Origin of sexual reproduction. The direction of the evolution of sexual reproduction The emergence of a new organism during sexual reproduction

Reproduction is the reproduction by an organism of similar organisms. Thanks to him, continuity of life is ensured. There are two ways to form new organisms: asexual and sexual reproduction. Asexuality, in which only one organism participates, occurs through cell division in half, sporulation, budding, or vegetatively. It is characteristic mainly of primitive organisms. In asexual reproduction, the new organisms are a copy of the parent. Sexual reproduction occurs with the help of sex cells called gametes. It mainly involves two organisms, which contributes to the emergence of new individuals that differ from the parent ones. Many animals are characterized by alternating asexual and sexual reproduction.

Types of sexual reproduction

There are the following types of sexual reproduction:

bisexual;
hermaphrodite;
parthenogenesis, or virgin reproduction.

Dioecious reproduction

Dioecious reproduction is characterized by the fusion of haploid gametes, which is called fertilization. Fertilization results in a diploid zygote containing genetic information from both parents. Dioecious reproduction is characterized by the presence of a sexual process.

Types of sexual process

There are three types of sexual process:

Isogamy. It is characterized by the fact that all gametes are mobile and have the same size.
Anisogamy or heterogamy. Gametes have different sizes; there are macrogametes and microgametes. But both gametes are capable of movement.
Oogamy. It is characterized by the presence of a large immobile egg and a small sperm capable of movement.

Hermaphroditism

Parthenogenesis

Some organisms are able to develop from an unfertilized cell. This sexual reproduction is called parthenogenesis. With its help, ants, bees, wasps, aphids and some plants reproduce. A type of parthenogenesis is pedogenesis. It is characterized by virgin reproduction of larvae. Some dipterans and beetles reproduce using pedogenesis. Parthenogenesis ensures a rapid increase in population size.

Plant propagation

Plants, like animals, can reproduce asexually and sexually. The difference is that sexual reproduction in angiosperms occurs through double fertilization. What is it? In double fertilization, discovered by S.G. Navashin, two sperm take part in the fertilization of the egg. One of them unites with the egg. This produces a diploid zygote. The second sperm fuses with the diploid central cell to form a triploid endosperm containing a supply of nutrients.

Biological meaning of sexual reproduction

Sexual reproduction makes organisms resistant to changing and unfavorable environmental conditions and increases their viability. This is facilitated by the diversity of offspring born as a result of the combination of heredity of two organisms.

Abstract: Asexual and sexual reproduction

Introduction

The ability to reproduce, that is, to produce a new generation of individuals of the same species, is one of the main characteristics of living organisms. During the process of reproduction, genetic material is transferred from the parent generation to the next generation, which ensures the reproduction of characteristics not only of a given species, but of specific parent individuals. For a species, the meaning of reproduction is to replace those of its representatives who die, which ensures the continuity of the existence of the species; in addition, under suitable conditions, reproduction makes it possible to increase the total number of the species.

Each new individual, before reaching the stage at which it is capable of reproduction, must go through a number of stages of growth and development. Some individuals die before reaching the reproductive stage (or sexual maturity) as a result of destruction by predators, diseases and various random events; therefore, the species can survive only on the condition that each generation produces more offspring than there were parent individuals who took part in reproduction. Population sizes fluctuate depending on the balance between reproduction and extinction of individuals. There are a number of different propagation strategies, each with distinct advantages and disadvantages; all of them will be described in this abstract.

And the purpose of my work is to consider some types of reproduction.

Asexual and sexual reproduction

There are two main types of reproduction - asexual and sexual. Asexual reproduction occurs without the formation of gametes and involves only one organism. Asexual reproduction usually produces identical offspring, and the only source of genetic variation is random mutations.

Genetic variability is beneficial to the species, since it supplies “raw materials” for natural selection, and therefore for evolution. The offspring that are most adapted to their environment will have an advantage in competition with other members of the same species and will have a greater chance of surviving and passing on their genes to the next generation. Thanks to this, species are able to change, i.e., the process of speciation is possible. Increased variation can be achieved by mixing the genes of two different individuals, a process called genetic recombination, which is an important feature of sexual reproduction; In a primitive form, genetic recombination is already found in some bacteria.

Asexual reproduction

In asexual reproduction, offspring come from one organism, without the fusion of gametes. Meiosis is not involved in the process of asexual reproduction (unless we talk about plant organisms with alternating generations), and the descendants are identical to the parent individual. Identical offspring descended from the same parent are called clones. Members of the same clone can be genetically different only if a random mutation occurs. Higher animals are not capable of asexual reproduction, but several successful attempts have recently been made to clone some species artificially; we will look at them later.

Division

Spore formation (sporulation)

A spore is a single-celled reproductive unit, usually microscopic in size, consisting of a small amount of cytoplasm and a nucleus. The formation of spores is observed in bacteria, protozoa, representatives of all groups of green plants and all groups of fungi. Spores can vary in type and function and are often formed in special structures.

Often, spores are formed in large quantities and have negligible weight, which makes them easier to spread by wind, as well as by animals, mainly insects. Due to their small size, the spore usually contains only minimal nutrient reserves; Because many spores do not reach a suitable location for germination, spore losses are very high. The main advantage of such spores is the ability to quickly reproduce and spread species, especially fungi.

Bacterial spores, strictly speaking, do not serve for reproduction, but to survive under unfavorable conditions, since each bacterium produces only one spore. Bacterial spores are among the most resistant: for example, they can often withstand treatment with strong disinfectants and boiling in water.

Budding

Budding is one of the forms of asexual reproduction, in which a new individual is formed in the form of an outgrowth (bud) on the body of the parent individual, and then separates from it, turning into an independent organism, completely identical to the parent. Budding occurs in different groups of organisms, especially in coelenterates such as Hydra (Fig. 1) and in single-celled fungi such as yeast. In the latter case, budding differs from fission (which is also observed in yeast) in that the two resulting parts have different sizes.

An unusual form of budding is described in the succulent plant bryophyllum, a xerophyte often grown as an ornamental houseplant: miniature plants equipped with small roots develop along the edges of its leaves (Fig. 2); these “buds” eventually fall off and begin to exist as independent plants.

Reproduction by fragments (fragmentation)

Fragmentation is the division of an individual into two or more parts, each of which grows and forms a new individual. Fragmentation occurs, for example, in filamentous algae such as Spirogyra. The spirogyra thread can break into two parts anywhere.

Fragmentation is also observed in some lower animals, which, unlike more highly organized forms, retain a significant ability to regenerate from relatively poorly differentiated cells. For example, the body of nemerteans (a group of primitive worms, mainly marine) is especially easily torn into many parts, each of which can give rise to a new individual as a result of regeneration. In this case, regeneration is a normal and regulated process; however, in some animals (for example, starfish), restoration from individual parts occurs only after accidental fragmentation. Animals capable of regeneration serve as objects for experimental study of this process; Often a free-living planarian worm is used. Such experiments help to understand the differentiation process.

Vegetative propagation

Vegetative propagation is a form of asexual propagation in which a relatively large, usually differentiated part is separated from the plant and develops into an independent plant. Essentially, vegetative propagation is similar to budding. Often, plants form structures specifically designed for this purpose: bulbs, corms, rhizomes, stolons and tubers. Some of these structures also serve to store nutrients, allowing the plant to survive periods of unfavorable conditions such as cold or drought. Storage organs allow the plant to survive the winter and produce flowers and fruits the following year (biennial plants) or survive for a number of years (perennial plants). These organs, called overwintering organs, include bulbs, corms, rhizomes and tubers.

Overwintering organs can also be stems, roots or entire shoots (buds), but in all cases the nutrients they contain are created mainly during the process of photosynthesis occurring in the leaves of the current year. The resulting nutrients are transferred to the storage organ and are then usually converted into some insoluble storage material, such as starch. When unfavorable conditions occur, the above-ground parts of the plant die, and the underground hibernating organ goes into a dormant state. At the beginning of the next growing season, nutrient reserves are mobilized with the help of enzymes: the buds awaken, and the processes of active growth and development begin in them due to the stored nutrients. If more than one bud sprouts, then we can assume that reproduction has occurred.

In some cases, special organs are formed that serve for vegetative propagation. These are the modified parts of the stem - potato tubers, onion bulbs, garlic bulbs, bulblets in the leaf axils of the bluegrass, shoots of the young, etc. Strawberries reproduce with “mustaches” (Fig. 3). Adventitious roots are formed at the nodes of the shoots, and shoots with leaves are formed from the axillary buds. Subsequently, the internodes die off, and the new plant loses its connection with the mother plant.

In agricultural practice, vegetative propagation of plants is used quite widely.

Cloning of higher plants and animals

As already mentioned, obtaining identical offspring through asexual reproduction is called cloning. In the early sixties, methods were developed that made it possible to successfully clone some higher plants and animals. These methods arose as a result of attempts to prove that the nuclei of mature cells, having completed their development, contain all the information necessary to encode all the characteristics of an organism, and that cell specialization is due to the turning on and off of certain genes, and not the loss of some of them. The first success was achieved by prof. Steward at Cornell University, who showed that by growing individual carrot root cells (the edible part) in a medium containing the right nutrients and hormones, cell division could be induced, leading to the formation of new carrot plants.

Soon after, Gurdon, working at Oxford University, achieved the first cloning of a vertebrate animal. Vertebrates do not form clones under natural conditions; however, by transplanting a nucleus taken from a frog's intestinal cell into an egg whose own nucleus had previously been destroyed by ultraviolet irradiation, Gurdon managed to grow a tadpole, and then a frog, identical to the individual from which the nucleus was taken.

Experiments of this kind not only prove that differentiated (specialized) cells contain all the information necessary for the development of the whole organism, but also allow us to expect that similar methods can be used for cloning vertebrates at higher stages of development, including humans . Cloning the desired animals, such as breeding bulls, racehorses, etc., may be as profitable as cloning plants, which, as stated, is already being done. However, the application of cloning methods to humans is associated with serious moral problems. Theoretically, it is possible to create any number of genetically identical copies of a given man or woman. At first glance, it might seem that talented scientists or artists could be reproduced in this way. However, we must remember that the degree of influence exerted on development by the environment is not yet entirely clear, and yet any cloned cell must again go through all stages of development, i.e. in the case of a person, the stages of embryo, fetus, infant, etc. .

Sexual reproduction.

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 is an important stage in life cycles involving sexual reproduction, as it leads to a reduction in the amount of genetic material by half. 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) represents the genetic basis of intraspecific variation. 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 in monoecious species male and female flowers are formed on the same plant, as, for example, in cucumber and hazel, then in dioecious species some plants bear only male, and others only female flowers, as in holly or yew.

Hermaphroditism

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.

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, in the honey bee, the queen lays fertilized eggs (2n = 32), which develop into females (queens or workers), and unfertilized eggs (n = 16), which produce males (drones) that produce sperm by mitosis, and not 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 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 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 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, 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

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 ( rice. 57), 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 the motile sperm of lower plants, the sperm of 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.

Double fertilization in flowering plants

Secrets of gender [Man and woman in the mirror of evolution] Butovskaya Marina Lvovna

Mechanisms of sexual reproduction

In animals that reproduce sexually, only two types of gametes are produced in the genital organs - male (small and mobile) and female (large and immobile). Under no circumstances are sex cells intermediate in type, combining the properties of male and female gametes.

Why did the process of evolution create two sexes - male and female? Why not three, four or more? And why, in fact, cannot sex cells be of an intermediate size? L. Miele, R. Trivers and others give the following explanation. The fact is that sexual reproduction was formed under the influence of a special form of natural selection (sexual selection), in which individuals producing sex cells of intermediate size were successively eliminated from the original population (Fig. 1.2). This happened because individuals producing small gametes were selected only on the condition that they had sexual relations with individuals carrying large gametes and vice versa. Selection for gamete size occurred in conjunction with selection for sexual partner selectivity.

Rice. 1.2. Evolution of anisogamy through disruptive selection for gamete size. The abscissa axis shows the size of the gametes, and the ordinate axis shows the frequency of occurrence of the parental type of gametes. (Given from Mealey. 2000).

Suppose there is a species of animal that reproduces sexually, in which some individuals produce large, nutrient-rich gametes, others produce small and mobile ones, and still others produce gametes of an intermediate type. Individuals that produce small gametes can produce significantly more of them than individuals that produce large or medium-sized gametes. They are able to reproduce much more frequently than producers of large and medium-sized gametes. Therefore, gradually in the population of this species there should be an increase in the proportion of individuals producing small, nutrient-poor germ cells.

However, small gametes have one significant drawback: combining with germ cells of the same size gives the zygote virtually no chance of survival. Even if such “proto-males” mate significantly more often than “proto-females” that produce large gametes, their success in leaving offspring is low. In a population abundant in proto-males, any proto-females will receive significant advantages: after all, they have plenty of “gentlemen”, and the chances of survival of their fertilized, large-sized egg are the greatest. As a result, the selection vector in the population shifts in a different direction - individuals that produce large gametes begin to be selected. Against this background, medium-sized gametes do not receive any advantage in any case and are gradually washed out of the population.

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Biology report. Sexual reproduction. Oogamy

Asexual and sexual reproduction

Sexual reproduction

Types of sexual process

There are several types of sexual process: isogamy, anisogamy, oogamy.

In isogamy, merging (copulating) gametes do not differ morphologically. This process is widespread in algae, as well as in lower fungi and many protozoa (rhizopods, radiolarians, lower gregarines), but is absent in multicellular animals. In isogamy, gametes that differ in biochemical and physiological properties copulate.

Anisogamy is a type of sexual process in which the fusion of gametes that differ in size, shape or behavior occurs. This process is characteristic, for example, of gametes in the alga Eudarina.

Anisogamy reaches its highest degree in multicellular plants and animals: fertilization of a stationary egg with a motile sperm. This process is already called oogamy, when, during fertilization, gametes merge to form a zygote, sharply different in size, shape and behavior. Oogamy is characteristic of all multicellular animals, many lower and all higher plants.

In oogamy, as a rule, two parent individuals take part, each of which participates in the formation of a new organism, introducing only one reproductive cell - a gamete (egg or sperm), which has half the number of chromosomes than non-sexual ones, i.e. somatic, parental cells. As a result of the fusion of gametes, a fertilized egg is formed - a zygote, which carries the hereditary inclinations of both parents, due to which the offspring develop new combinations of genes that are not characteristic of the parent individuals.

Sexual reproduction is another protozoan invention that has had profound effects on more complex organisms. It should be noted that the sexual process and reproduction are two different phenomena that can exist separately from each other. Reproduction is the emergence of new individuals, the sexual process is the creation of new combinations of genes originating from two different individuals. Reproduction in the absence of the sexual process is characteristic of those organisms that reproduce by simple division: when an amoeba divides or new hydra individuals budding, no redistribution of genes is observed. In unicellular organisms it is often

INTRODUCTION TO THE STUDY OF ANIMAL DEVELOPMENT___________________________________________ 21

There is also a sexual process without reproduction. Bacteria can transfer genes from one individual to another using special sexual villi called sex saws or fimbriae(Fig. 1.12). This transmission occurs independently of reproduction. Protozoa are also capable of redistributing genes independently of reproduction. So, for example, in paramecia, reproduction occurs by simple division in two, and the sexual process by conjugation(Figure 1.13). Two paramecia are connected by their mouthparts, and a cytoplasmic bridge appears between them. In both paramecia, the macronucleus (which regulates metabolic processes) is destroyed, while the micronucleus undergoes meiosis, followed by mitosis; Eight haploid micronuclei arise, all of which, with the exception of one, are destroyed. The remaining micronucleus divides again and forms two micronuclei, stationary and migrating. Each migrating micronucleus moves along the cytoplasmic bridge to the neighboring conjugate and merges with its stationary micronucleus (“fertilizes” it), due to which a new diploid nucleus appears in both cells. When the conjugating partners separate, this diploid nucleus divides, giving rise to a new micronucleus and a new macronucleus. In this case, paramecia do not reproduce, only the sexual process occurs.

In unicellular eukaryotes, a combination of these two independent phenomena, the sexual process and reproduction, is also observed; in this case we talk about sexual reproduction. In Fig. 1.14 shows the life cycle of Chlamydomonas ( Chlamydomonas). This organism usually exists in a haploid form like mammalian gametes, i.e. Each chromosome in Chlamydomonas is singular. However, individuals of each species are divided into two groups based on mating behavior - plus And minus. When individuals of different groups meet, their cytoplasm unites and their nuclei merge to form a diploid zygote. This zygote is the only diploid cell in the life cycle of Chlamydomonas: ultimately carrying out meiosis, it forms four new cells. Here we are dealing with sexual reproduction, since during the process of meiosis, chromosomes are redistributed, and at the same time a larger number of individuals arises. Please note that with this type of sexual reproduction of protozoa, the gametes are morphologically identical - differences between sperm and egg have not yet arisen.

With the emergence of sexual reproduction in the evolution, progress was made in two respects. First, the mechanism of meiosis arose (Fig. 1.15). by which the diploid set of chromosomes is reduced to the haploid state (this process will be discussed in detail in Chapter 22). Secondly, a mechanism has arisen through which individuals of two types, differing in sex, recognize each other. Recognition occurs initially at the level of flagellar membranes (Fig. 1.16; Goodenough. Weiss. 1975; Bergman et al.. 1975). Agglutination of flagella makes it possible to establish contact between special areas on cell membranes. These specialized areas contain components specific to individuals of different types, due to which the cytoplasm of these individuals merges. After agglutination of flagella plus- individuals initiate fusion, forming a “fertilization tube” similar to the one we find in sperm. This tube contacts and merges with a specialized area on the surface minus-individuals Interestingly, the same mechanism that is used to extend this tube

Gilbert S. Developmental biology: In 3 vols. T. I: Transl. from English - M.: Mir, 1993. - 228 p.

22________________ CHAPTER 1_______________________________________________________________________________

polymerization of actin protein - also acts in the formation of outgrowths in sea urchin sperm and eggs. In the next chapter we will see that the recognition and fusion of sperm and egg is strikingly similar to the processes described in Chlamydomonas.

Unicellular eukaryotes are apparently characterized by the basic elements of developmental processes that are characteristic of more complex organisms belonging to other types of animals: 1) synthesis processes in the cell are controlled at the transcriptional, translational and post-translational levels; 2) there is a mechanism that ensures the release of RNA through the nuclear envelope; 3) the structure of individual genes and chromosomes inherent in unicellular eukaryotes is preserved throughout the entire evolution of eukaryotes; 4) mitosis and meiosis become more advanced in the process of evolution: 5) sexual reproduction includes cooperation between individual cells, which becomes even more important in multicellular organisms.