Natural parthenogenesis as a method of reproduction is characteristic of. Parthenogenesis

Most animal representatives and flora divided into males and females. As a result of mixing the genetic material of parents, the offspring have a greater chance of surviving and adapting to the conditions of an ever-changing environment. However, there is also a way back. Sometimes females, when reproducing offspring, manage on their own, so to speak, without a “dad.” We will not describe all the methods of asexual reproduction of organisms, but will focus on one of the methods of sexual reproduction - parthenogenesis. What it is? What types of this phenomenon are there? We will talk about this in the article.

Two decks or one

To explain the difference between asexual (mitosis) and sexual (meiosis) cell division, we will use the association with card games. The gene set of all nuclear (eukaryotic) organisms consists of two decks of cards - one received from the mother, the other from the father (diploid set). Paired deck cards are alleles of the same gene. It is this shuffling of genetic material that makes it possible for evolution and increases the chances of successful adaptation of organisms to the environment. During mitosis (simple division), the chromosome set of the descendants is completely identical to that of the parent cell. During meiosis the final product divisions will result in sex cells (gametes) with a half haploid chromosome set - each with one deck of cards, and with different “backs”.

Two parents or one

During sexual reproduction, female and male gametes merge and form a zygote (embryo) with a complete diploid set of chromosomes (one set from dad, the other from mom), characteristic of a particular organism. But in some cases, a zygote is formed without the participation of one of the parents. Parthenogenesis is a method of reproduction of organisms when female gametes form an embryo without fertilization, without fusion with male gametes. The term is derived from the Greek words “parthenos” - “virgin” and “genesis” - “birth, development”. In nature, parthenogenetic reproduction is not so common and is called natural. What is artificial parthenogenesis? This is the division of an egg caused by various agents and normally requiring fertilization.

Types of parthenogenesis

The classification of parthenogenesis is based on various comparison criteria.

I can do it myself, I can do it with a partner

When the criterion is taken to be the presence in life cycle body different forms reproduction, then three types of parthenogenesis are classified: obligate, cyclic and facultative. Obligate, or constant parthenogenesis, is reproduction that is unique to a given organism. Cyclic is the one that alternates with the actual sexual one. What is facultative parthenogenesis? This is a backup way to leave offspring or has become an exception for this species.

Parthenogenesis in bees

Facultative, complete and meiotic parthenogenesis can be illustrated using the example of well-known bees. In early spring The queen hatches from the pupa and takes off on a mating flight when she is fertilized by many males (drones). But their sperm accumulates in the spermatheca of the queen bee, and it is with this that she will fertilize the eggs she lays throughout her life. Or it won't. When an egg passes through the female's oviduct, the spermatic duct opens and fertilizes it - a female emerges from the diploid embryo, and whether she becomes a queen or a worker bee depends on what the worker bees feed the larva. If the vas deferens does not open, the egg will remain unfertilized and will develop into a haploid male drone. A similar cycle occurs in aphids and ants.

Biological advantages

Despite the undeniable advantages of sexual reproduction, parthenogenesis has its advantages. If environmental conditions are favorable and sufficient food is available, then this method of reproduction, when each individual leaves offspring, provides advantages expressed in the speed of colonization of specific biotopes. When environmental conditions change in an unfavorable direction, you can sacrifice quantity, but improve the quality of the offspring by switching to sexual reproduction. This is what facultative parthenogenesis is. It is characteristic of arthropods, amphibians, reptiles and birds.

Lonely mother shark

It rarely happens that parthenogenesis becomes a real miracle. For example, in the case of sharks, only one method of reproduction was known - sexually. But in 2001, a hammerhead shark from the Nebraska Zoo in the USA suddenly gave birth to a baby shark, and this despite the fact that she had lived alone in the aquarium for many years. This event baffled biologists. Made it possible to clarify the situation accidental death a baby shark that was stung by a poisonous stingray. Genetic analysis showed that the cub was born through true parthenogenesis. Apparently, the mother shark’s body activated mechanisms unknown to science for preserving the species at the boundaries of its range. Or maybe the mother shark was very lonely.

Competing with God

The topic of the immaculate, virgin conception has not left the media for many years. Maybe the story of the birth of Jesus by the Virgin Mary is an example of parthenogenesis in humans? Geneticists unequivocally and categorically say: “No!” After all, if this was parthenogenetic reproduction, Jesus would have to be... a girl. And in general, natural parthenogenesis in mammals, including humans - as the highest phylogenetic group - is simply impossible. And that's why. In mammals, the development of many traits is linked to sex-linked genes (sex markers). This means that the inclusion of certain genes depends on the quality of the genetic material of both the mother and father. Of course, if genetic engineering specialists don’t get down to business.

It was Japanese specialists who, having conducted more than 600 experiments, of which 24 ended in pregnancy, and only 2 of them in childbirth, and only one cub survived, in 2004 received a mouse as a result of the “immaculate conception” of the mouse mother.

Surely each of you knows the story described in the Holy Scriptures. Mary, being God's chosen one, brought into the world an immaculately conceived child. Whether this actually happened or was just the result of the wild imagination of the authors of those times is difficult to say today. But let it be known to you that the virgin birth is quite common in our world. What is parthenogenesis and what is its essence?

Amazing world

Perhaps one of the greatest mysteries of our universe is the origin of life. Where it came from and who is the creator of everything is a sealed mystery. But whoever our creator was, he did a great job of ensuring that life on the blue planet never runs out. Its various forms inhabiting the Earth are capable of reproducing their own kind in a wide variety of, sometimes very unexpected, ways.

Parthenogenesis

What is parthenogenesis? This is the ability of a female to give birth to a new generation without the participation of a sexual partner - a male. This does not mean that males are not needed at all; they are, of course, important. Parthenogenesis is not a method asexual reproduction, as in some plants (budding, for example). But if it happens that a female for some reason could not find a partner for mating and fertilization of the egg did not occur, she will still be able to produce full-fledged offspring without his participation. This ability provides the species with very good survival. When numbers fall, females can replenish the population within a short time and continue the race. This is the essence of parthenogenesis.

Another important feature of such reproduction is the regulation of the ratio of the number of females and males. So, for example, in bees, drones (males) emerge from unfertilized eggs, and from fertilized eggs, workers emerge, which are all female.

Types of parthenogenesis

What is parthenogenesis and how can it occur in certain animals? In some species it is considered the main method of reproduction (obligate). For other forms, it is cyclical, that is, periodically offspring appear from unfertilized eggs, but more often with the participation of a male. The facultative, or emergency method of reproduction provides the species with survival in the most difficult living conditions; this is the essence of parthenogenesis for them. These cases are rather an exception, because usually such animals adhere to bisexual reproduction.

Parthenogenesis in animals

What is parthenogenesis? This is the process in which the parent egg, being unfertilized, begins to develop in order to subsequently turn into a full-fledged adult. Living being. Parthenogenesis may vary significantly between different types. So, for example, reproduction by parthenogenesis in bees is significantly different from the reproduction of other insects, say, ants.

Knowledge of what parthenogenesis is and how it occurs has significantly influenced the development of science and given impetus to the emergence of certain trends in the industry. Thus, scientists learned that silkworm parthenogenesis starts after exposure to certain temperatures. This significantly accelerated the process of breeding these insects.

The essence of parthenogenesis is well known to beekeepers and silk producers; many invertebrates use just this method. Some species of lizards and fish often practice this; the process is well known to representatives of the plant world; there are even parthenogenetic turkeys.

Representatives of science are working tirelessly to study this feature. Many attempts have been made to induce parthenogenesis in warm-blooded animals. Unfortunately, it is impossible to give examples, since in some cases cell growth and embryo development occurred, but before final stage it never came to fruition. There is also considerable interest from the medical side. A survey was conducted, after which it became known that the majority of married couples who cannot have a baby would happily decide on such an immaculate conception. Who knows, maybe over time the veil of secrecy will be lifted. And a miracle will come true - parthenogenesis will be able to give life to a human baby.

Parthenogenesis (from the Greek words parthenos - virgin and genesis - origin) is the development of an organism from only one egg without fertilization. Exist various shapes parthenogenesis in animals and plants.

In the 18th century Swiss scientist C. Bonnet described amazing phenomenon: Well-known aphids in summer are usually represented only by wingless females that give birth to live young. Only in autumn do males appear among the aphids. Fertilized eggs that survive the winter give rise to winged females. They scatter among food plants and establish new colonies of wingless parthenogenetic females. A similar development cycle has been described in many insects, as well as in small crustaceans - daphnia and microscopic aquatic animals - rotifers. In some species of rotifers and insects, no males were found at all - the sexual process was completely absent in them; all of them are represented by parthenogenetic females.

In plants, parthenogenesis was discovered later - first in the famous Australian plant Alhornea. This is a dioecious plant: on some specimens flowers with stamens develop, on others - with pistils. At Kew Botanic Gardens near London, only female plants with pistillate flowers grew. To the surprise of botanists, in 1839 they suddenly brought a bountiful harvest of seeds. It turned out that parthenogenesis occurs even more often in plants than in animals. In plants it is called apomixis. Many representatives of the Compositae and Poaceae, Rosaceae, cruciferous and other families (for example, many varieties of raspberries, common dandelions) are apomictic.

There are somatic and generative parthenogenesis. In the first case, the egg develops from a diploid cell of the body, with a double set of chromosomes, in the second, from cells that have undergone meiosis, i.e., with a halved number of chromosomes. Generative parthenogenesis is common in insects: bee drones, for example, develop from unfertilized eggs. Sometimes the number of chromosomes doubles during embryo development.

Peculiar forms of parthenogenesis are gynogenesis and androgenesis. During gynogenesis, the egg is stimulated to develop by the sperm of a male individual, even of a different species. Then the sperm dissolves without a trace in the cytoplasm of the egg, which begins development. As a result, single-sex populations consisting of only females appear. Gynogenesis has been described in the small tropical fish mollyfish, our silver crucian carp (its eggs develop when stimulated by the sperm of carp, minnow and other simultaneously spawning fish, in this case, when the zygote is fragmented, the paternal DNA is destroyed without affecting the characteristics of the offspring), as well as in some salamanders . It can be induced artificially by exposing mature eggs to sperm killed by X-ray irradiation. Naturally, the offspring produce exact genetic copies of the females.

During androgenesis, on the contrary, the nucleus of the egg does not develop. The development of the organism occurs due to two fused nuclei of spermatozoa that enter it (naturally, only one male is produced in the offspring). The Soviet scientist B.L. Astaurov obtained androgenetic male silkworms by fertilizing eggs with the sperm of a normal male, the nuclei of which were killed by irradiation or high temperature. Together with V. A. Strunnikov, he developed methods for artificially obtaining androgenetic offspring from the silkworm, which has a great practical significance, since male caterpillars produce more silk when forming cocoons than females.

Parthenogenesis is more common in lower animals. In more highly organized people, it is sometimes possible to induce it artificially, by the influence of some factors on unfertilized eggs. It was first caused in 1885 by the Russian zoologist A. A. Tikhomirov from the silkworm.

Nevertheless, in higher animals, parthenogenetic development most often does not go to completion and the developing embryo eventually dies. But some species and breeds of vertebrates are more capable of parthenogenesis. For example, parthenogenetic species of lizards are known. IN Lately A breed of turkeys was developed whose unfertilized eggs are highly likely to develop to completion. It is curious that in this case the offspring are male (usually parthenogenesis produces females). The riddle is easy to solve: if, for example, a person and a fruit fly have a set of sex chromosomes in the female sex XX (two X chromosomes), and in the male XY (chromosomes X and Y), in birds it is the other way around - the male has two identical chromosomes ZZ, and the female has different chromosomes (WZ). Half of the unfertilized eggs have one W chromosome, half have a Z chromosome (see Meiosis). In the developing parthenogenetic egg, the number of chromosomes doubles. But the W W combination is not viable, and such embryos die, but the ZZ combination gives a completely normal turkey.

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.).

In sexual reproduction, offspring are produced by the fusion of genetic material from haploid nuclei. Usually these nuclei are contained in specialized germ cells - gametes; During fertilization, the gametes fuse to form a diploid zygote, which during development produces a mature organism. Gametes are haploid - they contain one set of chromosomes resulting from meiosis; they serve as a link between this generation and the next (during sexual reproduction of flowering plants, not cells, but nuclei, merge, but usually these nuclei are also called gametes).

Meiosis - important stage life cycles, including sexual reproduction, since it leads to a halving of the amount of genetic material. Thanks to this, in a series of generations that reproduce sexually, this number remains constant, although during fertilization it doubles each time. During meiosis, as a result of random divergence of chromosomes (independent distribution) and the exchange of genetic material between homologous chromosomes (crossing over), new combinations of genes appear in one gamete, and such shuffling increases genetic diversity. The fusion of haploid nuclei contained in gametes is called fertilization or syngamy; it leads to the formation of a diploid zygote, that is, a cell containing one chromosome set from each parent. This combination of two sets of chromosomes in the zygote (genetic recombination) is genetic basis intraspecific variability. The zygote grows and develops into a mature organism of the next generation. Thus, during sexual reproduction in the life cycle, an alternation of diploid and haploid phases occurs, and in different organisms these phases take different forms.

Gametes usually come in two types, male and female, but some primitive organisms produce only one type of gamete. In organisms that produce two types of gametes, they can be produced by male and female parents, respectively, or it may be that the same individual has both male and female reproductive organs. Species in which there are separate male and female individuals are called dioecious; such are most animals and humans. Among flowering plants there are also dioecious species; if monoecious species have male and female flowers are formed on the same plant, as, for example, in cucumber and hazel, then in dioecious plants some plants bear only male, and others only female, flowers, like holly or yew.

Parthenogenesis

Parthenogenesis is one of the modifications of sexual reproduction in which the female gamete develops into a new individual without fertilization by the male gamete. Parthenogenetic reproduction occurs in both the animal and plant kingdoms and has the advantage of increasing the rate of reproduction in some cases.

A distinction is made between natural parthenogenesis, the normal method of reproduction of some organisms in nature, and artificial parthenogenesis, caused experimentally by the action of various stimuli on an unfertilized egg, which normally requires fertilization. Classification of parthenogenesis:

Obligate - when it is the only way of reproduction

Cyclic - parthenogenesis naturally alternates with other methods of reproduction in the life cycle (for example, in daphnia and rotifers).

Facultative - occurring as an exception or as a backup method of reproduction in forms that are normally bisexual.

There are two types of parthenogenesis - haploid and diploid, depending on the number of chromosomes in the female gamete. In many insects, including ants, bees and wasps, various castes of organisms arise within a given community as a result of haploid parthenogenesis. In these species, meiosis occurs and haploid gametes are formed. Some eggs are fertilized and develop into diploid females, while unfertilized eggs develop into fertile haploid males. For example, at honey bee The queen lays fertilized eggs (2n = 32), which develop to produce females (queens or workers), and unfertilized eggs (n = 16), which produce males (drones), which produce sperm by mitosis rather than meiosis. This mechanism of reproduction in social insects has adaptive significance, since it makes it possible to regulate the number of descendants of each type. In aphids, diploid parthenogenesis occurs, in which the female oocytes undergo a special form of meiosis without chromosome segregation - all chromosomes pass into the egg, and the polar bodies do not receive a single chromosome. The eggs develop in the mother's body, so that young females are born fully formed, rather than hatching from eggs. This process is called viviparity. It can continue for several generations, especially in the summer, until almost complete non-divergence occurs in one of the cells, resulting in a cell containing all pairs of autosomes and one X chromosome. From this cell the male develops parthenogenetically. These autumn males and parthenogenetic females produce haploid gametes through meiosis that participate in sexual reproduction. Fertilized females lay diploid eggs that overwinter, and in the spring they hatch into females that reproduce parthenogenetically and give birth to live offspring. Several parthenogenetic generations are followed by a generation resulting from normal sexual reproduction, which introduces genetic diversity into the population through recombination. The main advantage that parthenogenesis gives to aphids is the rapid growth of the population, since all its mature members are capable of laying eggs. This is especially important during periods when environmental conditions are favorable for the existence of a large population, i.e. during the summer months.

Parthenogenesis is widespread in plants, where it takes various forms. One of them, apomixis, is parthenogenesis, simulating sexual reproduction. Apomixis is observed in some flowering plants in which the diploid ovule cell, or nucellus cell, or megaspore develops into a functional embryo without the participation of a male gamete. The rest of the ovule forms the seed, and the ovary develops into the fruit. In other cases, the presence of a pollen grain is required, which stimulates parthenogenesis, although it does not germinate; the pollen grain induces hormonal changes necessary for the development of the embryo, and in practice such cases are difficult to distinguish from true sexual reproduction.

Fertilization occurs in a unique way in flowering plants. After fertilization, the ovule produces a seed containing an embryo and a supply of nutrients. How is the supply of nutrients formed in the seed?

In flowering plants, double fertilization occurs. During pollination, the pollen grain lands on the stigma of the pistil and germinates, forming a pollen tube. It is formed from a vegetative cell and grows quickly, reaching the ovary. At the end of the pollen tube there are two sperm cells.

Unlike motile sperm lower plants Sperm in flowering plants are immobile and can penetrate to the egg only through the pollen tube.

The pollen tube grows into the ovule, its tip ruptures, and the sperm enter the embryo sac. One of them fuses with the egg. A diploid cell is formed - a zygote. The second sperm fuses with the diploid secondary nucleus of the embryo sac. As a result, a cell is formed with a triple set of chromosomes, from which endosperm is formed through repeated mitoses - tissue containing a supply of nutrients.

Hermaphroditism

Conjugation

Conjugation (Latin “conjugatio” - connection) is a form of sexual process without the participation of gametes. Characteristic of Escherichia coli (division Bacteria), slipper ciliates (type Protozoa), in which two single-celled individuals come together and exchange genetic material through a cytoplasmic bridge.

Fig.4

As a result of conjugation, bacteria do not increase the number of individuals. In the green alga Spirogyra, conjugation occurs differently: two multicellular filaments stand parallel to each other, form opposing cytoplasmic bridges, along which the protoplast of a physiologically male individual flows into the female filament. As a result, many zygotes are formed.

Copulation

Some single-celled organisms experience a type of sexual process called copulation. Copulation (from the Latin “copulatio” - connection) is the process of fusion of two germ cells.

During copulation (in protozoa), the formation of sexual elements and their pairwise fusion occur. In this case, two individuals acquire sexual differences and completely merge, forming a zygote. Combination and recombination of hereditary material occurs, so individuals are genetically different from their parents.