In animals and plants, there are so-called irregular types of sexual reproduction. This is, first of all, apomixis (from the Greek "apo" - without, "mixis" - mixing), i.e. sexual reproduction without fertilization. Apomixis is opposite to amphimixis ("amphi" - divided), ie, sexual reproduction, which occurs through the fusion of different-quality gametes. A synonym for apomixis is parthenogenesis, i.e. virgin reproduction from the Greek. "Parthenos" - a virgin). The term apomixis is more commonly used in relation to plants, and parthenogenesis - in relation to animals.

Along with parthenogenesis, the development of the ovum is also observed, activated by the sperm, which does not participate in fertilization. The male pronucleus dies, and the body develops at the expense of the female pronucleus. This phenomenon is called gynogenesis, which occurs in hermaphroditic roundworms and in some fish.

The opposite of gynogenesis is androgenesis - development only due to the male pronucleus in the event of the death of the female pronucleus. Haploid androgenesis is very rare. The development of androgenic individuals to adulthood was observed only in the rider Habrobracon and in the silkworm.

In the silkworm, during fertilization, several spermatozoa penetrate into the egg, but the nucleus of only one of them merges with the nucleus of the egg, the rest die. If unfertilized eggs are activated with a temperature shock, as described above, and irradiated with X-rays, the egg nucleus will die. If further such enucleated eggs are inseminated, then two male pronuclei that have penetrated into the ovum merge with each other. Due to the formed diploid nucleus, a zygote develops. As shown by B. JI. Astaurov, such androgenetic zygotes always turn into males, since they carry two identical sex chromosomes - ZZ. Obtaining purely male offspring from a silkworm is economically beneficial, since males are more productive than females.

TO irregular types sexual reproduction can be attributed to:

  • parthenogenetic,
  • gynogenetic,
  • androgenetic

reproduction of animals and plants.

Parthenogenesis - This is the development of an embryo from an unfertilized egg. The phenomenon of natural parthenogenesis is characteristic of lower crustaceans, rotifers, hymenoptera (bees, wasps), etc. It is also known in birds (turkeys). Parthenogenesis can be artificially stimulated by inducing the activation of infertile eggs by exposure to various agents. Distinguish parthenogenesis:

  • somatic, or diploid,
  • generative, or haploid.

At somatic In parthenogenesis, the egg cell does not undergo reduction division, or if it does, then two haploid nuclei, merging together, restore the diploid set of chromosomes (autokaryogamy); thus, a diploid set of chromosomes is preserved in the cells of the tissues of the embryo. At generative During parthenogenesis, the embryo develops from a haploid egg. For example, in the honey bee (Apis mellifera), drones develop from unfertilized haploid eggs by parthenogenesis.

Gynogenesis ... Gynogenetic reproduction is very similar to parthenogenesis. Unlike parthenogenesis, gynogenesis involves sperm as egg development stimulants(pseudogamy), but fertilization (karyogamy) in this case does not occur; the development of the embryo is carried out exclusively through female core... Gynogenesis was found in roundworms, viviparous fish Molliensia formosa, goldfish (Platypoecilus) and some plants - buttercup (Ranunculus auricomus), bluegrass (genus Poa pratensis), etc. Gynogenetic development can be caused artificially if sperm or pollen are irradiated with X-rays, treated with chemicals, or exposed to high temperatures before fertilization. In this case, the nucleus of the male gamete is destroyed and the ability to karyogamy is lost, but the ability to activate the egg remains.

The phenomenon of gynogenetic reproduction is of great importance for the study heredity, since in this case the offspring receives hereditary information only from mothers... Thus, with asexual reproduction, parthenogenesis and gynogenesis, the offspring should be similar only to the maternal organism.

Androgenesis ... The direct opposite of gynogenesis is androgenesis. In androgenesis, the development of the egg is carried out only at the expense of male nuclei and maternal cytoplasm... Androgenesis can take place when the maternal nucleus for some reason dies before fertilization. If one sperm gets into the egg, then the developing embryo with a haploid set of chromosomes turns out to be unviable or unviable. Viability androgenic zygotes are normalized if the diploid set of chromosomes is restored.

In animals and plants, there are so-called irregular types of sexual reproduction. This is primarily apomixis(from the Greek. aro- from, from, mixis- mixing), i.e. sexual reproduction without fertilization. Apomixis is the opposite of amphimixis (from the Greek. amphi- at both sides, mixis- mixing), i.e. sexual reproduction, occurring by the fusion of different-quality gametes. Synonym for apomixis - parthenogenesis, i.e. virgin reproduction (from the Greek. parihenos- virgin and genesis). The term apomixis is more commonly used in relation to plants, and parthenogenesis - in relation to animals.

In 1958 I.S. Darevsky described populations of lizards of the genus Lacerta, consisting of some females and reproducing parthenogenetic. Then a similar phenomenon was found in lizards of the genus Cnemidaphorus. It turned out that before meiosis, endomitotic doubling of the number of chromosomes occurs in gonial cells. Then these cells go through the normal cycle of meiosis, and as a result, diploid eggs are formed, which, without fertilization, give rise to a new generation consisting only of females.

The phenomenon of apomixis in plants is associated with dramatic pages in the history of genetics. On the unsuccessful advice of K. Nageli, G. Mendel after 1865 began to check the patterns he discovered in hawks (Hieracium). Crossing different species of this plant, he discovered splitting in F] and complete uniformity in F 2. Having obtained this result, G. Mendel published it in the work "On some bastards Hieracium obtained by artificial insemination "(1869) and gave up hybridization. Only 40 years later did it become clear that G. Mendel encountered apomictic reproduction. Obviously, the selected forms were optionally apomictic. In the generation, which G. Mendel considered the first hybrid, splitting of the initially heterozygous forms took place. It is now known that facultative apomicts, after hybridization, acquire the ability to sustain apomixis.

The patterns of parthenogenesis and the chromosomal mechanism of sex determination in silkworms were used by B.L. Astaurov for the selection of the most productive lines. If you extract from females unfertilized eggs that have not passed meiosis, then when they are warmed up to 46 ° C, there is no meiosis. These eggs contain the Z and Y chromosomes. They develop parthenogenetically and give rise only to females, since in the silkworm the female sex is heterogametic. In this way, valuable breeding material can be quickly multiplied.

Along with parthenogenesis, the development of the ovum is also observed, activated by the sperm, which does not participate in fertilization. The male pronucleus dies and the body develops at the expense of the female pronucleus. This phenomenon is called gynogenesis and is found in hermaphroditic roundworms and some fish.

The opposite of gynogenesis is androgenesis- development only due to the male pronucleus in the event of the death of the female pronucleus. Haploid androgenesis is very rare. Development of androgenic individuals to adulthood was observed only in the rider Habrobracon and silkworm.

In the silkworm, during fertilization, several spermatozoa penetrate into the egg, but the nucleus of only one of them merges with the nucleus of the egg, the rest die. If unfertilized eggs are activated with temperature shock, as described above, and irradiated with X-rays, the egg nucleus will die. If further such enucleated eggs are inseminated, then two male pronuclei that have penetrated into the ovum merge with each other. Due to the formed diploid nucleus, a zygote develops. As B.L. Astaurov, such androgenetic zygotes always turn into males, since they carry two identical sex chromosomes - ZZ. Obtaining purely male offspring from a silkworm is economically beneficial, since males are more productive than females.

Sexual reproduction found mainly in higher organisms. It provides significant genetic diversity and, consequently, great phenotypic variability of the offspring; organisms receive great evolutionary possibilities, material for natural selection arises.

Besides sexual reproduction, there is a sexual process. Its essence is that the exchange of genetic information between individuals occurs, but without increasing the number of individuals. The formation of gametes in multicellular organisms is preceded by meiosis. The sexual process consists in combining hereditary material from two different sources (parents).

During sexual reproduction, the offspring is genetically different from their parents, since there is an exchange of genetic information between the parents.

The basis of sexual reproduction is meiosis. The parents are two individuals - male and female, they produce different germ cells. This is a manifestation of sexual dimorphism, which reflects the difference in the tasks performed during sexual reproduction by male and female organisms.

Sexual reproduction is carried out through gametes - sex cells that have a haploid set of chromosomes and are produced in parental organisms. The fusion of parental cells leads to the formation of a zygote, from which a descendant organism is subsequently formed. Sex cells are formed in the gonads - the sex glands (in the ovaries in females and testes in males).

The process of formation of germ cells is called gametogenesis (ovogenesis in females and spermatogenesis in males).

If male and female gametes are formed in the body of one individual, then it is called hermaphrodite. Hermaphroditism is true (an individual has gonads of both sexes) and false hermaphroditism (an individual has gonads of the same type - male or female, and the external genital organs and secondary sexual characteristics of both sexes).

Parthenogenesis (virgin reproduction)

Types of parthenogenesis:

1.obligate (obligatory) parthenogenesis. It is found in populations consisting exclusively of females. At the same time, the probability of meeting opposite-sex individuals is minimal.

2. Cyclic (seasonal) parthenogenesis (in aphids, daphnia, rotifers). Occurs in populations that have historically died out in large numbers at certain times of the year. In these species, parthenogenesis is combined with sexual reproduction. Moreover, in the summer there are only females that lay two types of eggs - large and small. From large eggs, females emerge parthenogenetically, and males from small eggs, which fertilize eggs that lie on the bottom in winter. Of these, only females appear;

3. optional (optional) parthenogenesis. Found in social insects (wasps, bees, ants). In a population of bees, females (worker bees and queens) hatch from fertilized eggs, and males (drones) from unfertilized eggs.


Gynogenesis (in teleost fish and some amphibians). The sperm cell enters the egg and only stimulates its development. At the same time, the nucleus of the sperm does not merge with the nucleus of the egg and dies, and the DNA of the nucleus of the egg serves as the source of hereditary material for the development of the offspring.

Androgenesis. In the development of the embryo, the male nucleus is involved, introduced into the egg, and the nucleus of the egg dies. The ovum provides only nutrients to its cytoplasm.

Polyembryony ... The zygote (embryo) is divided into several parts asexually, each of which develops into an independent organism. Found in insects (riders), armadillos. In armadillos, the cellular material of initially one embryo at the blastula stage is evenly divided between 4–8 embryos, each of which subsequently gives rise to a full-fledged individual.

In unicellular organisms, two forms of sexual reproduction are distinguished - copulation and conjugation .

When conjugated (for example, in ciliates), special germ cells are not formed. During this process, there is no increase in the number of individuals, therefore, they talk about the sexual process, and not about sexual reproduction. However, there is an exchange (recombination) of hereditary information, so the offspring are genetically different from their parents.

During copulation (in protozoa), the formation of genital elements and their pairwise fusion occur. In this case, two individuals acquire gender differences and completely merge, forming a zygote. The unification and recombination of the hereditary material takes place, therefore the individuals are genetically different from the parental ones.

The different types of sexual reproduction include parthenogenetic, gynogenetic and androgenetic reproduction of animals and plants.

The listed types of sexual reproduction arose as a result of complete or partial loss of meiosis and its replacement by mitosis in the cycle of sexual reproduction. Asexual reproduction in this case is secondary.

We have already pointed out that in most species of animals and plants, during sexual reproduction, two gametes merge - male and female. In a number of animal and plant species, virgin reproduction takes place without the participation of the sperm. The development of an embryo from an unfertilized egg is called parthenogenesis.

Parthenogenesis is subdivided into natural and artificial. During natural parthenogenesis, an egg that has undergone or has not undergone maturation division, under the influence of internal or external reasons, begins to split and develops into a normal embryo without any participation of the spermatozoon. The phenomenon of natural parthenogenesis is characteristic of lower crustaceans, rotifers, hymenoptera (bees, wasps), etc. It is also known in birds (turkeys).

Parthenogenesis can be permanent (obligate parthenogenesis) or partial (facultative parthenogenesis). In some animals, only females can develop from unfertilized eggs, and males from fertilized ones, in others, both sexes, in others, only males develop from unfertilized eggs, and females from fertilized eggs.

Parthenogenetic reproduction can be interspersed in generations with sexual (cyclic parthenogenesis). In lower crustaceans (daphnia), as well as in aphids, rotifers and other animals, a change of generations is observed, which developed from normally fertilized and parthenogenetic eggs. In Daphnia, females are diploid, males are haploid. Under favorable external conditions, daphnia reproduce parthenogenetically. In this case, only one sex appears - female, since the eggs do not undergo meiotic division. The reason for this phenomenon will become clear when the genetic and cytological mechanisms of sex determination are considered. The onset of unfavorable external conditions, for example, a decrease in temperature or a lack of food, leads to the fact that females begin to lay haploid eggs. From these eggs, males develop parthenogenetically. After mating and normal fertilization, the sexual generation of female organisms with a diploid chromosome number is restored. Fertilized eggs in cysts can tolerate overwintering and unfavorable external conditions. A similar pattern is observed in grass aphids and other insects.

Distinguish between somatic or diploid parthenogenesis and generative or haploid parthenogenesis. During somatic parthenogenesis, the egg does not undergo reduction division, or, if it does, two haploid nuclei, merging together, restore the diploid set of chromosomes (autokaryogamy); thus, a diploid set of chromosomes is preserved in the cells of the tissues of the embryo. In some cases, the somatic cells of such organisms can have a multiplied number of chromosomes due to the nondisjunction of whole sets of chromosomes. In generative parthenogenesis, the embryo develops from a haploid egg. As a rule, males develop from such eggs (bees, worms, ticks). For example, in a honey bee, drones develop from unfertilized haploid eggs by parthenogenesis. During the development of germ cells, their meiosis is replaced by mitosis, and therefore the spermatozoa have a haploid set of chromosomes. In contrast to the primordial pathway, a diploid set of chromosomes can be restored in the catfish of such animals.

Artificial parthenogenesis is the experimentally induced activation of unfertilized eggs. The honor of this discovery belongs to the Russian zoologist A.A.Tikhomirov, who first carried out artificial parthenogenesis in 1885 on silkworm eggs. Artificial parthenogenesis can be induced by the action of heat, acids, light, and other agents. The possibility of artificial parthenogenesis has been proven for many aquatic and terrestrial invertebrates (sea urchins, stars, insects, etc.) and vertebrates (amphibians).

In artificial parthenogenesis, the normal development of the embryo is often inhibited. However, using an improved technique for processing silkworm eggs with high temperatures at a certain exposure and at a certain stage of development, BL Astaurov managed to obtain a large number of female parthenogenetic butterflies. At present, artificial parthenogenesis has also been carried out in frogs and rabbits. Artificial parthenogenesis has also been obtained in plants (algae, fungi, and higher plants: cereals, legumes, etc.). It is stimulated by irritation of the stigma with alien or killed pollen, as well as talc, chalk, etc. In this case, as in animals with parthenogenetic development, inheritance is carried out only along the maternal line.

Gynogenetic reproduction, that is, the development of the embryo exclusively at the expense of the female nucleus, can also be attributed to the type of parthenogenetic reproduction. Unlike parthenogenesis, in this case, the participation of the sperm is necessary to stimulate the development of the egg (pseudogamy), but fertilization (karyogamy) in this case does not occur. Gynogenesis has been found in hermaphroditic roundworms, the viviparous fish Mollienisia formosa, and the goldfish found in our Far East. Gynogenesis, as a rule, is found in individuals of a species at the boundaries of its range as a mechanism that guarantees the preservation of the species here.

Gynogenetic development of eggs can be induced artificially if sperm are irradiated with X-rays, treated with chemicals or exposed to high temperatures before fertilization. This destroys the nucleus of the sperm, and it loses the ability to karyogamy, but such a sperm can activate the egg. Natural and artificial gynogenesis is also found in plants and is caused by the same factors as in animals. In the case of natural gynogenesis, developing individuals contain a normal diploid number of chromosomes. Artificial gynogenesis is often associated with haploidy, so such embryos are not very viable.

The phenomenon of parthenogenetic and gynogenetic reproduction is of great importance for the study of heredity, since in this case the offspring is completely similar to the mother's organism. The study of parthenogenesis, like gynogenesis, is also important for solving a number of practical issues, in particular, for obtaining individuals of the same sex in some practically important objects, for example, for breeding valuable fish species, as well as chickens.

Sometimes under artificial conditions, when the female nucleus with part of the cytoplasm is removed, the remaining part of the egg, after the sperm penetrates into it, begins cleavage, which soon stops. Such an experimentally induced initial development of the embryo from a part of the cytoplasm of the egg without the participation of the female nucleus is called merogenesis. In some cases (in the sea urchin), egg crushing can be obtained in the complete absence of the kernel. Nuclear-free parthenogenetic merogons developed to abortive morula or blastula, which soon died.

Reproduction of plants and animals without fertilization is called apomixis with fertilization (karyogamy) - amphimixis... Some researchers (S. S. Khokhlov and others) put a broader meaning in the concept of apomixis - any asexual reproduction, including all its species. The terms "apomixis" and "amphimixis" apply equally to plants and animals.

But since apomixis is especially widespread in the plant kingdom and is of great importance for the study of inheritance, let us consider its features. The types of apomixis in plants are extremely diverse, but there is still no generally accepted classification for them.

The most common is the type of parthenogenetic formation of the embryo from the egg. In this case, in the case of diploid parthenogenesis, meiosis completely disappears, and in the case of haploid parthenogenesis, normal megasporogenesis proceeds and a haploid embryo sac with a reduced set of chromosomes in the nuclei is formed. In diploid parthenogenesis, the constancy of the number of chromosomes of the double set is maintained by mitosis, and the inheritance of the traits of the endosperm and the embryo occurs only along the maternal line.

In haploid parthenogenesis, an embryo is formed from a haploid egg cell also without fertilization. Plants developing from such an embryo are sterile, weak and small-leaved. Such a plant can only reproduce vegetatively. In this case, for the formation of full-fledged seeds, pseudogamy is necessary - the activation of the embryo sac by the pollen tube. One sperm from the tube, reaching the embryo sac, is destroyed, while the other merges with the central nucleus and participates only in the formation of endosperm tissue (Potent ilia species, raspberries Rubus, etc.). Inheritance will work a little fine here. The signs of the embryo and the plant are inherited only through the maternal line, and the signs of the endosperm can appear both paternal and maternal.

The emergence of the embryo not from the ovum, but from other cells of the female gametophyte (synergids and antipodes), which have undergone and have not undergone meiosis, is called apogametic. In addition to these two types of apomixis, there are others, but they are of more special interest, in particular for plant embryologists.

All types of apomictic reproduction (excluding vegetative reproduction) arose, apparently, as a result of various simplifications of the normal sexual process. But no matter how diverse the types of apomixis are, if meiosis falls out, it is replaced by mitosis, and fertilization is completely excluded. In some types of apomixis, meiosis and a reduction in the number of chromosomes remain in the embryo sac, then fertilization is replaced by autogamy,

that is, the development of the embryo is carried out due to the fusion of the haploid nuclei of the embryo sac itself. The latter is necessary to maintain the constancy of the species number of chromosomes. With any type of apomixis, the principle of the constancy of the number of chromosomes of a species cannot be violated, since otherwise the number of chromosomes in cells will either increase or decrease with each generation. Knowledge of the type of apomixis is essential for the study of genetic phenomena in individuals that reproduce in this way. An example of this is the episode that took place in the experiments of G. Mendel.

Mendel, wishing to find out the universality of the patterns of inheritance discovered by him on peas, conducted many years of experiments with one of the species of the genus Hieracium. But this plant has widespread apomictic reproduction. Crossing different races of the hawk, Mendel did not receive confirmation of the desired results precisely due to the lack of a fertilization process in this object. This circumstance was one of the reasons that prevented Mendel from extending the laws he discovered to other organisms.

Natural parthenogenesis and apomixis play an important role in evolution as special mechanisms providing hereditary diversity.

Currently, genetic techniques are being developed using apomixis for practical purposes to consolidate heterosis, maintain valuable mutations, etc.

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