals, such as flatworms or leeches, one individual contains both ovaries and testes; however, even in the case of simultaneous maturation of eggs and sperm, the animal avoids self-fertilization and usually mates with another individual (the exception is, for example, tapeworms, which live alone in the intestine). Hermaphroditism is most common in worms and mollusks and is rare in more highly organized forms - echinoderms, arthropods and vertebrates; on the other hand, it is quite rare in such ancient multicellular organisms as coelenterates and in particular jellyfish.

Already in some worms and mollusks, in addition to the gonads, reproductive ducts have formed - the vas deferens and the oviducts. Gonads and genital ducts constitute the main functional parts of the internal genital organs, and they are present in all more highly organized animals.

Insemination.

The genital organs ensure the production and release of germ cells, and thereby insemination, i.e. bringing together the eggs and sperm of two individuals. The process of insemination precedes fertilization - the fusion of gametes. There are two methods of insemination (and, accordingly, fertilization): external and internal. In external insemination, eggs and sperm are released into the water, where the sperm, by actively swimming, can connect with the egg and produce fertilization. It is clear that this method can only be characteristic of aquatic (or, like amphibians, those that have retained a connection with the aquatic environment) animals, and indeed, it is observed in most of them. External insemination is usually not associated with a complex structure of the reproductive system, although some animals develop additional adaptations, for example, for the coupling of two individuals during the release of reproductive products.

Greater independence from external factors(in particular, from the aquatic environment) and more economical production of gametes is provided by another method of insemination - internal, in which sperm are introduced directly into the female genital tract. There is also a known option for internal insemination using spermatophores - capsules filled with sperm. This type of insemination is sometimes called external-internal. In a salamander, for example, the female captures the spermatophore secreted by the male with her cloaca, into which the genital ducts open; the males of many arachnids, with the help of their claw-shaped chelicerae (the first pair of head limbs), transfer the spermatophore directly into the genital opening of the female; the male cephalopod captures the spermatophore with a special modified tentacle and transfers it to the mantle cavity of the female. But in any case, fertilization occurs inside the female’s body, usually in the oviducts. Fertilized eggs are laid in external environment(in most species) or develop in utero. Internal insemination is characteristic of a number of aquatic animals and all terrestrial ones. It appeared already at a very early stage of evolution, namely in flatworms.

Complication of the reproductive system.

The transition to internal insemination and fertilization was accompanied by a complication of the reproductive system and the formation of additional genital organs. Thus, glands were formed, for example, secreting fluid in which sperm are located and which they need for movement, or, in females, forming outer shell eggs Flatworms and a number of other animals, especially insects, have developed spermatic receptacles to store sperm supplied during insemination. Since sperm can remain viable for a long time, the presence of sperm receptacles makes fertilization less dependent on the meeting of partners: many insects reproduce successfully by mating only once in their lives. Accordingly, the time between mating and egg laying can vary widely.

The females of a number of insects (dragonflies, cicadas, grasshoppers, ichneumon fly, etc.) have developed an additional sexual organ called an ovipositor, which serves for laying eggs in cells, the ground, or in the tissue of plants or animals.

Copulative (copulating) organs also arose as a device for internal insemination. They were formed in different groups of animals in different ways: many of them have lower section the reproductive duct, but, for example, in crustaceans - by modifying one pair of legs, in flies and other dipterous insects - from the terminal segments of the abdomen, in viviparous fish - from outgrowths of the fins. However, a number of animals, for example many birds, do not have special copulatory organs.

If in some oviparous animals the apparatus for laying eggs was improved, then in animals that switched to viviparity, primarily in mammals, other changes in the reproductive system occurred; the most significant of them is the transformation of the middle section of the oviduct into the uterus, where the embryo develops.

Pairing.

One of the conditions for successful reproduction is the simultaneous maturation of gametes in male and female individuals. Some animals are able to reproduce all year round, but for many, especially those living in middle and high latitudes, reproduction is seasonal. In this case, the onset of the breeding season depends on external factors: length of daylight hours, air temperature, availability of food, etc. The effect of these factors on the reproductive system, as a rule, is not direct, but indirect – most often by hormones that regulate the functional activity of the gonads and/or the level of metabolism. Thus, in vertebrates with seasonal reproduction, changes in illumination affect the secretion of pituitary hormones, which “turn on” the function of the gonads, and thereby determine the timing of reproduction.

However, these physiological mechanisms may not be sufficient to ensure mating. Sexual selection of the strongest and fittest individuals, usually males, who are able to attract a female and defend their right to reproduce, often comes into play. Tournament fights between males, courtship before mating, protection of their breeding territory, as well as, apparently, the mating attire of males are all means of achieving success in the reproduction of the most viable individuals. Sexual behavior reaches great complexity in highly organized animals with their developed neuroendocrine system

Most animals do not form permanent pairs, and the problem of finding a partner for mating arises regularly. However, among birds and mammals there are monogamous species, i.e. forming strong pairs (for example, wolves, swans, parrots). Examples of polygamy are also known; Thus, fur seals, seals, some other mammals and birds create a stable group of one male, stronger than his competitors, and a whole harem of females.

Methods of reproduction.

Different groups of animals have developed not only different ways fertilization; They produce offspring differently. Depending on how this happens, there are three methods of reproduction.

Oviparity.

The vast majority of animal species lay eggs from which young hatch. Such animals are called oviparous or oviparous. These include almost all marine invertebrates, insects, many fish, amphibians, reptiles, birds and monotreme mammals.

Viviparity.

In viviparous animals, the fertilized egg develops in the body of the female, receiving nutrition from her until the birth of the baby. All mammals are viviparous except for monotremes - the platypus and the echidna. Viviparity also occurs in other groups, for example, in some reptiles and in more primitive animals.

Ovoviviparity.

There is also an intermediate form of reproduction: the egg develops while continuing to remain in the female’s body, but the nutrition of the embryo is provided by the yolk of the egg, and not by the mother’s body. Ovoviviparity is characteristic of some sharks and other fish, a number of amphibians, and many lizards and snakes.

Reproduction strategies.

These methods of reproduction are also associated with different strategies.

At one pole is the strategy of economical reproduction, characterized by the slow reproduction of small offspring and their careful rearing (feeding, caring, protecting, teaching); on the other - wasteful, excessive reproduction with the production of a very large number of eggs and the reproduction of numerous offspring in the absence of care for them. If in the first case the probability of survival of the offspring is very significant, then in the second the chances of preserving eggs and surviving each individual offspring are extremely small, so that only intensive reproduction can compensate for the high percentage of death of eggs and young at all stages of development. The first strategy is generally characteristic of highly organized animals - mammals and birds. However, within these groups the basic strategy can be expressed in varying degrees. For example, in primates it manifests itself in to the greatest extent: they bear the fetus for a long time and usually give birth to one, still very helpless, baby, which they feed and raise for a long time. On the other hand, mice, rats or rabbits can breed several times a year, giving birth to up to a dozen babies each time, which quickly become independent. As a result of such intensive reproduction, the probability of death of offspring increases due to lack of food or due to rapid growth populations - due to the spread of diseases and the proliferation of predators. Thus, when comparing rodents with primates, their reproductive strategy should be considered uneconomical. Nevertheless, the extravagance of energy during the reproduction of rodents is not comparable to what is observed in various species of oviparous fish, for example, fish, many of which lay hundreds of thousands and millions of eggs.

Many animals are concerned about the safety of their eggs: some lay them in silt, soil and various secluded places, others (in particular, some crustaceans and brittle stars, among fish the pipefish and seahorse, among amphibians midwife toads and pipas) carry eggs on themselves , and the number of eggs in this case is much less than when sweeping them into water. Ovoviviparous animals took this strategy even further.

Social insects, such as ants and social bees, have chosen a unique reproductive strategy. They build nests, protect eggs and provide food for larvae, but leave the reproductive function to only one (in bees) or several (in ants) females in the community. The breeding female, called the queen or queen, lays numerous eggs. Males appear only on a short time and die after mating.

Parthenogenesis.

The eggs of some organisms are able to develop without fertilization, i.e. without the participation of sperm. This process of same-sex reproduction is called parthenogenesis, or virgin reproduction. It is considered as a reduced form of sexual reproduction.

There are no known examples of natural parthenogenesis in mammals; they occur occasionally in lower vertebrates and are quite common in invertebrates, especially insects. There are two types of parthenogenesis: obligate (i.e. obligatory) and facultative. The first is characteristic of species in which males either do not exist at all, or they are rare and are not able to function. These species include some aphids, stick insects, crickets, butterflies; Maleless populations are occasionally found in fish such as goldfish. With facultative parthenogenesis, eggs can develop both parthenogenetically and as a result of fertilization, and parthenogenetic reproduction can predominate in conditions where contacts of opposite-sex individuals are too rare, for example, at the border of the species' distribution area.

Cyclic parthenogenesis is also known, in which reproduction involving both sexes alternates with parthenogenetic. For example, many species of aphids produce several parthenogenetic generations during a short warm period of summer, and in the winter they lay fertilized eggs, which are covered with a dense shell and are able to overwinter; In the spring, only females emerge from them, but in the fall a generation appears with a certain number of males - and the cycle resumes. Some other species with high seasonal mortality, such as rotifers, reproduce similarly. Cyclic parthenogenesis is also observed in species with larval reproduction; in this case, fertilized eggs are usually laid only by mature individuals, and in larvae they develop parthenogenetically.

Harmful changes are also fixed for a long period of time. In addition, in unfavorable, changing environmental conditions, almost all individuals will die, since on average they are almost identical to one parent individual. It should be noted that the ability of a species to reproduce asexually does not exclude the ability to undergo the sexual process, but then these events are separated in time.

The most common method of reproduction of single-celled organisms is by dividing into two parts, forming two separate individuals.

Amitosis

Mitosis is one of the fundamental processes of ontogenesis. Mitotic division ensures the growth of multicellular eukaryotes by increasing tissue cell populations. As a result of mitotic division of meristem cells, the number of plant tissue cells increases. The fragmentation of a fertilized egg and the growth of most tissues in animals also occurs through mitotic divisions.

Based morphological features Mitosis is conventionally divided into stages: prophase, prometaphase, metaphase, anaphase, telophase. The first descriptions of the phases of mitosis and the establishment of their sequence were undertaken in the 70-80s of the 19th century. In the late 1870s and early 1880s, German histologist Walter Flemming coined the term “mitosis” to refer to the process of indirect cell division.

The average duration of mitosis is 1-2 hours. Mitosis of animal cells, as a rule, lasts 30-60 minutes, and plants - 2-3 hours. Over the course of 70 years, a total of about 10 14 cell divisions occur in the human body.

Meiosis

With a decrease in the number of chromosomes as a result of meiosis, a transition from the diploid phase to the haploid phase occurs in the life cycle. Restoration of ploidy (transition from the haploid phase to the diploid phase) occurs as a result of the sexual process.

Due to the fact that in the prophase of the first, reduction stage, pairwise fusion (conjugation) of homologous chromosomes occurs, the correct course of meiosis is possible only in diploid cells or in even polyploids (tetra-, hexaploid, etc. cells). Meiosis can also occur in odd polyploids (tri-, pentaploid, etc. cells), but in them, due to the inability to ensure pairwise fusion of chromosomes in prophase I, chromosome divergence occurs with disturbances that jeopardize the viability of the cell or developing from it a multicellular haploid organism.

The same mechanism underlies the sterility of interspecific hybrids. Since interspecific hybrids combine in the cell nucleus the chromosomes of parents belonging to various types, chromosomes usually cannot enter into conjugation. This leads to disturbances in chromosome segregation during meiosis and, ultimately, to the non-viability of germ cells, or gametes. Certain restrictions on the conjugation of chromosomes are also imposed by chromosomal rearrangements (large-scale deletions, duplications, inversions or translocations).

Reproduction by spores

Asexual reproduction with the help of unicellular spores is also characteristic of various fungi and algae. Spores in many cases are formed by mitosis (mitospores), and sometimes (especially in fungi) in huge quantities; upon germination, they reproduce the mother's organism. Some fungi, such as the harmful plant pest Phytophthora, form motile spores equipped with flagella, called zoospores or wanderers. After floating in droplets of moisture for some time, such a wanderer “calms down”, loses its flagella, becomes covered with a dense shell and then, under favorable conditions, germinates.

Vegetative propagation

In some plants, shoots (in willows) or leaves separated from the mother plant may take root. In animals, vegetative reproduction (often called asexual by zoologists) occurs either by division or by budding.

Vegetative propagation is based on processes similar to regeneration processes; As a rule, in the absence of the ability to regenerate in a given group of organisms (for example, rotifers, nematodes, leeches), vegetative reproduction is also absent, and in the presence of a developed regenerative ability (annelis, hydroids, flatworms, echinoderms), vegetative reproduction also occurs.

Budding

Some species of unicellular organisms are characterized by a form of asexual reproduction called budding. In this case, mitotic nuclear division occurs. One of the resulting nuclei moves into the emerging local protrusion of the mother cell, and then this fragment buds off. The daughter cell is significantly smaller than the mother cell, and it takes some time for it to grow and complete the missing structures, after which it takes on the appearance characteristic of a mature organism. Budding is a type of vegetative propagation. Many lower fungi, such as yeast and even multicellular animals, such as freshwater hydra, reproduce by budding. When yeast budding, a thickening forms on the cell, which gradually turns into a full-fledged daughter yeast cell. On the hydra’s body, several cells begin to divide, and gradually a small hydra grows on the mother individual, which forms a mouth with tentacles and an intestinal cavity connected to the intestinal cavity of the “mother.”

Fragmentation (body division)

Some organisms can reproduce by dividing the body into several parts, and from each part a full-fledged organism grows, similar in all respects to the parent individual (flatworms, annelids, echinoderms).

Sexual reproduction

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

During sexual reproduction, gametes, or sex cells, are formed. These cells have a haploid (single) set of chromosomes. Animals are characterized by a double set of chromosomes in ordinary (somatic) cells, therefore gamete formation in animals occurs during the process of meiosis. In many algae and all higher plants, gametes develop in the gametophyte, which already has a single set of chromosomes, and are obtained by simple mitotic division.

Based on the similarities and differences between the resulting gametes, several types of gamete formation are distinguished:

isogamy - gametes of the same size and structure, with flagella
anisogamy - gametes of different sizes, but similar structure, with flagella
oogamy - gametes of different sizes and structures. Small male gametes with flagella are called sperm, and large female gametes without flagella are called eggs.

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

Hermaphroditism

An animal that has both male and female gonads is called a hermaphrodite (from the name of Hermaphrodite - a mythical bisexual creature). Hermaphroditism is widespread among lower animals and to a lesser extent among higher ones. A similar trait in plants is called monoecy (as opposed to dioecy) and is associated with the general evolutionary advancement of the species to a lesser extent than in animals.

Parthenogenesis and apomixis

Parthenogenesis, which is a sexual but unisexual type of reproduction, arose during the process of evolution in dioecious organisms. In cases where some species are represented only by females (always or periodically), one of the main biological advantages of parthenogenesis is to accelerate the rate of reproduction of the species. Parthenogenetically reproducing unisexual taxa often occupy the periphery of species' ranges, where hybridization and competition with bisexual populations does not prevent the establishment and spread of unisexual (female) populations. Parthenogenesis has been described for aphids, daphnia, lizards, some fish and other animals. Parthenogenesis does not occur in mammals, in which parthenogenetic embryos die in the early stages of embryogenesis.

Progenesis

Progenesis is gametogenesis during the larval stage. It is divided into:

In many algae, in all higher plants, in some protozoa and coelenterates life cycle There is an alternation of generations that reproduce sexually and asexually, respectively - metagenesis. In some worms and insects, heterogony is observed - alternation of different sexual generations, for example, alternation of dioecious generations with hermaphrodite, or with reproducing ones

4. Forms of reproduction of organisms

The succession of generations of organisms in nature is carried out through reproduction. Reproduction- This is the ability of an organism to reproduce its own kind. In nature, there are two types of reproduction: asexual and sexual.

Types of asexual reproduction

Asexual reproduction- the formation of a new organism from one cell or group of cells of the original maternal organism. In this case, only one parent individual participates in reproduction, which passes on its hereditary information to its daughter individuals. Asexual reproduction produces identical offspring. The only source of variability is random hereditary changes that can arise during the process of individual development.

Asexual reproduction is based on mitosis. There are several types of asexual reproduction.

Asexual reproduction in bacteria is interesting (Fig. 7).

Rice. 7. Asexual reproduction of bacteria: A - general scheme reproduction; B - cell division diagram

The circular DNA molecule attaches to the cell membrane and replicates. A transverse partition begins to form in the cell on the side where DNA molecules attach. The transverse septum then bifurcates, moving the anchored DNA to different parts of the cell. Ribosomes are evenly distributed between the two daughter cells, and a constriction is formed that divides the cell into two daughter cells.

Budding - This is a form of asexual reproduction in which a small outgrowth (bud) is separated from the parent individual and a daughter organism is formed. A new organism develops from a group of cells of the original organism. This type of asexual reproduction is characteristic of coelenterates (hydra) and some other animals and plants. Single-celled fungi - yeasts - also reproduce by budding. In contrast to simple division, during budding the mother cell is divided into unequal parts, budding a constantly smaller daughter cell (Fig. 8, B).

Rice. 8. Types of asexual reproduction: A - simple division in two of green euglena (longitudinal); B - budding of yeast and hydra; B - sporulation of mosses; G - vegetative propagation by begonia leaves

Reproduction by spores (sporulation) is typical for spore-bearing plants (algae, mosses, ferns). Reproduction occurs with the help of special cells - spores formed in the mother's body (Fig. 8, B). A spore is a small cell consisting of a nucleus and a small amount of cytoplasm. They are formed in large quantities in the original maternal organism. Each spore, germinating, gives rise to a new organism. Since they are microscopically small, they are easily transported by wind, water or other organisms, which facilitates the spread of these plants. Fungi, such as penicillum and cap mushrooms, also reproduce by spores.

Vegetative propagation- reproduction by individual organs, parts of organs or the body. Vegetative propagation most often occurs in plants that can reproduce by roots, shoots and parts of shoots (stems, leaves), modified shoots. Methods of vegetative propagation of plants are very diverse. This is propagation by bulbs (tulip), underground stolons - tubers (potatoes), rhizomes (wheatgrass), root cones (dahlia), layering (currants), root suckers (raspberries), leaves (begonia, violet), aboveground stolons - tendrils (strawberries ) etc. (Fig. 8, D).

Fragmentation- this is the division of an individual into two or more parts, each of which can give rise to a new organism. This method is based on regeneration- the ability of organisms to restore missing parts of the body. It is characteristic of lower invertebrate animals (coelenterates, flatworms, starfish, etc.). The animal's body, divided into separate parts, completes the missing fragments. For example, under unfavorable conditions, the planarian flatworm breaks up into separate parts, each of which, when favorable conditions can give rise to a new organism.

Fragmentation also occurs in plants; for example, multicellular algae can reproduce in parts of the thallus.

Cloning. Artificial method reproduction, which appeared relatively recently, in the early 60s. XX century It is based on obtaining a new organism from one cell of the original one. Since the cell nucleus contains the entire set of chromosomes, and therefore genes, under certain conditions it can be forced to divide, which will lead to the formation of a new organism. The formation of a clone is based on mitosis. To clone plants, the cells of the educational tissue are separated and grown on special nutrient media. A plant cell, dividing successively, gives rise to a whole organism. This method is currently widely used to obtain valuable plant varieties.

There is experience in animal cloning. It was first put forward by the English biologist D. Gurdon and gave positive results in experiments with the South American toad. Tadpole intestinal cells were used as a nuclear donor. The nuclei of the recipient eggs were destroyed by ultraviolet rays and the intestinal epithelial nuclei were transplanted into these cells. As a result of the experiment, it was possible to obtain several cloned toad individuals, completely identical to each other. In 1995, English scientists managed to obtain a clone of sheep that were similar to the original maternal individual. However, the lambs died at an early age, before reaching nine months.

In 1997, Dolly the sheep was obtained by cloning. To do this, the nuclei of mammary gland cells from a sheep of one breed (nucleus donor) were taken and transplanted into eggs with previously destroyed nuclei from a sheep of another breed (recipient). The cloned sheep was no different from the nuclear donor, but very different from the recipient.

The use of the cloning method will make it possible not only to preserve economically valuable animals, but also to reproduce them without limit. Currently, work is underway on human cloning, which causes heated debate not only among scientists, but also various groups population. However, using this method, it is intended to reproduce only individual organs and tissues for subsequent transplantation into the donor’s body, and not to create individual individuals. This method will solve the problem of incompatibility of tissues of different organisms.

Features of sexual reproduction

Sexual reproduction - This is the formation of a new organism with the participation of two parent individuals. The new organism carries hereditary information from two parents, and the resulting offspring differ genetically from each other and from their parents. This process is characteristic of all groups of organisms; in its simplest form, it occurs even in prokaryotes.

During sexual reproduction, special cells are formed in the body sex cells - gametes male and female types that are capable of merging. Male gametes - spermatozoa, or sperm(if they are motionless). Female gamete - egg. Gametes are different from all other cells in the body, which are called somatic(from lat. soma - body). They always have haploid set of chromosomes (n).

As a result of the fusion of two gametes diploid set chromosomes are restored again. In this case, half of all chromosomes are paternal, and the other half are maternal. For example, a person has 46 chromosomes, of which 23 are received from the mother and 23 from the father.

Sexual reproduction has a number of advantages. As a result of this process, a change in hereditary information occurs, and new individuals combine the characteristics of two parents. This leads to the emergence of new combinations of traits and genes. Sexual reproduction makes the organism more competitive and adapted to changing environmental conditions, as it increases the chances of survival. In the process of evolution, sexual reproduction turned out to be more preferable and progressive.

Questions for self-control

1. What types of reproduction occur in organisms? How are they different from each other?

2. What type of cell division underlies asexual reproduction?

3. Compare reproduction by spores and vegetative reproduction in plants. What are their similarities and differences?

4. What advantage does spore reproduction provide to the body?

5. Describe the features of each type of asexual reproduction.

6. What are the features of sexual reproduction? What advantages does this type of reproduction provide?

7. What cells are called gametes? What makes them special?

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4.1. Types of reproduction In the process of the evolution of living organisms, there was also an evolution of methods of reproduction, the diversity of which is observed in living species. All reproduction options can be divided into two fundamentally different types - asexual and

Sexual reproduction involves the formation of new individuals not from parts of the parent organism, as in asexual reproduction, but from a zygote formed by the fusion of male and female reproductive cells. Sexual reproduction in nature occurs in most species and has advantages over asexual reproduction, since it combines the hereditary material of parent organisms.

Gametes

Sex cells, or gametes, differ in structure from other cells in the body.

Gametes have a halved amount of hereditary information (the number of chromosomes). This is achieved through meiosis, a special type of division characteristic of developing gametes.

In plants, the organs in which the development of germ cells (gametogenesis) occur are called gametangia. The plant itself on which gametes develop is called a gametophyte.

Female gametes are called eggs, and male gametes are called sperm or spermatozoa (if they have a flagellum).

Rice. 1. Sex cells.

In male animals, gametes develop in gonads called testes, and in female animals - ovaries.

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Gametes from different types vary in size and ability to move. In mammals and humans, eggs are large and immobile, while sperm are small and motile.

Fertilization

Mature gametes can combine with gametes of the other sex. This process is called fertilization and in different animals it occurs in two forms:

external fertilization , occurring outside the body (amphibians, fish);
internal when gametes meet inside the female's body.

A fertilized egg (zygote) has a full set of chromosomes, half from the father and half from the mother.

Dioecy and hermaphroditism

In some species of plants and animals, both male and female gametes develop in the body of one individual. Such species are called hermaphrodites.

Examples of hermaphroditic species are:

sea bass;
large pond snail;
earthworm;
bull tapeworm.

If a species has separate male and female organisms, which is the case in most cases, then these animals are said to be dioecious.

When male and female organisms of the same species have noticeable differences in external structure or color, they say that this species is characterized by sexual dimorphism.

Rice. 2. Sexual dimorphism.

Types of sexual reproduction

In addition to sexual reproduction itself, with the fusion of germ cells there are other types:

parthenogenesis;
fusion of single-celled organisms;
conjugation.

In parthenogenesis, offspring develop from unfertilized eggs.

Parthenogenesis occurs in

ants;
aphids;
bees;
crucian carp, etc.

During parthenogenesis, there is no exchange of hereditary material and all offspring are similar to the maternal organism.

Conjugation is sexual reproduction without the formation of germ cells. Characteristic, for example, of algae. Cells of different individuals grow together for a while and exchange genetic material.

In unicellular algae, fusion of entire parent cells occurs, followed by division into 4 cells.

Rice. 3. Sexual reproduction of algae.

In plants, sexual reproduction is usually combined with vegetative reproduction. For example, onions are usually propagated by shoots - bulbs, but onions are also characterized by sexual reproduction, they bloom and form seeds after pollination.

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