What organisms reproduce by parthenogenesis? Types of sexual reproduction of multicellular organisms
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 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, early autumn (i.e. most years) 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 promotes rapid reproduction 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 as a stimulating factor (chemical, mechanical or short-term influences of high or low temperatures etc.). 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 sex silkworm 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 central position in the egg. The egg developed further parthenogenetically, but with a sperm nucleus. Formed 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.
In animals, dioeciousness is more common, i.e. the presence of male and female individuals (males) and (females), which often differ in size and appearance (sexual dimorphism).
Sex cells are formed in special organs - gonads. Small, equipped with a flagellum, motile spermatozoa are formed in testes, and large ones are motionless eggs(eggs) - V ovaries.
The process of fertilization multicellular organisms, as in unicellular organisms, consists in the fusion of male and female gametes. As a rule, then the fusion of their nuclei immediately occurs with the formation of a diploid zygote (fertilized egg) (Fig. 1).
Rice. 1. Diagram illustrating the mechanism of maintaining the diploid set of chromosomes during sexual reproduction
The formed zygote combines in its nucleus the haploid sets of chromosomes of the parent organisms. In a daughter organism developing from a zygote, the hereditary characteristics of both parents are combined.
In multicellular organisms there are external fertilization(when gametes fuse outside the body) and internal fertilization , occurring inside the parent organism. External application can only be carried out in aquatic environment, so it is most widely found in aquatic organisms(algae, coelenterates, fish). Terrestrial organisms are more often characterized by internal fertilization (higher seed plants, insects, higher vertebrates).
Atypical sexual reproduction
We will talk about parthenogenesis, gynogenesis, androgenesis, polyembryony, double fertilization in angiosperms.
Parthenogenesis (virgin reproduction)
Opened in the middle of the 18th century. Swiss naturalist C. Bonnet. Parthenogenesis occurs in plants and animals. With it, the development of a daughter organism is carried out from an unfertilized egg. Moreover, the resulting daughter individuals, as a rule, are either male (drones in bees) or female (in Caucasian rock lizards), in addition, descendants of both sexes (aphids, daphnia) can be born. The number of chromosomes in parthenogenetic organisms can be haploid (male bees) or diploid (aphids, daphnia).
The meaning of parthenogenesis:
1) reproduction is possible with rare contacts different-sex individuals;
2) the population size increases sharply, since the offspring are usually numerous;
3) occurs in populations with high mortality rate within one season.
Types of parthenogenesis:
1) obligate (obligatory) parthenogenesis. 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;
2) cyclic (seasonal) parthenogenesis (in aphids, daphnia, rotifers). Occurs in populations that have historically gone extinct in large quantities V certain time of the year. In these species, parthenogenesis is combined with sexual reproduction. At the same time, in summer time There are only females that 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 emerge from them; facultative (optional) parthenogenesis. 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 species, parthenogenesis exists to regulate the sex ratio in the population.
There are also natural (exists in natural populations) and artificial (used by humans) parthenogenesis. This type of parthenogenesis was studied by V.N. Tikhomirov. He achieved the development of unfertilized silkworm eggs by irritating them with a thin brush or immersing them in sulfuric acid for a few seconds (only females are known to produce silk thread).
Gynogenesis(y bony fish and some amphibians). The sperm penetrates the egg and only stimulates its development. In this case, the sperm nucleus does not merge with the egg nucleus and dies, and the source of hereditary material for the development of the offspring is the DNA of the egg nucleus.
Androgenesis. The development of the embryo involves the male nucleus introduced into the egg, and the nucleus of the egg dies. The egg cell provides only nutrients from 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.
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.
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. The development of individuals of these three types in the honey bee is schematically presented in Fig. 4. This mechanism of reproduction in social insects has adaptive significance, since it allows you to regulate the number of descendants of each type.
In aphids, diploid parthenogenesis occurs, in which the female's oocytes undergo special form meiosis without chromosome segregation - all chromosomes pass into the egg, and 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 nondisjunction 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, which overwinter, and in the spring they hatch into females that reproduce parthenogenetically and give birth to living 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 fast growth population size, since all its sexually mature members are capable of laying eggs. This is especially important during periods when environmental conditions are favorable for the existence large population, i.e. in the summer months.
Parthenogenesis is widespread in plants, where it takes various shapes. One of them, apomixis, is parthenogenesis, simulating sexual reproduction. Apomixis is observed in some flowering plants in which the diploid ovule cell, either a nucellus cell or a 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.
To the beginning individual development precedes the emergence of germ cells, i.e. gametogenesis, which can be considered progenesis during individual development.
The process of development of female germ cells is called ovogenesis (oogenesis). Unlike spermatogenesis, it has some features. The course of oogenesis and its differences from the development of male gametes are shown in Fig. 3.
There are 3 periods in oogenesis: reproduction, growth and maturation. Undifferentiated female germ cells - oogonia - reproduce in the same way as spermatogonia, through normal mitosis. After division, they become first-order oocytes and enter the growth period.
The growth of oocytes lasts a very long time - weeks, months and even years. During the growth period, two stages are distinguished: small, or slow growth, when new substances are assimilated and the cytoplasm is enriched with them, and large, or fast growth, when nutrient yolk accumulates in the cell. The nucleus also undergoes profound changes during the growth period; it swells greatly, its contents seem to blur. Cell sizes increase enormously (for example, perch eggs increase almost a million times).
Then the first order oocyte enters the period of maturation, or meiosis. Here, too, reduction and equational divisions take place. The division processes in the nucleus proceed in the same way as during meiosis of spermatocytes, but the fate of the cytoplasm is completely different. During reduction division, one nucleus carries with it most of the cytoplasm, and only a small part of it remains for the share of the other. Therefore, only one full-fledged cell is formed - an oocyte of the second order, and a second tiny one - a directional, or reduction, body, which can be divided into two reduction bodies.
During the second, equational division, the asymmetrical distribution of the cytoplasm is repeated and again one large cell is formed - the ovotide and the third polar body. The ovotide, in terms of its nuclear composition and functionality, is a completely mature germ cell.
The formation period, unlike spermatogenesis, is absent in oogenesis. Thus, in oogenesis, only one mature egg arises from one oogonia. Polar bodies remain underdeveloped and soon die and are phagocytosed by other cells. Mature female gametes are called ova or eggs, and those deposited in water are called caviar.
Features of oogenesis in humans are presented in Fig. 5. The development of female germ cells occurs in the ovaries. The period of reproduction begins in oogonia while still in the embryo and stops by the time the girl is born. The period of growth during oogenesis is longer, because in addition to preparation for meiosis, accumulation of reserves occurs nutrients, which will be necessary in the future for the first divisions of the zygote. In the small growth phase, formation occurs large quantity different types RNA. Rapid accumulation of RNA occurs due to special mechanism– gene amplification (multiple copying of individual DNA sections encoding ribosomal RNA). Fast increase The mRNA proceeds through the formation of “lampbrush” chromosomes. As a result, more than a thousand additional nucleoli are formed, which are a necessary structure for the synthesis of rRNA, from which ribosomes involved in protein synthesis are subsequently formed. During the same period, meiotic chromosome transformations occur in the oocyte, characteristic of the prophase of the first division.
During tall ovarian follicular cells form several layers around the first-order oocyte, which facilitates the transfer of nutrients synthesized elsewhere into the cytoplasm of the oocyte.
In humans, the growth period of oocytes can be 12–50 years. After completion of the growth period, the first order oocyte enters the maturation period.
During the period of oocyte maturation (as well as during spermatogenesis), meiotic cell division occurs. During the first reduction division, from an oocyte of the first order, one oocyte of the second order (1n2C) and one polar body (1n2C) are formed. During the second equational division, a mature egg cell (1n1C) is formed from a second-order oocyte, which has retained almost all the accumulated substances in the cytoplasm, and a second polar body of small size (1n1C). At the same time, division of the first polar body occurs, giving rise to two second polar bodies (1n1C).
As a result, during oogenesis, 4 cells are obtained, of which only one will later become an egg, and the remaining 3 (polar bodies) are reduced. The biological significance of this stage of oogenesis is to preserve all the accumulated substances of the cytoplasm around one haploid nucleus to ensure normal nutrition and development of the fertilized egg.
During oogenesis in women, at the stage of the second metaphase, a block is formed, which is removed during fertilization, and the maturation phase ends only after the sperm penetrates the egg.
The process of oogenesis in women is a cyclical process, repeating approximately every 28 days (from the period of growth until just after fertilization). This cycle is called menstrual.
Distinctive features spermatogenesis and oogenesis in humans are presented in the table. The most obvious distinguishing feature eggs are hers big sizes. A typical egg cell is spherical or oval shape, and its diameter in humans is about 100 microns (the size of a typical somatic cell is about 20 microns). The size of the nucleus can be just as impressive; in anticipation of the rapid divisions immediately following fertilization, reserves of proteins are deposited in the nucleus.
The cell's need for nutrients is satisfied mainly by the yolk, a protoplasmic material rich in lipids and proteins. It is usually found in discrete structures called yolk granules. Another important specific structure of the egg is the outer egg membrane - a covering of a special non-cellular substance consisting mainly of glycoprotein molecules, some of which are secreted by the egg itself, and the other part by surrounding cells. In many species, the membrane has an inner layer directly adjacent to the plasma membrane of the egg and is called the zona pellucida in mammals and the vitelline layer in other animals. This layer protects the egg from mechanical damage, in some eggs it also acts as a species-specific barrier to sperm, allowing only sperm of the same species or very closely related species to penetrate.
Many eggs (including mammals) contain specialized secretory vesicles located under plasma membrane in the outer, or cortical, layer of the cytoplasm. When the egg is activated by sperm, these cortical granules release the contents by exocytosis, as a result of which the properties of the egg membrane change in such a way that other sperm cannot penetrate through it. The process of formation of male germ cells is spermatogenesis. As a result, sperm are formed.
Somatic cells, having reached a certain mature physiological state, divide mitotically (sometimes by amitosis), while germ cells in their development undergo special phases of transformation until they mature and become capable of fertilization. This difference has a deep biological meaning. Somatic cells must retain the entire amount of hereditary information during divisions so that the daughter cells remain the same as the mother cells. The transfer of information is ensured during mitosis by the precise distribution of chromosomes between dividing cells: the number of chromosomes, their biological structure, DNA content and, consequently, the hereditary information contained in it are preserved in a number of cell generations, ensuring the constancy of the structure of the individual and the species.
During fertilization, the nuclei of male and female germ cells unite into a common nucleus, and if there were as many chromosomes in each as in somatic cells, then in the zygote it would double, and such a double number would pass into all cells of the developing embryo. In the future, during the development of the germ cells of the next generations of young organisms, there will be a sequential accumulation of chromosomes in the cells, and the species could not preserve its hereditary characteristics unchanged. In addition, the nuclear-plasma coefficient in favor of the nucleus would gradually be disrupted, and after several generations a moment would come when the addition of chromosomes to the nucleus would lead to the inevitable death of the cell. As a result, fertilization would serve not to preserve, but to destroy organisms. However, this does not happen, since the process of gametogenesis includes two special divisions, during which the number of chromosomes in the nuclei of both the male and female germ cells is halved. Intracellular processes associated with a decrease in the number of chromosomes constitute the essence of maturation of germ cells - the essence of meiosis. During fertilization, half the number of chromosomes in the nuclei of the father's cells and half the number of chromosomes in the nuclei of the mother's cells combine, and the characteristic properties of the zygote are restored. this species set of chromosomes.
There are 4 periods in spermatogenesis: reproduction, growth, maturation (meiosis) and formation (Fig. 3).
During the reproduction period, the original undifferentiated germ cells - spermatogonia, or gonia - divide through normal mitosis. After making several such divisions, they enter a period of growth. At this stage, they are called first-order spermatocytes (or I spermatocytes). They intensively assimilate nutrients, enlarge, undergo a deep physical and chemical restructuring, as a result of which they prepare for the third period - maturation, or meiosis.
In meiosis, spermatocytes I undergo two processes of cell division. In the first division (reduction), the number of chromosomes decreases (reduction). As a result, from one cyte I, two equal-sized cells arise - spermatocytes of the second order, or cytes II. Then comes the second division of maturation. It proceeds like ordinary somatic mitosis, but with a haploid number of chromosomes. Such a division is called equational (“equatio” - equality), since two identical divisions are formed, i.e. completely equivalent cells called spermatids.
In the fourth period - formation - the rounded spermatid takes on the shape of a mature male reproductive cell: a flagellum grows, the nucleus becomes denser, and a shell is formed. As a result of the entire process of spermatogenesis, from each initial undifferentiated spermatogonia, 4 mature germ cells are obtained, each containing a haploid set of chromosomes.
In Fig. Figure 4 shows a diagram of the processes of spermatogenesis and spermiogenesis in humans. Spermatogenesis occurs in the convoluted seminiferous tubules of the testes. The development of sperm begins during the period of prenatal development during the laying of generative tissues, then resumes during the onset of puberty and continues until old age.
During the breeding season, a series of successive mitoses occur, resulting in an increase in the number of cells called spermatogonia. Some spermatogonia enter a period of growth and are called first-order spermatocytes.
The period of growth corresponds to the period of interphase of the cell cycle, in which the hereditary material of first order spermatocytes (2n4C) is doubled, and then they enter prophase I of meiotic division. During prophase I, conjugation of homologous chromosomes and exchange between homologous chromatids (crossing over) occurs. Crossing over has important genetic significance because it results in genetic differences between individuals.
Rice. 3. Scheme of gametogenesis:
1st – reproduction period: cells divide mitotically, the set of chromosomes in them is 2n; 2nd – growth period: accumulation of nutrients in cells, a set of chromosomes 2n; 3rd – period of maturation – meiosis: a) 1st, or reduction, division, formation from diploid cells with a set of chromosomes equal to 2n, cells with a haploid set equal to n; b) 2nd division of meiosis, proceeds as mitosis, but in cells with a haploid set of chromosomes; 4th – period of formation – occurs only in spermatogenesis
The maturation period occurs in two stages, which corresponds to the I meiotic (reduction) and II meiotic (equational) divisions. In this case, from one first-order spermatocyte, two second-order spermatocytes (1n2C) are first obtained, then 4 spermatids (1n1C). Spermatids differ from each other in the set of chromosomes: they all contain 22 autosomes, but half of the cells contain an X chromosome, and the other half a Y chromosome. Autosomes differ from each other and from their parents in a different combination of alleles, since an exchange occurred during crossing over.
During the formation period, the number of cells and the number of chromosomes in them does not change, because During this period, 4 spermatids are formed from 4 spermatozoa, in which morphological reorganization of cellular structures occurs and a tail is formed. In humans, this phase lasts 14 days.
Male germ cells do not develop singly; they grow in clones and are interconnected by cytoplasmic bridges. Cytoplasmic bridges exist between spermatogonia, spermatocytes and spermatids. At the end of the formation phase, spermatozoa are freed from cytoplasmic bridges.
In humans, the maximum daily sperm productivity is 10 8, the duration of sperm existence in the vagina is up to 2.5 hours, and in the cervix up to 48 hours.
The sperm is an elongated, motile cell. The main spermatozoa are the nucleus, which occupies the main volume of the head, and the organ of movement – the flagellum, which makes up the tail. The spermatozoon is a motile nucleus. The structure of the sperm is determined primarily by its functions.
The sperm has very little cytoplasm, but there are several supporting structures:
1) mitochondria, which provide it with energy
2) acrosome, an organelle similar to a lysosome and containing enzymes necessary for the penetration of sperm into the egg
3) centriole - the beginning of the flagellum, during fertilization it is used during the first division of the zygote
The acrosome is located in front of the nucleus in the head, and the centriole and mitochondria are in the middle part of the cell. The nucleus contains a haploid set of chromosomes (see also CELL), it is dense, condensed; the long flagellum is similar in structure to the flagella of protozoa and the cilia of the ciliated epithelium of multicellular animals.
Sperm are very tenacious cells and under appropriate conditions (in the uterus) they remain viable for up to five days.
DIFFERENCES IN SPERMATOGENESIS FROM OVOGENESIS IN HUMANS
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.).
Parthenogenesis- form sexual reproduction, in which the eggs of females develop into a new organism without prior fertilization.
Terminology
Previously, many authors (for example, B.N. Shvanvich) defined parthenogenesis as a variant of the asexual form, although this contradicted generally accepted biological terminology. Asexuality is the emergence of new individuals from the somatic cells of the mother’s body, and not from the sexual ones, as happens during parthenogenesis. Thus, at present, parthenogenesis is usually classified as sexual, since in its process daughter individuals are formed from, and not from parts of the “mother’s” body, as, for example, with simple division bacteria, yeast budding, body segmentation flatworms etc.
The phenomenon of parthenogenesis is in most cases observed in primitive organisms, although, in general, it is found among many representatives of the animal world: Arthropods, Mollusks, Fish and even Reptiles. An interesting assumption about the existence of parthenogenesis in humans: according to unconfirmed data, there were cases when dead women discovered pregnancy early dates, and when examining the fetus, it turned out that the embryo represents a complete genetic copy of the mother. However, even if such a phenomenon is possible in higher animals (meaning, in natural conditions), then complete development of the egg never occurs; it usually stops at the blastula stage. (author's note) (photo)
This phenomenon is observed relatively often among insects. For the most part, these creatures are dioecious, which can be guessed even at first glance by the sex of individuals of many species, but sometimes parthenogenesis is combined with classical sex or even completely replaces it.
Parthenogenesis as a biological process
The cytological basis of this phenomenon varies. In some cases, there is a “disturbance” in the development of a normal egg, for example, a change in the number of divisions of the genetic material. In others, other structures take on the role of sperm. For example, there is such a formation as a directional (polar) body. It is attached to the egg and contains a small amount of cytoplasm and genetic material. In the “norm”, that is, during sexual intercourse, it is separated from after a certain number of meiotic divisions. In some parthenogenetic individuals, for example, the Lecanium scale insect, the body does not degenerate or detach, but penetrates inside and merges with the nucleus of the egg, imitating the penetration of the sperm and giving impetus to the development of the embryo.
Parthenogenesis seems to be a phenomenon that does not depend on the “will” of the insect. However, in some cases, individuals themselves control the forms of their own. In some hymenoptera (honey bees), as well as in the Californian race of scale insects, sperm are stored in a special chamber, from where the female may or may not release them onto the egg, depending on the “purpose” of laying eggs. (photo)
Varieties of parthenogenesis
Parthenogenesis is a very heterogeneous phenomenon that is divided into several categories.
Sporadic: most of the time, bisexual individuals reproduce in the “normal” way, but when certain conditions are created (decrease in population size, absence of males) they can switch to parthenogenesis. This phenomenon characteristic of the Poplar Hawk Moth and other insects, primarily Lepidoptera. In rare cases, sporadic parthenogenesis is observed in spiders, for example, tropical harvestmen, but usually their unfertilized ones die without completing their development.
Constant: observed all the time, along with the sexual form. Typical example- social hymenoptera, in which males always develop from unfertilized ones, and females - from fertilized ones. In some cases, parthenogenesis completely or almost completely replaces sexuality. Thus, in some species of stick insects, scale insects, gall moths and sawflies, males are either rare or completely unknown. A similar phenomenon occurs among ticks.
There are organisms in which the frequency of occurrence of males varies depending on the habitat. For example, males of Cysts (centipedes) are often found in France (42% of individuals), while in Holland they are only 39%, in Denmark - 8%, and in further promotion to the north there are none at all.
Cyclical: there is a correct alternation of sexual and asexual generations, as, for example, in. In them, the fertilized one survives the winter, after which a virgin female emerges from it, giving rise to another series that also reproduce parthenogenetically. In the fall, males also hatch, mate and lay eggs, starting new round life cycle. (photo)
Artificial: This category can be considered a type of sporadic parthenogenesis, but does not occur in nature. The essence of this form is that individuals that reproduce “normally” sexually switch to parthenogenesis when exposed to special physical (electricity, temperature) and chemical factors. This phenomenon was first discovered in 1886.
Pedogenesis: a type of parthenogenesis in which virgin