Aphid reproduction parthenogenesis. Parthenogenesis, forms and meaning. See what "Parthenogenesis" is in other dictionaries
Most representatives of the animal and plant world are divided into males and females. As a result of the mixing of the genetic material of the parents, the offspring are more likely to survive and adapt to the conditions of a constantly changing environment. However, there is also a reverse way. Sometimes female individuals, 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 sexual methods - parthenogenesis. What it is? What are the types of this phenomenon? We will talk about this in the article.
Two decks or one
To explain the difference between asexual (mitosis) and sexual (meiosis) cell division, let's 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 one 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 descendants is completely identical to that of the parent cell. During meiosis, the final product of division will be germ cells (gametes) with a half haploid chromosome set - each with one deck of cards, and with different "shirts".
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 deck from the father, the other from the mother), characteristic of a particular organism. But in some cases, the 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" - "genesis, development." In nature, parthenogenetic reproduction is not so common and is called natural. What is artificial parthenogenesis? This is the division of the egg, caused by exposure to various agents and normally requiring fertilization.
Types of parthenogenesis
The classification of parthenogenesis is based on various criteria for comparison.
I can do it myself, I can do it with a partner
When the presence of different forms of reproduction in the life cycle of an organism is taken as a criterion, three types of parthenogenesis are classified: obligate, cyclic and facultative. Obligate, or permanent parthenogenesis, is the only reproduction inherent in a given organism. Cyclic is the one that alternates with the actual sexual. 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 well-known bees as an example. In early spring, the uterus hatches from the chrysalis and rushes into the mating flight, when it is fertilized by many males (drones). But their sperm accumulates in the spermatheca of the queen bee, and it is with it that she will fertilize the eggs she lays all her life. Or it won't. When an egg passes through the female's oviduct, the duct of the seed receptacle opens and fertilizes it - a female appears from a diploid embryo, and whether she becomes a queen or a worker bee depends on what the worker bees will feed the larva. If the spermatic duct does not open, the egg will remain unfertilized and a haploid male drone will grow out of it. A similar cycle occurs in aphids and ants.
Biological benefits
Despite the undeniable advantages of sexual reproduction, parthenogenesis has its advantages. If environmental conditions are favorable and sufficient food is available, then such a method of reproduction, when each individual leaves offspring, provides advantages, expressed in the rate of settlement of specific biotopes. When environmental conditions change in an unfavorable direction, you can sacrifice the quantity, but improve the quality of the offspring by switching to sexual reproduction. This is what facultative parthenogenesis is. It is inherent in arthropods, amphibians, reptiles and birds.
Lonely mother shark
It is very rare that parthenogenesis becomes a real miracle. For example, in the case of sharks, only one method of reproduction was known - sexual. But in 2001, a hammerhead shark from the Nebraska Zoo in the United States suddenly gave birth to a baby shark, and this despite the fact that she lived alone in the aquarium for many years. Biologists were baffled by this event. The accidental death of a shark, which was stung by a poisonous stingray, allowed to clarify the situation. Genetic analysis showed that the cub was born through true parthenogenesis. Apparently, in the body of the mother shark, mechanisms unknown to science for the preservation of the species at the boundaries of the range were turned on. Or the mother shark was very lonely.
Competing with God
The topic of the immaculate, virginal 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 human parthenogenesis? Geneticists unequivocally and categorically say: "No!" After all, if it was parthenogenetic reproduction, Jesus must have been ... a girl. 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 associated with sex-related genes (sex markers). This means that the inclusion of certain genes depends on the quality of the gene material of both the mother and the father. Of course, if specialists in genetic engineering do not take up the matter.
It was the Japanese experts who, having conducted more than 600 experiments, of which 24 ended in pregnancy, and of which only 2 ended in childbirth, and only one cub survived, in 2004 they received a mouse as a result of the “immaculate conception” of a mouse mother.
Parthenogenesis is also called virgin reproduction, this process is typical for species in which a short life cycle is accompanied by pronounced seasonal changes.
Androgenesis and gynogenesis
In the process of adrogenesis, the female germ cell does not take part in the development of a new organism, which appears as a result of the fusion of two nuclei of male germ cells - spermatozoa. In this case, only males are present in the offspring. In nature, androgenesis occurs in Hymenoptera insects.
During gynogenesis, the sperm nucleus does not merge with the nucleus of the egg, it can only stimulate its development, the so-called false fertilization occurs. This process is characteristic of bony fish and roundworms, while the offspring consists only of females.
Haploid and diploid parthenogenesis
In haploid parthenogenesis, the organism develops from a haploid egg, and individuals can be female, male, or both, it all depends on the chromosomal sex determination in this species. In ants, bees and wasps, as a result of parthenogenesis, males appear from unfertilized eggs, and females from fertilized ones. Due to this, organisms are divided into castes, the process allows you to regulate the number of descendants of a certain type.
In some lizards, aphids and rotifers, diploid parthenogenesis is observed, it is also called somatic. In this case, diploid eggs are formed in females. This process allows to maintain the number of individuals if the meeting of individuals of different sexes is difficult.
Natural and artificial parthenogenesis
Parthenogenesis is cyclical in rotifers, aphids, and daphnia. In summer, only females exist, they develop parthenogenetically, and in autumn, reproduction takes place with fertilization.
Parthenogenesis can be artificially induced, for example, by irritating the surface of silkworm eggs, by heating or by exposure to various acids, and crushing of the egg can be achieved without fertilization. Parthenogenetically managed to obtain adult rabbits and frogs.
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 the generally accepted biological terminology. Asexual is the appearance of new individuals from the somatic cells of the mother's organism, and not from the sex cells, as happens during parthenogenesis. Thus, at present, parthenogenesis is usually attributed to sexual, since in its process, daughter individuals are formed from, and not from parts of the body of the “mother”, as, for example, in the simple division of bacteria, budding of yeast, body segmentation in flatworms, etc.
The phenomenon of parthenogenesis in most cases is 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 reports, there were cases when dead women were found to have early pregnancy, and when examining the fetus, it turned out that the embryo represents a complete genetic copy of the mother. However, if such a phenomenon is possible in higher animals (meaning, under natural conditions), then the full development of the egg never occurs, it usually stops at the blastula stage. (author's note) (photo)
Relatively often, this phenomenon is observed among insects. For the most part, these creatures are dioecious, which even at first glance can be guessed from the sex of many species, but sometimes parthenogenesis is combined with the classical sexual or even completely replaces it.
Parthenogenesis as a biological process
The cytological basis of this phenomenon is different. In some cases, there is a "violation" of 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 spermatozoa. For example, there is such a formation as a directional (polar) body. It is attached to the egg, contains a small amount of cytoplasm and genetic material. In the "normal", that is, during sexual, it is separated from after a certain number of meiotic divisions. In some parthenogenetic individuals, for example, the worm Lecanium, the body does not degenerate and does not detach, but penetrates and merges with the nucleus of the egg, imitating the penetration of the spermatozoon 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 worms, spermatozoa are stored in a special chamber, from where the female may or may not release them on the egg, depending on the “purpose” of laying. (photo)
Varieties of parthenogenesis
Parthenogenesis is a very heterogeneous phenomenon, which is divided into several categories.
Sporadic: most of the time, bisexual individuals reproduce in the “usual” way, but when certain conditions are created (population decline, lack of males), they can also switch to parthenogenesis. This phenomenon is typical for 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 die without completing their development.
Constant: observed all the time, along with the sexual form. A typical example is the social Hymenoptera, in which males always develop from unfertilized ones, and females from fertilized ones. In some cases, parthenogenesis completely or almost completely replaces the sexual. So, in some species of stick insects, mealybugs, gall wasps 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, male Kisteviks (centipedes) are often found in France (42% of individuals), while in Holland they are only 39%, in Denmark - 8%, and further north they are not at all.
Cyclical: there is a correct alternation of sexual and asexual generations, as, for example, in. In them, the fertilized survives the winter, after which a virgin female emerges from it, giving another row, also breeding parthenogenetically. In autumn, males also hatch, which mate and lay, starting a new round of the life cycle. (photo)
Artificial: this category can be considered as a kind of sporadic parthenogenesis, but it does not occur in nature. The essence of this form lies in the fact that individuals that reproduce by the “normal” sexual way 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 the virgin
The concept of parthenogenesis
During fertilization, the spermatozoon brings the egg out of its dormant state and it begins to develop. But in nature, cases are known when an organism developed from an unfertilized egg.
Definition 1
The 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 (in lizards) can exist. For other species, parthenogenesis is the only mode of reproduction (in stick insects). In ground beetles and daphnia, sexual and parthenogenetic generations naturally alternate.
Some scientists consider parthenogenesis a separate form of asexual reproduction, since there is no sexual process (copulation) here. Others consider it a variant of sexual reproduction, since it is the sex cells that take part in it.
Diploid parthenogenesis
There are a number of animal species in which, over a certain period, the development of unfertilized eggs occurs. In this case, the number of chromosomes in the nucleus of the egg is doubled and they become diploid (or meiosis does not occur during the formation of the egg).
Example 1
For example, in the already mentioned above ground beetles and daphnia during spring, summer, 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 takes place. The fertilized eggs endure the winter. In the spring, females capable of parthenogenetic reproduction again develop from them.
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. Of these, working individuals (underdeveloped females) and uterus are formed. Drones (parthenogenetic males) develop from unfertilized eggs. Male cells have a haploid set of chromosomes. During the formation of spermatozoa, meiosis does not occur and the number of chromosomes in spermatozoa does not decrease. Therefore, during fertilization, organisms receive a diploid set of chromosomes.
artificial parthenogenesis
Scientists-embryologists in the course of research was able to stimulate the development of the egg without fertilization. They used some stimuli as a stimulating factor (chemical, mechanical or short-term effects 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.
Remark 1
The phenomenon of artificial parthenogenesis is actively used, for example, to adjust the sex of the silkworm in sericulture.
Androgenesis
In science, there are cases when the nucleus of the 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 cell developed further parthenogenetically, but with a spermatozoon nucleus. The resulting new organism had only paternal characteristics. This phenomenon is known in science as androgenesis .
The phenomenon of parthenogenesis arose, probably, as a reaction of the organism to abrupt 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 has allowed species to survive in unfamiliar and changing conditions. The method of parthenogenesis can be very useful in breeding work.
Parthenogenesis ( Parthenogenesis- from Greek. parthenos- girl, virgin + genesis- genesis) - a form of sexual reproduction in which the development of the body occurs from the female germ cell (ovum) without fertilization of its male (spermatozoon).
In cases where parthenogenetic species are represented (always or periodically) only by females, one of the main biological advantages of parthenogenesis is to accelerate the rate of reproduction of the species, since all individuals of such species are able to leave offspring. In cases where females develop from fertilized eggs, and males develop from unfertilized eggs, parthenogenesis contributes to the regulation of the numerical ratios of sexes (for example, in bees).
Parthenogenesis must be distinguished from asexual reproduction, which is always carried out with the help of somatic organs and cells (reproduction by division, budding, etc.).
Distinguish parthenogenesis natural- a normal way of reproduction of some organisms in nature and artificial, caused experimentally by the action of various stimuli on an unfertilized egg, which normally needs to be fertilized.
Parthenogenesis in animals
The original form of parthenogenesis - rudimentary, or rudimentary parthenogenesis - is characteristic of many animal species in cases where their eggs remain unfertilized. As a rule, rudimentary parthenogenesis is limited to the initial stages of embryonic development; however, sometimes development reaches the final stages.
At androgenesis the nucleus of the female germ cell (ovum) does not participate in development, and the new organism develops from two merged nuclei of the male germ cells (spermatozoa). Natural androgenesis occurs in nature, for example, in Hymenoptera insects - riders. Artificial androgenesis is used to obtain offspring from the silkworm: during androgenesis, only males are obtained in the offspring, and male cocoons contain significantly more silk than female cocoons.
When gynogenesis the sperm nucleus does not merge with the nucleus of the egg, but only stimulates its development (false fertilization). Gynogenesis is characteristic of roundworms, bony fish and amphibians. In this case, only females are obtained in the offspring.
At human there are 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 sources, there are 16 cases of immaculate conception in history that took place in Africa and European countries).
The material was prepared on the basis of information from open sources
