Reproduction means the production of new individuals from the old ones which die or are eaten or destroyed by their enemies. Reproduction is not necessary to maintain life in any single individual, but it is essential only for the continued existence of the species.
Protozoa reproduce at short or long intervals between periods of nutrition and growth. The mode of reproduction in protozoa is variable among different groups, however it is basically a cell division. In protozoa, reproduction takes place by asexual (involving mitosis) and sexual (involving meiosis); however, some reproduce asexually only.
Asexual Reproduction:
Asexual reproduction involves the mitotic cell division of parent body into two or more young individuals that develop into nature organisms. It always involves a single parent and neither meiosis nor fertilization occurs.
Protozoa reproduce asexually by following methods:
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1. Binary fission
2. Multiple fission
3. Plasmotomy
4. Budding
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1. Binary Fission:
Division of the parent cell into two or more daughter cells is called fission. Binary fission is the commonest type of asexual reproduction in Protozoa. It involves the division of the parent body into two nearly equal parts or daughter individuals. During this process first the nucleus divides usually mitotically (karyokinesis) then the cytoplasm divides (cytokinesis) by a constriction. The plane of fission differs in the different groups of Protozoa, and bears a definite relation to the body form.
(i) Simple Binary Fission:
In Sarcodina, e.g. Amoeba Entamoeba, the plane of fission cannot be recognized.
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(ii) Transverse Binary Fission:
In ciliates e.g. Paramecium, animal divides transversely resulting into two daughter paramecia.
(iii) Longitudinal Binary Fission:
In flagellates e.g., Euglena and Vorticella, the plane of division is longitudinal, i.e., along the longitudinal axis of the body
(iv) Oblique Binary Fission:
In Dinoflagellates, e.g., Ceratium the division plane is oblique.
In Chlamydomonas and Microgromia etc., repeated equal binary fissions result in four or more individuals which do not separate till the whole process is completed. The microgamete formation of Volvox also occurs in a similar manner.
Usually the daughter individuals retain their polarity but it may be reversed in a few cases. The products of binary fission are generally similar in size and structure, but fission may be unequal. In stalked Ciliophora (e.g., Vorticella), one of the daughters is smaller and called telotroch. It develops a posterior girdle of cilia becomes free-swimming for a while, but eventually secretes a stalk and grows into a normal adult. Certain ciliates like Colpoda and Tillina, undergo fission only in an encysted condition.
(v) Repeated Binary Fission:
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Chlamydomonas divide by binary fission to produce two new individuals, which fail to separate the undergoing further binary fission. Thus four or more individuals are produced.
2. Multiple Fission or Sporulation:
During multiple fission or sporulation, nuclear division is not followed immediately by the division of the cytoplasm. First the nucleus undergoes a series of divisions either by repeated binary fissions (e.g., Plasmodium) or by simultaneous multiple divisions (e.g., Aggregata).
The body thus becomes multinucleate. Later the body cytoplasm divides into as many parts as there are daughter nuclei which usually arranged themselves at the periphery, each getting surrounded by a fragment of cytoplasm. The parent body thus simultaneously divides into as many daughter individuals as there are nuclei, usually leaving behind some residual cytoplasm which disintegrated afterwards. The number of offspring greatly varies among different and the same species and sometimes runs into thousands.
Products of multiple fission of a zygote generally form spores, sometimes the products of any multiple fission are called spores. A spore may be enclosed in a spore case or they may be naked. The naked spores may be amoeboid or flagellated (chlamydomonas) or ciliated. Spores may be gametes or serve for the distribution of the species.
All types of fission occur within a cyst or without encystment. Cyst formation is common in fresh water and parasitic protozoa, though all protozoa do not form cysts. In cyst formation the animal becomes rounded, loses its organelles of locomotion. It rejects the food vacuoles and contractile vacuoles disappear.
The animal then secretes a gelatinous covering which hardens into a chitinous epicyst. Inside this a membranous endocyst is secreted. The cyst may have more than two layers. The functions of the cyst are protection of the animal against unfavourable conditions of environment, or it may be reproduction. The cyst can be carried by wind or some other agent and are thus important in dispersal.
Protozoa have the following kinds of cyst:
(i) Resting cysts enable an organism to proceed undistributed in its normal activities (Euglena).
(ii) Resistance cysts are formed against unfavourable conditions of environment (Amoeba).
(iii) Gamocysts are those in which union of gametes takes place for reproduction (Gregaring).
(iv) Oocysts contain a zygote (Plasmodium).
(v) Sporocysts are those in which multiple fission occurs to form sporozoites (monocystis).
Finally excystment takes place on return of favourable conditions. But the individual leaving the cyst is never the same as the one that underwent encystment. It has a complement of new organelles and renewed vigour. The excystment may be through a minute pore in the cyst, but more usually it is due to the protozoan secreting some enzymes which rupture the cyst wall.
Multiple fission is quite common in Foraminifera, Radiolaria, Sporozoa and certain Mastigophora. The process receives different names according to the particular period in the cycle when it occurs.
Multiple fission may be distinguished:
(i) Schizogony or Agamogony:
It is a kind of multiple fission taking place during sexual cycle. The resulting individuals are called Schizogamont or agamonts which grow into adults, e.g., Plasmodium.
(ii) Gamogony:
It occurs in Monocystis. The nucleus of gametocyte undergoes multiple fissions to produce uninucleate bodies called gametes. Thus production of gametes by multiple fission in asexual reproduction and is called gamogony.
(iii) Sporogony:
When zygote nucleus undergoes multiple fission, it is called sporogony and the cells thus produced are called spores.
3. Plasmotomy:
This is binary fission is multinucleate form such as Opalina. The multinucleate body divides into two daughter individuals without any nuclear division. Each of the offspring regains the normal number of nuclei by the division of parental nuclei.
4. Budding:
It is also a type of fission in which one or more smaller individuals are produced from the parent. Later they separate from the mother cell. Each bud differentiates, either before or after separation to form the adult. It is a regular feature in Suctoria. When a parent body forms a single bud, it is called monotonic (e.g., Vorticella) and if several buds are produced it is called multiple budding (e.g., Suctoria).
Budding can be of following two types:
(i) Exogenous Budding:
Buds formed from body surface, e.g. Noctiluca, Ephilota, etc.
(ii) Endogenous Budding:
Budding takes place within the body such as in brood chamber or pouches e.g., Suctoria and Testacea etc.
Sexual Reproduction:
Asexual reproduction is interrupted at regular intervals by sexual reproduction. Sexual reproduction involves meiotic nuclear division that results in a change from diploid to haploid and the union of gametes to restore diploidy. It may be amphimictic, involving the union of gametes from different parents, or automictic, which the gametes arise from the same parent.
In either case the uniting gametes may be whole organisms or nuclei only. Where whole organisms (gametes) unite, the union is termed syngamy. Where only nuclei unite, the process is called conjugation. Conjugation occurs only among ciliates whereas syngamy occurs in all other groups where sexual reproduction occurs.
The sexual reproduction in Protozoa takes place in the following ways:
1. Syngamy
2. Conjugation
3. Autogamy
4. Endomixis
5. Hemixis
1. Syngamy:
Syngamy is the complete and permanent fusion of two specialized protozoan individuals, or gametes, resulting in the formation of a fertilized cell or zygote. The nuclei of the gametes fuse to form the zygote nucleus or synkaryon. The zygote develops into the adult either directly or through encystment and fission of various types in different forms. The fusing gametes may be similar or dissimilar from each other.
On this basis syngamy can be of following types:
(i) Isogamy:
When the gametes are similar in shape, size and structure, they are called isogametes and their fusion is called isogamy. It is found in Foraminifera, Phytomonadida (Chlamydomonas) and Gregarindia (Monocysts).
(ii) Anisogamy:
When the fusing gametes differ from each other in shape, structure and behaviour, they are called anisogametes and their union is called anisogamy. In it mal is smaller and motile called microgamete, while larger and nonmotile gamete is called macrogamete. It is found in Sporozoa (Plasmodium) and Phytomanadida (Volvox).
(iii) Hologamy:
It is the fusion of two mature individuals behaving as gametes to form zygote. It is found in certain Rhizopoda and Flagellata.
(iv) Autogamy:
Fusion of two gametes produced by the same parent cells is called autogamy, e.g. Actinophrys and Actinosphaerium.
(v) Paedogamy:
It is the fusion of two young individuals.
(vi) Microgamy:
Certain protozoans, e.g. foraminifers and Arcella, produce two kinds of microgametes. Microgamy is the fusion of such gametes to form synkaryon.
(vii) Macrogamy:
It is the fusion of two macrogametes of a species as is found in Heliozoa (Actinophrys).
(viii) Exogamy:
When two nuclei derived from two different parent cells of the same species fuse, the phenomenon is called exogamy.
Effect of Syngamy:
Morphologically, syngamy brings about a periodic reorganization of the nucleus.
Physiologically, it has four distinct effects:
1. In higher animals it activates or initiates the development in the egg.
2. It brings into close association the two previously separate lines of heredity.
3. In increases the external diversity in the offspring.
4. It invigorates or renews the vital power which becomes exhausted due to repeated asexual multiplication.
2. Conjugation:
Conjugation is the temporary union of two individuals of different mating types of a species to facilitate exchange of the nuclear material. These individuals separate out after nuclear exchange. The pairing gametes (individuals) are called conjungants and may be either isogamous (equal) as in Paramecium or anisogamous (unequal) as in Vorticella.
In case of Vorticella, the two members of a conjugating pair are of striking different sizes. Such “dioecious” macro and micro-conjugants represent an adaptation for conjugation in sessile species. The macro-conjugant remains attached while the small bell of the micro-conjugant breaks free from its stalk and swims about. A synkaryon forms only in the macro conjugant from one gametic nucleus contributed by each conjugant. However, there is no separation after conjugation, and little migratory conjugant degenerates.
In the Suctoria, conjugation takes place between two attached individuals that happen to be located side by side.
Significance of Conjugation:
Conjugation is an episode in reproduction, not a mode of multiplication. It fulfills the same results as syngamy or sexual fertilization, i.e., the combination of nuclear materials from two individuals, thus promoting variability in the stock and warding off senescence. Early works of Maupas, Hertwig and Calkins led to the result that in cultures of ciliates after a certain number of asexual generations, a clone falls into an unhealthy condition or depression.
It gradually declines in vitality and fission rate with increasing degeneration of organs and functions, stops digestion and finally dies. These workers therefore, concluded that Protozoa, like Metazoa, are subject to periodical waxing and waning of vitality, called senescence, resulting in death unless conjugation intervenes. The effect of conjugation was regarded to be rejuvenation or invigoration.
Later Woodruff, Enriques, Sonneborn and others proved by experiments that many ciliates under adequate environmental conditions propagate indefinitely by binary fission without undergoing conjugation or endomixis. At present it seem clear that conjugation, whether or not it is essential, can produce a physiological stimulation in at least certain strains. The stimulating effect is not due to union of nuclei but to the accompanying renewal of the mega-nucleus, which probably has become worn out.
Jenning’s experiment shows the importance of conjugation in the production of diversities.
Factors for Conjugation:
(i) Maupas believed in sexual maturity as a prerequisite for conjugation. According to Maupas individuals undergo a certain number of asexual generations before they conjugate.
(ii) In some cases conjugation could be induced by starvation following rich feeding.
(iii) Conjugation occurs after a period of rapid fission shortly before the population reached the maximum.
(iv) Various environmental factors have been found to affect conjugation, such as temperature or a particular bacterial diet. Darkness favours and light suppresses it in P. aurelia. Addition of chemicals, such as iron and aluminium chlorides to the medium, is also doubtfully suggested to induce conjugation in P. caudatum. According to Wichtermann (1936), Nyctotherus cordiformis undergoes conjugation only during the metamorphosis of the tadpole.
3. Automixis:
It is a process of self-fertilization or autogamy that results in the homozygosis. Automixids is the fusion of two gametic nuclei originating by the division of single nucleus of an individual. It is conjugation-like method of nuclear reorganization. The macronucleus degenerates and micronucleus divides a number of times of form eight or more nuclei.
Two of these nuclei fuse to form a synkaryon; the others degenerate and disappear. The synkaryon then divides to form a new micronucleus and macronuclei as occurs in conjugation. If the two nuclei which fuse are present in a single cell, then the process is called autogamy.
But if the two fusing nuclei are present in two different cells then the process is called paedogamy. Autogamy takes place in a single P. aurelia which provides both the fusing nuclei to form a synkaryon. Paedogamy occurs in Actinosphaerium and Actinophrys in which two cells of a zygote which reproduces by binary fission.
4. Endomixis:
Woodruff and Erdmann (1914) described in Paramecium aurelia a regularly recurring internal process of nuclear reorganization called endomixis. It differs from conjugation in that it occurs only within a single individual and there is no meiosis and fusion of nuclei.
But it resembles conjugation in that a new mega-nucleus is formed from the micronuclear material, thus bringing about a physiological activation of the cell. However, Diller (1936) doubts the validity of process in Paramecium, thinking that the original reporters have probably confused the stages of autogamy and hemixis into one scheme. Endomixis does occur in some of lower ciliates.
5. Hemixis:
It also occurs in Paramecium aurelia. The macronucleus throws off its fragments which are latter absorbed by the cytoplasm. The remaining part of macronucleus functions as a normal macronucleus of the cell. The micronucleus undergoes no change and remains inactive. Diller believed that the fragments of macronucleus probably contain waste matter. Hence it is a kind of purification act.
Other Methods of Reproduction:
1. Parthenogenesis:
In Actinophrys and Chlamydomonas, fertilization by gametes or syngamy does not take place and they develop parthenogenetically into adults.
Endomixis results into the formation of new macronucleus from micronucleus which accelerates metabolic activities with new vigour and increase the vitality.
2. Regeneration:
The Protoplasm of the body of Protozoa has the power of regeneration of the lost parts. But it is usually not found in parasitic protozoans. A small part of the body having cytoplasm and a piece of nucleus develop into an individual.
3. Plasmogamy:
In some Rhizopoda, two individuals unite partially without the fusion of their nuclei. Later both separate from each other resulting into multinucleated mass of protoplast.