Facultative parthenogenesis. Parthenogenesis is a technique of immaculate conception. Haploid and diploid parthenogenesis

The concept of parthenogenesis

During fertilization, the sperm releases the egg from its dormant state and it begins to develop. But in nature there are cases when an organism developed from an unfertilized egg.

Definition 1

The phenomenon of development of an organism from an unfertilized egg is called parthenogenesis .

In the case of parthenogenesis, the new generation has an unchanged parental genotype. In some species, both parthenogenetic and bisexual populations can exist (in lizards). For other species, parthenogenesis is the only method of reproduction (in stick insects). In ground beetles and daphnia, sexual and parthenogenetic generations naturally alternate.

Some scientists consider parthenogenesis to be a separate form of asexual reproduction, since there is no sexual process (copulation). Others consider it a variant of sexual reproduction, since it is the germ cells that take part in it.

Diploid parthenogenesis

There are a number of animal species in which unfertilized eggs develop over a certain period. In the nucleus of the egg, the number of chromosomes doubles and they become diploid (or meiosis does not occur during the formation of the egg).

Example 1

For example, in the above-mentioned ground beetles and daphnia, throughout spring, summer, and early autumn (that is, most of the year), reproduction occurs only parthenogenetically. Only females develop from unfertilized eggs. In autumn, males appear and the process of fertilization occurs. Fertilized eggs survive the winter. In the spring, they again develop into females capable of parthenogenetic reproduction.

Diploid parthenogenesis contributes to the rapid reproduction of populations of these species.

Haploid parthenogenesis

In bees and some other insects, females develop from fertilized eggs. From them, workers (underdeveloped females) and queens are formed. Drones (parthenogenetic males) develop from unfertilized eggs. Male cells have a haploid set of chromosomes. During the formation of sperm, meiosis does not occur and the number of chromosomes in sperm does not decrease. Therefore, upon fertilization, organisms receive a diploid set of chromosomes.

Artificial parthenogenesis

During research, embryologists were able to stimulate the development of an egg without fertilization. They used certain stimuli (chemical, mechanical or short-term effects of high or low temperatures, etc.) as a stimulating factor. The influence of these stimuli contributed to the excitation of the egg and the beginning of the formation of fertilization membranes.

Note 1

The phenomenon of artificial parthenogenesis is actively used, for example, to regulate the sex of silkworms in sericulture.

Androgenesis

There are cases in science when the nucleus of an egg was destroyed. At the same time, the egg itself retained the ability to fertilize. Then the sperm nucleus occupied a central position in the egg. The egg developed further parthenogenetically, but with a sperm nucleus. The resulting new organism had only paternal characteristics. This phenomenon is called in science androgenesis .

The phenomenon of parthenogenesis probably arose as a reaction of the body to sudden changes in environmental conditions. These changes led to the impossibility of fertilization. Therefore, individuals survived. In which the egg began to develop independently. This adaptation allowed the species to survive in unusual and changing conditions. The parthenogenesis method can be very useful in breeding work.

Recently, scientists from the Institute for the Development of Cell Technology (Massachusetts USA) obtained full-fledged embryos from unfertilized eggs from 4 of 28 test macaques. They were able to achieve such results thanks to a chemical drug under the influence of which a special method of reproduction occurs - “parthenogenesis”. What is parthenogenesis?

Parthenogenesis(Parthenogenesis - from the Greek parthenos - girl, virgin + genesis - generation) - a form of sexual reproduction in which the development of an organism occurs from a female reproductive cell (egg) without fertilization by a male one (sperm).

Parthenogenesis should be distinguished from asexual reproduction when development occurs not from germ cells, but from somatic cells or organs by division, budding.

It's sexual, but same-sex reproduction, which arose during the evolution of organisms in dioecious forms. In cases where parthenogenetic species are represented only by females, one of the main biological advantages of parthenogenesis is the acceleration of the rate of reproduction of the species, since all individuals of such species are capable of leaving offspring. If a female develops from fertilized eggs, and a male from unfertilized eggs, parthenogenesis contributes to the regulation of numbers and sex ratios(for example, in bees, males - drones - develop parthenogenetically, and from fertilized ones - females - queens and worker bees).

Parthenogenesis - natural a normal method of reproduction for some species of animals and plants. Complete natural parthenogenesis occurs in invertebrate animals of all types, but most often in arthropods. Of the vertebrates, these are fish, some species of amphibians, reptiles, and certain species of birds (turkeys) reproduce parthenogenetically. In mammals, only cases of embryonic parthenogenesis are known; isolated cases of full development were observed in the rabbit during artificial parthenogenesis. In humans there are known cases when, under the influence of stressful situations of high temperatures and in other extreme situations, a female egg can begin to divide, even if it is not fertilized, but in 99.9% of cases it soon dies (according to some data, 16 cases of immaculate conception that took place in history are known in history Africa and European countries).

Parthenogenesis may be obligate, when the eggs are only capable of parthenogenetic development, and optional in which eggs can develop both through parthenogenesis and as a result of fertilization (in bees). Often reproduction through parthenogenesis alternates with bisexual reproduction - the so-called cyclic parthenogenesis. Thus, in some species of aphids, two-toed generations (winged forms) are replaced by parthenogenetic (wingless females), while different generations use different types of food plants.

Parthenogenetically, either an egg that has undergone meiosis and contains a haploid set of chromosomes (n) (generative, haploid, or meiotic parthenogenetic) can develop, or an egg from one of the premeiotic stages of oogenesis with preservation of the chromosome set characteristic of this species - diploid (2n) or polyploid (3n, 4n, 5n rarely 6n, 8n) (ameiotic parthenogenesis). In some forms of parthenogenesis, the fusion of the haploid nucleus of the egg with the haploid nucleus of the directional (polar) body leads to restoration of diploidity (automictic parthenogenesis). The genotype, sex of the parthenogenetic offspring, as well as the preservation or loss of heterozygosity, acquisition of homozygosity, etc., depend on these features of parthenogenesis.

Artificial parthenogenesis in animals was first obtained by a Russian zoologist A. A. Tikhomirov in 1886 by exposing unfertilized silkworm eggs to various physicochemical irritants (solutions of strong acids, friction, etc.) Subsequently, artificial parthenogenesis was obtained by J. Lebon and other scientists in many animals, for example, in marine invertebrates (sea urchins, stars, worms , mollusks), as well as in some amphibians (frogs), and even in mammals.

Artificial parthenogenesis is caused by the effect on the eggs of a hypertonic solution (hypotonic - osmotic parthenogenesis), the injection of an egg with a needle moistened with hemolymph (the so-called Traumatic parthenogenesis of amphibians), sudden heating or cooling (Temperature parthenogenesis), as well as the action of alkali acids, etc.

With the help of artificial parthenogenesis, it is usually possible to obtain only the initial stages of the development of an organism; complete parthenogenesis is rarely achieved.

Method of mass parthenogenesis developed for silkworms in 1936 by B. L. Astaurov. This method is based on precisely dosed short-term heating (up to 46 o C for 18 minutes) of unfertilized eggs removed from the female. The method makes it possible to obtain only female individuals, hereditarily identical with the maternal female (initial), as well as similar to each other, giving an increased yield of silk fiber of the highest quality.

Parthenogenesis also includes peculiar ways of development of animals and plants - gynogenesis and androgenesis, in which the eggs are activated for development by penetrating sperm of their own or a similar species, but the nuclei of the egg and sperm do not fuse. Fertilization turns out to be false, and the embryo develops with only a female (gynogenesis) or only a male (androgenesis) nucleus.

Thus, if American scientists’ experiment ends successfully, that is, healthy baby macaques are born, then experiments with human eggs will probably begin soon, especially since There are enough women volunteers.

Infertile and lesbian couples are most interested in this. However, they must take into account that only girls can be born. For now, scientists do not intend to conduct similar experiments on humans, since there is a serious risk of developing cancer and metabolic disorders in newborns, and also because we have genes that only work if they are received from the father. It is in them that the proteins necessary for the development of tissues and organs are encoded. Therefore, it is unlikely that women will be able to do without men, at least in the near future, while man is not able to outwit nature.

On the eve of the Christian holiday of Easter, I would like to consider a topic that scientifically approaches one New Testament miracle.

According to legend, the Blessed Virgin Mary, without any fertilization, became pregnant and gave birth to the King of the Jews Jesus Christ - the Messiah, whose coming was predicted in the Old Testament.

“Without fertilization? Impossible!" – some will object. But such a phenomenon is possible. The Virgin Mary is called "Agni Parthene" in Greek, translated as "Pure Virgin".

Here from the word “ parthenay"- maiden, virgin - the term parthenogenesis was formed.

Reproduction by parthenogenesis

Parthenogenesis- This is the process by which reproduction occurs from unfertilized.

But this should not be confused with reproduction.

Reproduction by parthenogenesis- This is a form of sexual reproduction, as female gametes are formed.

One of the first to study parthenogenesis was the Swedish naturalist Charles Bonnet and German zoologist Karl Siebold.

Parthenogenesis is divided into two types: meiotic And ameiotic .

At ameiotic parthenogenesis the eggs remain diploid because they do not undergo meiosis.

At meiotic parthenogenesis the organism develops either from haploid egg, and is itself haploid, or egg restores diploidity and the organism becomes diploid.

Restoration of diploidity can occur in different ways: the egg can fuse with a polar body (this is similar to gamete copulation) or it can occur endomitosis.

Endomitosis – doubling process. As with, but the nuclear membrane does not dissolve and the cell does not divide.

What organisms can reproduce by parthenogenesis?

Here are some classic examples

Aphids. In this way, they quickly increase their numbers without much expense. Parthenogenetically breed in summer. The result is only females. This is a kind of preparation for unfavorable conditions, aimed at ensuring that as many individuals as possible survive. As autumn approaches, a different type of gamete is born, from which both males and females can emerge. And insects begin to reproduce through ordinary sexual intercourse.

Daphnia. During the summer they reproduce by ameiotic parthenogenesis. When the temperature of the reservoir drops and daylight hours shorten, haploid males appear. The population switches to ordinary sexual reproduction.

Rotifers. Don't be surprised if this name is not familiar to you; as far as I know, they are not in the school curriculum. In short: rotifers are a whole separate type. They are multicellular organisms, but their sizes are very small. Rotifers, like aphids and daphnia, reproduce by parthenogenesis in favorable conditions, and when unfavorable conditions occur, they switch to ordinary sexual reproduction. There are even some species such as rotifers that have achieved “perfection”: these species are formed only by females that reproduce by parthenogenesis. In such cases, when parthenogenesis is the only method of reproduction, it is called obligate. And when there is an alternation of parthenogenesis and another method of reproduction, parthenogenesis is called cyclical(like daphnia and aphids).

Bees. In bees, egg development follows two patterns: some are fertilized, some are not. From unfertilized eggs (1n), males develop - drones. Therefore, the somatic cells of drones are haploid ( You shouldn’t forget about this if you suddenly come across something on this topic in a problem on genetics).

From fertilized eggs, females develop - worker bees or a queen. In this case, when eggs can develop both as a result of fertilization and parthenogenetically, parthenogenesis is called facultative.

Thanks to the ability for facultative parthenogenesis in bees, the number of individuals of each caste (workers, drones) is controlled.

Rod Rock includes several species capable of parthenogenesis. Before the germ cells of these lizards undergo a mitotic increase in the number of chromosomes, therefore, after a normal meiotic cycle, the eggs become diploid and are ready to form a new organism. Rock lizards live on rocks and sometimes moving from one to another is problematic; in such conditions, parthenogenesis is precisely what is needed.

Parthenogenesis has been discovered in komodo dragons. Females have sex chromosomes: ZW, and males: ZZ. Therefore, as a result of parthenogenesis, organisms should be obtained: ZZ or WW, but WW are not viable. Therefore, in Komodo dragons, only males can develop as a result of parthenogenesis.

Fertilization , the fusion of a male reproductive cell (sperm) with a female (egg, ovum), leading to the formation of a zygote - a new single-celled organism. The biological meaning of fertilization is the unification of the nuclear material of male and female gametes, which leads to the unification of paternal and maternal genes, restoration of the diploid set of chromosomes, as well as activation of the egg, that is, stimulation of its embryonic development. The union of the egg with the sperm usually occurs in the funnel-shaped dilated part of the fallopian tube during the first 12 hours after ovulation. Seminal fluid (sperm), entering a woman’s vagina during sexual intercourse (coitus), usually contains from 60 to 150 million sperm, which, thanks to movements at a speed of 2 - 3 mm per minute, constant wave-like contractions of the uterus and tubes and an alkaline environment, already after 1 - 2 minutes after sexual intercourse they reach the uterus, and after 2 - 3 hours - the end sections of the fallopian tubes, where fusion with the egg usually occurs.

There are monospermic (one sperm penetrates the egg) and polyspermic (two or more sperm penetrate the egg, but only one sperm nucleus fuses with the egg nucleus). The preservation of sperm activity while passing through the woman’s genital tract is facilitated by the slightly alkaline environment of the cervical canal of the uterus, filled with a mucus plug. During orgasm during sexual intercourse, the mucous plug from the cervical canal is partially pushed out and then retracted into it again, thereby facilitating the faster entry of sperm from the vagina (where normally in a healthy woman the environment is slightly acidic) into the more favorable environment of the cervix and uterine cavity. The passage of sperm through the mucous plug of the cervical canal is also facilitated by the sharply increasing mucus permeability on the days of ovulation. On the remaining days of the menstrual cycle, the mucus plug has significantly less permeability to sperm.

Many sperm found in a woman's genital tract can retain the ability to fertilize for 48 - 72 hours (sometimes even up to 4 - 5 days). An ovulated egg remains viable for approximately 24 hours. Taking this into account, the most favorable time for fertilization is considered to be the period of rupture of a mature follicle followed by the birth of an egg, as well as the 2nd - 3rd day after ovulation. Soon after fertilization, the zygote begins to fragment and form an embryo.

Parthenogenesis(from the Greek παρθενος - virgin and γενεσις - birth, in plants - apomixis) - the so-called “virgin reproduction”, one of the forms of sexual reproduction of organisms, in which female reproductive cells (eggs) develop into an adult organism without fertilization. Although parthenogenetic reproduction does not involve the fusion of male and female gametes, parthenogenesis is still considered sexual reproduction, since the organism develops from a germ cell. It is believed that parthenogenesis arose during the evolution of organisms in dioecious forms.

In cases where parthenogenetic species are represented (always or periodically) only by females, one of the main biological advantages parthenogenesis consists in accelerating the rate of reproduction of the species, since all individuals of similar species are capable of leaving offspring. This method of reproduction is used by some animals (although relatively primitive organisms resort to it more often). In cases where females develop from fertilized eggs, and males from unfertilized eggs, parthenogenesis contributes to the regulation of numerical sex ratios (for example, in bees). Often parthenogenetic species and races are polyploid and arise as a result of distant hybridization, displaying heterosis and high viability in this regard. Parthenogenesis should be classified as sexual reproduction and should be distinguished from asexual reproduction, which is always carried out with the help of somatic organs and cells (reproduction by division, budding, etc.).

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GBOU VPO Tver State Medical Academy of the Ministry of Health of Russia

Essay

On the topic " Parthenogenesis,forms and meaning"

(Department of Biology)

Completed by: Smirnova Ksenia

Parthenogenesis - (“virgin reproduction”), one of the forms of sexual reproduction of organisms, in which female reproductive cells (eggs) develop into an adult organism without fertilization. Although parthenogenetic reproduction does not involve the fusion of male and female gametes, parthenogenesis is considered sexual reproduction because the organism develops from a germ cell. It is believed that parthenogenesis arose during the evolution of organisms in dioecious forms. Opened in the middle of the 18th century. Swiss naturalist C. Bonnet (1720-1793).

Parthenogenesis can be haploid or diploid:

In aphids, daphnia, rotifers, and some lizards, diploid (somatic) parthenogenesis is observed, in which the female's oocytes form diploid eggs. For example, in Daphnia, females are diploid and males are haploid. Under favorable conditions, meiosis does not occur in Daphnia: diploid eggs develop without fertilization and give rise to females. In rock lizards, before meiosis, a mitotic increase in the number of chromosomes in the cells of the gonads occurs. The cells then undergo a normal meiosis cycle, resulting in the formation of diploid eggs, which, without fertilization, give rise to a new generation consisting only of females. This makes it possible to maintain the number of individuals in conditions where it is difficult to meet individuals of different sexes.

Classification of parthenogenetic reproduction:

By method of reproduction :

Natural - a normal method of reproduction for some organisms in nature. Opened Sh.Bonnet in 1762, who studied the life of aphids: a female aphid gave birth to live young, also females, which after a few days reached normal sizes and, without fertilization, became the ancestors of a new generation. Natural parthenogenesis exists in a number of plants, worms, and insects.

Artificial- caused experimentally by the action of various stimuli on an unfertilized egg, which normally requires fertilization. Artificial parthenogenesis in animals was first obtained by a Russian zoologist A.A.Tikhomirovin 1886G, by exposing unfertilized silkworm eggs to various physical and chemical irritants (solutions of strong acids, friction, etc.)

Artificial parthenogenesis is caused by the action on eggs of a hypertonic solution (hypotonic - osmotic parthenogenesis), an injection of an egg with a needle moistened with hemolymph (the so-called Traumatic parthenogenesis of amphibians), sudden heating or cooling (Temperature parthenogenesis), as well as the action of acids, alkalis, etc. Using artificial parthenogenesis, it is usually possible to obtain only the initial stages of the development of an organism; complete parthenogenesis is rarely achieved.

Soviet biologist academician B.L.AstaurovV1936 G developed an industrial method for stimulating parthenogenesis for silkworms. This method is based on precisely dosed short-term heating (up to 46oC for 18 minutes) of unfertilized eggs removed from the female. The method makes it possible to obtain only female individuals, hereditarily identical with the maternal female (initial), as well as similar to each other and giving an increased yield of silk fiber of the highest quality.

Natural parthenogenesis can be:

Optional(optional), in which the egg is able to develop both without fertilization and after it. Found in social insects (wasps, bees, ants). In a population of bees, fertilized eggs produce females (worker bees and queens), while unfertilized eggs produce males (drones). In these animals, parthenogenesis occurs as an adaptation to regulate the sex ratio.

obligate(compulsory). It is found in populations consisting exclusively of female individuals (in the Caucasian rock lizard). At the same time, the probability of meeting different-sex individuals is minimal (the rocks are separated by deep gorges). Without parthenogenesis, the entire population would be on the verge of extinction

Cyclic(seasonal). 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 who lay two types of eggs - large and small. From large eggs, females emerge parthenogenetically, and from small eggs, males emerge, which fertilize the eggs lying on the bottom in winter. Only females appear from them. This type of parthenogenesis is characteristic of aphids, daphnia, and rotifers.

Obligate and cyclic parthenogenesis historically develops in those animal species that died in large numbers or in which it was difficult to meet individuals of different sexes.

According to the completeness of the course parthenogenesis is classified into :

Rudimentary (rudimentary)- unfertilized eggs begin to divide, but embryonic development stops in the early stages. At the same time, in some cases, it is also possible to continue development to the final stages (accidental or random parthenogenesis). parthenogenesis sexual reproduction

Full- the development of the egg leads to the formation of an adult. This type of parthenogenesis is observed in all types of invertebrates and in some vertebrates.

Depending on the sex of the organism parthenogenesis is divided into:

Gynogenesis is a form of unisexual development in which the sperm activates the egg, prompting it to develop, but its nucleus (male pronucleus) does not merge with the female one and does not participate in development. Known in several species of fish (golomyanka, silver crucian carp, etc.), amphibians, roundworms and plants of the amaryllis family.

In laboratory conditions, gynogenesis can be carried out using non-viable sperm.

Androgenesis-- a form of reproduction in which the male nucleus (introduced into the egg by the sperm) and the cytoplasm of the egg participate in the development of the embryo. Certain species of animals reproduce in this way (for example, equestrians Habrobracon) and some plants (corn, tobacco) in the event that the female nucleus dies before fertilization (therefore this process is actually false in them). Androgenesis can be induced artificially by mechanically removing the female nucleus from the egg or by resorting to its inactivation by physical or chemical agents. The embryos resulting from the “fertilization” of such eggs have a haploid (i.e., one) set of chromosomes - male - and are usually not viable.

Pedogenesis- a type of parthenogenesis in which virgin reproduction occurs among larvae, as, for example, in immature individuals of the Oligarces mosquito, in which this phenomenon looks very interesting and even somewhat ominous. Inside the individuals emerging from the egg, several daughter larvae develop, which use the internal organs of the mother’s body as food. When the latter dies, the larvae gnaw through its covers and come out, settling in places where they can have access to organic matter (in rotten stumps, etc.). The same thing is repeated several more times. Finally, when the number of larvae reaches a sufficiently large number, they pupate, and upon emerging from the pupa they turn into ordinary males and females.

The meaning of parthenogenesis

1. Increase in population growth rate. This is especially clearly noticeable in cases where females develop during the process of reproduction, because they are capable of bearing offspring, “supplementing” parthenogenetic reproduction with typical sexual reproduction.

2. Regulation of the ratio of males and females. For example, in order for a colony of bees to continue to exist, it is necessary that within it the individuals are divided into castes, the representatives of which are in certain numerical proportions. That is why, when laying eggs, the uterus allows only part of them to be fertilized.

3. Ensuring the continued existence of the species (however, this is also typical for other forms of reproduction).

4. Overcoming geographical barriers. Even if a very small number of individuals enter the new habitat, in conditions where females do not need to look for males, they will be able to reproduce and give rise to a new population. On this basis, it is assumed that parthenogenesis as a phenomenon spread after the Ice Age, which significantly reduced the habitats of animals. Having acquired the ability for parthenogenetic reproduction, they were able to again spread throughout the territories freed from ice.

5. Separately, it is worth mentioning the importance of artificial parthenogenesis, first identified in the silkworm, and then obtained in other organisms (the largest number of studies were carried out on marine invertebrates).

In humans There are known cases when, under the influence of stressful situations, high temperatures and other extreme situations, a female egg can begin to divide, even if it is not fertilized, but in 99.9% of cases it soon dies (according to some data, 16 cases of immaculate conception occurred in history in Africa and European countries).

Currently, the potential for the development of new organisms without fertilization is an attractive area for genetic engineering research. Encouraging results have already been obtained in mice and macaques; It seems that in the future this will help solve the problem of infertility in humans.

Literature

1. Shvanvich B.N. General entomology course. - M.L. Soviet science. 1949.--900 pp., ill.

2. N.A. Lemeza L.V. Kamlyuk N.D. Lisov "A manual on biology for those entering universities"

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