Asexual - by mitosis of the nucleus and division of the cell in two (in amoeba, euglena, ciliates), as well as by schizogony- multiple divisions (in sporozoans).

Sexual - copulation. The protozoan cell becomes a functional gamete; As a result of the fusion of gametes, a zygote is formed.

The sexual process is characteristic of ciliates - conjugation. It lies in the fact that cells exchange genetic information, but the number of individuals does not increase.

Many protozoa can exist in two forms - trophozoite(vegetative form, capable of active feeding and movement) and cysts, which is formed under unfavorable conditions. The cell is immobilized, dehydrated, covered with a dense membrane, and metabolism slows down sharply. In this form, protozoa are easily transported over long distances by animals, by wind, and dispersed. When hitting favorable conditions habitat, excystation occurs, the cell begins to function in the trophozoite state. Thus, encystment is not a method of reproduction, but helps the cell survive unfavorable environmental conditions.

For many representatives of the type Protozoa characterized by the presence life cycle consisting of a natural alternation of life forms. As a rule, there is a change of generations with asexual and sexual reproduction. Cyst formation is not part of the normal life cycle.

Time generation for protozoa it is 6-24 hours. This means that, once in the host’s body, the cells begin to multiply exponentially and theoretically can lead to the host’s death. However, this does not happen as the defense mechanisms host organism.

Medical significance have representatives of protozoa belonging to the classes Sarcodaceae, Flagellates, Ciliates and Sporozoans.

The phylum Protozoa includes organisms whose body consists of a single cell, which, however, functions as a whole organism. Protozoan cells are capable of independent nutrition, movement, protection from enemies and survival of unfavorable conditions.

The feeding of protozoa occurs with the help of digestive vacuoles, carried out by phago - or pinocytosis. The remains of undigested food are thrown out. Some protozoa contain chloroplasts and are able to feed themselves through photosynthesis.

Most protozoa have organelles of movement: flagella, cilia and pseudopodia (temporary mobile outgrowths of the cytoplasm). The forms of movement organelles underlie the taxonomy of protozoa.

Reproduction of protozoa is usually carried out in different forms divisions are varieties of mitosis. The sexual process is also characteristic: in the form of cell fusion - copulation - or the exchange of part of the hereditary material - conjugation.

Most protozoa have a single nucleus, but multinucleate forms are also found.

Protozoa belonging to the classes Sarcodaceae, Flagellates, Ciliates and Sporozoans are of medical importance.

Like any cell, protozoa have a nucleus.

In the nuclei of protozoa, as well as in the nuclei of multicellular organisms, there is a membrane, nuclear juice (karyolymph), chromatin (chromosomes) and nucleoli. However, different protozoa are very diverse in size and structure of the nucleus (Fig.). These differences are due to the ratio structural components nucleus: the amount of nuclear juice, the number and size of nucleoli (nucleoli), the degree of preservation of the chromosome structure in the interphase nucleus, etc.

Rice. Protozoan nuclei.
A - micronucleus of the ciliate Paramecium aurelia;
B - flagellate Trypanosoma brucei,
B - amoeba Amoeba sphaeronuclei;
G - armored flagellate Geratium fuscus;
D - radiolaria Aulacantha scolymantha;
E - macrogamete of coccidia Aggregata eberthi;
F - macronucleus of the ciliate Epidinium ecaudatum

Most protozoa have one nucleus. However, there are also multinucleate species of protozoa.

In some protozoa, namely ciliates and a few rhizomes - foraminifera, an interesting phenomenon of dualism (duality) of the nuclear apparatus is observed. It boils down to the fact that in the body of a protozoan there are two nuclei of two categories, differing both in their structure and in their physiological role in the cell. Ciliates, for example, have two types of nuclei: a large, chromatin-rich nucleus - a macronucleus and a small nucleus - a micronucleus. The first is associated with the performance of vegetative functions in the cell, the second with the sexual process.

Protozoa, like all organisms, reproduce. There are two main forms of protozoan reproduction: asexual and sexual. The basis of both is the process of cell division.

With asexual reproduction, the number of individuals increases as a result of division. For example, an amoeba during asexual reproduction is divided into two amoebas by constriction of the body. This process begins from the nucleus and then invades the cytoplasm. Sometimes asexual reproduction takes on the character of multiple divisions. In this case, the nucleus is pre-divided several times and the simplest one becomes multi-core. Following this, the cytoplasm breaks up into a number of sections corresponding to the number of nuclei.

As a result, the protozoan organism immediately gives rise to a significant number of small individuals. This is, for example, the asexual reproduction of Plasmodium falciparum, the causative agent of human malaria.

Sexual reproduction of protozoa is characterized by the fact that reproduction itself (increase in the number of individuals) is preceded by the sexual process, characteristic feature which is the fusion of two sex cells (gametes) or two sex nuclei, leading to the formation of one cell - a zygote, giving rise to a new generation.

The forms of the sexual process and sexual reproduction in protozoa are extremely diverse.

Protozoa reproduce by division. The most common is binary fission. With this method of division, two daughters are formed from one individual. The nucleus divides mitatically, after which the cytoplasm splits into two parts. Sporozoans and some other forms are characterized by multiple division, schizogony, during which the nucleus first divides repeatedly, after which the cytoplasm divides according to the number of nuclei formed.
Peculiar division, endodiogeny, consisting in the formation of two daughter individuals inside the mother, was found in Toxoplasma.

Sexual process not described in all protozoan species; it is not known for many amoebas, trypanosomes, leishmania, etc. The forms of the sexual process of protozoa are copulation (sarcodes, flagellates and sporozoans) and conjugation (ciliates). During copulation, two sex cells, gametes, merge; the fusion product is called a zygote. The gamete is always haploid, that is, its nucleus contains half as many chromosomes as the nucleus of a diploid zygote.

Number reduction chromosomes(meiotic division) in sporozoans occurs during the first division of the zygote and all other stages of the life cycle (sporozoites, schizonts and gametes) are haploid. This type of reduction is called zygotic. In flagellates of the order Nuper mastigida, most of the life cycle takes place in the diploid phase and meiotic division occurs only before the formation of gametes. This type of reduction is called gametic. It is common in multicellular organisms. In the life cycle of representatives of the order Foraminifera, generations alternate, reproducing in the haploid and diploid phases.

Copulating gametes are often indistinguishable morphologically (isogamous copulation), but sometimes there are physiological differences between them that allow us to speak conditionally about male and female gametes. In all sporozoans and representatives of a number of other groups, there are morphological differences between female and male gametes (anisogamous copulation).

During conjugation of ciliates fusion of the cytoplasm of the partners does not occur and the temporarily united individuals exchange one of the two haploid nuclei formed at the beginning of the process.

Methods of reproduction of protozoa

With a combination of 2 different levels organizations: zygotic, gametic nuclear cycles - there is a wide variety of methods of reproduction : asexual : monotomy(mitosis - longitudinal transverse division); polytomy(many cells) - schizogony (without growth stage) from the 8th class. very small daughter cells are formed; polytomy - growth precedes division. You get a lot of exactly the same cells. as original); budding (observed in multinucleate protozoa; the cytoplasm separates around individual nuclei and buds from the mother. Sexual: (gametogony - formation of gametes) - isogamy(exactly identical gametes in size and function are formed); anisogamy(formation of unequal gametes); conjugation (sexual process without reproduction - ciliates).

Features of the structure and reproduction of colonial protozoa

Among flagellates, a colonial form is observed; a colony can consist of several thousand cells. Colonial organisms differ from multicellular organisms in that each cell in the colony can perform all the functions inherent in an individual organism. Cl. cannot exist separately from the colony. Pandorin colony - part of the class. find. inside the colony and lose flagella; The endorin colony has a cavity; Volvox- the presence of a very large number of cells. in the colony. Based on the nature of development, monotomous and palintomic colonies are distinguished. Monotomy They call this method of asexual reproduction in which, after the act of separation, the daughter individuals grow and restore all the organelles characteristic of the maternal cell. Separate cells Colonies periodically divide, increasing the number of individuals composing the colony. From time to time, the colony itself, having reached the maximum size for a given species, is laced in half. This way the number of colonies increases. In polyntomic colonies, to which, for example. Plant flagellates belong to the Volvox family. Reproduction occurs differently. All class colonies, or only some of them, undergo successive polyntomic divisions, resulting in several young colonies at once. The entire mother colony breaks up into daughter colonies, the number of which is equal to the number of cells of the old colony. Volvox has only a few cells. the mother colony undergoes palintomic division. The resulting daughter cells. first placed inside the mother. After some time, the old colony is destroyed and those located inside it become free. In such a complex colony as Volvox, most of the cells. does not give rise to the next generation, but periodically die. Just like somatic (bodily) cells. multicellular organism. In colonial species there is a gradual transition to anisogamy. Copulation occurs in a colony between m. (microgametes) and f. (macrogametes) sexual cells. The vast majority of cells. remains asexual. In a colony, only 25-30 female cells produce gametes. and 5-10 males. Among Volvox there are dioecious species, in which only male or female gametes develop on one colony, and hermaphraditic ones. In which the colony gives rise to both categories of gametes. Each of the cells giving rise to microgametes by palentomy, produces 256 tiny biflagellate cells.. Cl. wives orientations do not divide, but grow and turn into a macrogamete, which can be called an egg cell. She is motionless. M. gametes are found. and merge with them. The fertilized macrogamete (zygote) is surrounded by a dense membrane. It gives rise to a colony through successive polytomous divisions; the old colony is falling apart and somatic cells. die. In all flagellates, the first 2 divisions of the zygote occur through meiosis, i.e. Only their zygote is diploid, and all other stages of the life cycle are haploid. Zygotic reduction is observed.

The main form of reproduction of protozoa is monotomy- simple division in two. Between two divisions, the animal acquires all the structures lost during the division process, actively feeds, grows, and by the beginning of a new division manages to reach the size of the mother individual (amoeba, euglena, ceratium, etc.).

A special form of protozoan reproduction is palintomy or re-division. The division is repeated many times without a subsequent period of nutrition and growth, and the intermediate products of palintomy remain undifferentiated, and their size decreases with each division. Only the last generation reaches full development, eats and grows. Palintomic division usually occurs under the cover of protective membranes (division of the oocyst in the malarial plasmodium, the zygote in the Volvox, etc.).

In protozoa, multiple division is also common - syntomy(schizogony). In this case, first, multiple divisions of the nucleus occur without dividing the cytoplasm, as a result of which the cell temporarily becomes polyenergetic, then the cytoplasm disintegrates immediately into many (corresponding to the number of nuclei) daughter cells (trypanosomes, malarial plasmodium, foraminifera).

Sexual process in protozoa

The sexual process can take place in protozoa in the form copulation And conjugation. At copulation There is a complete fusion of two sexual individuals - gametes. The process when gametes are no different from each other is called isogamy. In other cases, large macrogametes (female) and small microgametes (male) are formed; Anisogamy occurs. The process when macrogametes become immobile and accumulate spare nutrients, is called oogamy.

Conjugation is a more complex form of the sexual process and is found only in ciliates. In this case, only a temporary connection of two individuals and an exchange of parts of micronuclei occurs. In this case, real gametes are not formed, but the migrating nucleus is covered with a thin layer of cytoplasm and corresponds to a microgamete, and the rest of the body of the ciliate with a stationary nucleus is a macrogamete. The migrating (male) nucleus of each conjugant moves into the body of the partner and copulates with its stationary (female) nucleus - cross-fertilization occurs. Macronuclei are not involved in this process.

Nuclear cycles

The nuclear cycle is associated with the sexual process. Common to all nuclear cycles is the alternation of haploid and diploid generations. Gametes are always haploid; when they fuse, a diploid zygote is obtained; at some stage of the life cycle, reduction division (meiosis) occurs, which returns the nuclei to the haploid state.

There are three types of nuclear cycles, each of which is characterized by the place of reduction division in this cycle (Fig. 3).

1) Cycles with zygotic reduction. Most of the life cycle occurs in the haploid generation. The diploid generation is represented by one zygote, which divides in a reduction manner, and haploidy is restored. Such cycles are characteristic of Phytomonadida, Sporozoa, part of Polymastigida, part of Hypermastigida.

2) Cycles with gametic reduction. Most of the cycle occurs in the diploid generation. The haploid generation is represented only by gametes; reduction division occurs right before their formation. Such cycles are characteristic of the remaining Polymastigida and Hypermaatlgida, Heliozoa, Ciliophora, Opalinida, Cnidosporidia and almost all Metazoa.

3) Cycles with intermediate reduction. The haploid and diploid generations occupy approximately equal parts of the cycle: the zygote divides mitotically. More developed stages of the diploid generation (agamont) arise. At some point, reduction division occurs with the formation of mononuclear haploid agametes - agamous (asexual) reproduction. Agametes develop into more advanced stages of the haploid generation. Then this adult haploid generation (gamont) breaks up into gametes, which copulate - the sexual process occurs. With the formation of a zygote, the cycle closes. This cycle occurs only in foraminifera and, as a rare exception, in some rotifers.

Life cycles

The life cycles of protozoa are very diverse. There are protozoa that have a cycle without the sexual process, i.e. without a nuclear cycle (for example, in trypanoses, in which different vegetative stages alternate).

In the most differentiated life cycle, four periods are distinguished:

1) progamous, ending in the formation of gametes;

2) syngamous, which includes the copulation of gametes and the formation of a zygote;

3) metagamous, covering several generations of sporozoites;

4) vegetative, during which growth occurs and the mass of individuals increases.

At some part of the life cycle, a reduction in the number of chromosomes occurs.

For the convenience of comparing life cycles, we will draw up a table, which we will divide vertically into 4 parts, corresponding to the indicated four periods. In this table we will place several cycles of protozoa and the cycle of a multicellular organism, placing the periods of their cycles in the corresponding columns (Fig. 4). Let us choose the protozoa with the simplest and most dissected cycles.

Figure 4. Comparison of life cycles of various protozoa and metazoans (from K.V. Beklemishev, 1979):

A – indefinitely multi-linked cycle of lower phytomonads with a facultative sexual process; B - Volvox cycle with the division of the cycle into vegetative, progamous and metagamous parts; B – multicellular cycle with increased differences between parts of the cycle; D – cycle of ciliates (in the metagamous part of the cycle only nuclear division is shown, cell contours are not depicted). 3 – zygote, C – synkaryon, R! – reduction division, PN – pronuclei, P – parenchymula, rs – reproduction and growth of sporogonia, ro – reproduction and growth of oogonium, VM – adult multicellular, scI and scP – spermatocytes of the I and P order, ocI and otsP – oocytes of the I and P order

Of the variety of life cycles of protozoa with sexual processes, monoenergetic flagellates, for example, Chlamydomonas, have the simplest cycle. In some species of this genus, many asexual generations are observed, which are formed through monotomy; the sexual process occurs when living conditions deteriorate. Gametes morphologically differ little from individuals of asexual generations. After copulation, the zygote is dressed in a shell, under the cover of which, through palintomic division, 4–32 individuals are formed, which emerge from the shell and begin asexual reproduction. A reduction in the number of chromosomes occurs during the first divisions of the zygote. In this cycle, all stages are haploid, the only diploid stage is the zygote. This is a cycle with zygotic reduction. In other species of the genus Chlamydomonas, asexual reproduction occurs by palintomy, and gametes also arise palintomically.

Most dismembered life cycle in the “multicellular” protozoan – Volvox. In zoology courses, little attention is paid to this object, because it has an autotrophic type of nutrition. However, Volvox occupies an important place in the comparative anatomy of invertebrates, since its life cycle is closest to that of metazoans. The Volvox life cycle is characterized by a strictly fixed place of the sexual process, which is preceded by the progamous part of the cycle, during which gamogy occurs, i.e. transformation of special generative cells into gametes: macrogamont grows, accumulates nutrients and turns into a large immobile macrogamete, microgamont divides palintomically, giving rise a large number microgametes. This is followed by fusion of gametes (the syngamous part of the cycle) and reduction division. Next, the metagamous part, the zygote, divides palintomically, leading to the formation of a blastula-like colony, very reminiscent of the blastula stage of multicellular organisms; this colony grows, new daughter colonies are formed inside it - after the metagamous one, the vegetative part of the cycle also appears. In the life cycle of Volvox, attention should be paid to the fact that the development of a daughter colony from a zygote occurs first in a state of incubation (i.e., cells formed as a result of palintomic division of the zygote, with their flagellar ends facing inside the hollow ball), and only before the exit of the formed daughter colony excursion occurs from the mother - everting, as a result of which the cells take their final arrangement with flagella outward. Reduction division in Volvox, as in Sporozoans, occurs after the formation of the zygote - a cycle with zygotic reduction. The cycle of ciliates is very peculiar. Zoology textbooks give a description of the ciliate cycle without taking into account the nuclear cycle. Therefore, you should pay attention to the fact that in their progamous part only a micronucleus participates, which has undergone reduction division; of the four haploid nuclei formed, three are reduced, leaving one, which divides to form two pronuclei. The female pronucleus turns into a stationary nucleus, remains in the ciliate, and thus all of it corresponds to the macrogamete, and the male pronucleus turns into a migrating nucleus, corresponding to the microgamete, and passes into the partner, merging with its stationary nucleus (the syngamous part of the cycle). In the metagamous part of the ciliate cycle, palintomy is not observed; The synkaryon divides, leading, after a series of rearrangements, to the restoration of the binucleate nature of each individual and to the restoration of the polyploidy of the macronucleus. In the vegetative part of the cycle, ciliates grow and reproduce asexually. This is a cycle with gametic reduction.

In the life cycle of any multicellular organism, just like Volvox, there are progamous, syngamous, metagamous and vegetative parts. But the progamic part becomes more complicated due to the fact that in multicellular organisms during this period a complex restructuring of the nuclear apparatus is observed: the formation of tetraploid oo- and spermatocytes of the first order, of which diploid oo- and spermatocytes of the second order and, finally, reduction division, giving rise to haploid gametes .

The metagamous palintomic part of the metazoan cycle is very similar to that of phytomonads. However, the vegetative part of the cycle is different: it is characterized high degree integration of the body and a wide range of cell quality.

The table shows that of the protozoa considered, the Volvox cycle is closest to the life cycle of multicellular organisms. Its metagamous part ends with the formation of a spherical blastula-shaped stage; just like Metazoa, it has a vegetative part, during which the organism (or colony) grows, its cells differentiate, and only a small part of them specializes into generative ones. The main difference between the Volvox life cycle and the multicellular cycle is the place of reduction division. This detailed comparison the cycles of various protozoa and metazoans is necessary to understand the origin of multicellular animals. It should, however, be clearly understood that Volvox should in no case be considered as the direct ancestor of metazoans. This object is used only as a model convenient for understanding the processes that could have occurred in the evolution of colonial heterotrophic flagellates, most likely close to modern Choanoflagellata, which are currently considered the ancestors of metazoans. Among modern collared flagellates there are no species with a sexual process, but it is believed that it was precisely these colonial Choanoflagellata, which had a spherical shape and a Volvox-type life cycle, but with gametic reduction, that were the ancestors of metazoans.