Reproduction in Protozoa- Asexual & Sexual Reproduction
This involves the division of one individual into two approximately equal parts. The division is not a mere fragmentation but a complicated process of mitosis, during which nuclear division or karyokinesis is always followed by the division of cytoplasm or cytokinesis. Division or fission may be either in a transverse plane (e.g. Paramecium), or in a longitudinal plane (e.g. Euglena), or in an oblique plane (e.g. Ceratium) or in any plane (e.g. Amoeba). The two daughter organisms produced as a result of binary fission carry all the cytoplasmic organelles of the parent individual. Some organelles like mitochondria, divide the time of division, while others, like oral apparatus, flagella, and contractile vacuoles, are formed afresh by one of the daughters. In shelled sarcodina (e.g. Euglypha, Arcella) a mass of protoplasm extrudes from the opening of shell, which secretes a new shell. This double-shelled organism now divides into two. In ciliates (e.g. Paramecium), during fission mega or macronucleus divides amitotically and micronucleus by usual mitotic division. Some Protozoa divide only in the encysted stage (e.g. Colpoda, Tillina).
It is a special type of binary fission concerned with the division of multinucleate Protozoa into two or more smaller multinucleate daughter individuals. Plasmotomy takes place in Pelomyxa, opalinids and some other forms, etc.
In its simplest form budding implies modified fission resulting in a small daughter individual in the form of a bud. When the bud breaks off, it grows to full size. When a parental body produces only one bud it is monotonic (e.g. Vorticella), while in multiple budding several buds are formed simultaneously (e.g. Ephelota).
During multiple fission or sporulation, nuclear division is not followed immediately by division of cytoplasm. First, nucleus undergoes a series of divisions either by repeated binary fissions as in Plasmodium, or by simultaneous multiple divisions, as in Aggregata. The body thus becomes multinucleate. Later, the body cytoplasm divides into as many parts as there are daughter nuclei which usually arrange themselves at the periphery, each getting surrounded by a fragment of cytoplasm. Thus, the parent body simultaneously divides into as many daughter individuals as there are nuclei. Parent cell usually leaves behind some residual cytoplasm which disintegrates afterwards. Number of offspring greatly varies among different and the same species and sometimes runs into thousands. Multiple fission is quite common in Foraminifera, Radiolaria, Sporozoa and certain Mastigophora. The process receives different names according to the particular period in life cycle when it occurs.
(a) Schizogony– In this process, a series of nuclear divisions results into numerous daughter nuclei. This is followed by the formation of cytoplasmic buds, each containing a nucleus. The pinched off to grow directly into new organisms.
In certain Rhizopoda and Mycetozoa, two or more individuals may fuse by their cytoplasm to form a plasmodium, in which the nuclei remain distinct, and they separate again unchanged afterwards. The process, which is thus non-sexual and not syngamy, is called plasmogamy and sometimes serves the purpose of digestion of large prey.
In Protozoa, sexual reproduction takes place mainly by two processes: (i) syngamy or fusion of two sex cells, and (ii) conjugation or temporary contact of two protozoans with nuclear exchange.
Syngamy is the complete fusion of two sex cells or gametes, resulting in the formation of zygote. The fusion nucleus of zygote is called synkaryon. Depending upon the degree of differentiation displayed, by the fusing gametes, syngamy is of the following types :
(a) Hologamy– The two ordinary mature protozoan individuals do not form gametes but themselves behave as gametes and fuse together to form zygote. Hologamy occurs in a few Sarcodina and Mastigophora (e.g. Copromonas).
(b) Isogamy– When two fusing gametes are similar in size and shape but differ in behaviour, they are called isogametes and their union, isogamy. Isogametes are generally produced by multiple fission. Isogamy is common in Forminifera (e.g. Elphidium), Gregarinia (e.g. Monocystis) and Phytomonadida (e.g. Chlamydomonas).
(c) Anisogamy– When two fusing gametes, differ morphologically as well as in behaviour, they are called anisogametes. Usually small and motile gametes are the male or microgametes and large non-motile ones are the female or macrogametes. Fusion of such dissimilar gametes is anisogamy. This mode of sexual reproduction is widely seen in Sporozoa (e.g. Plasmodium) and Phytomonadida (e.g. Volvox).
(d) Autogamy- It is the fusion of gametes derived from the same parent cell, as in Actinophrys and Actinosphaerium. In Actinophrys, during sexual reproduction, pseudopodia are withdrawn and a cyst is formed. Now meiotic division takes place and two daughter nuclei with half number of chromosomes are formed. No cell division takes place. After sometime, gametic nuclei fuse to form a zygote nucleus.
This involves temporary union of two individuals, called conjugants, usually at oral or buccal regions of their body. It is characteristic of Suctoria and holotrich ciliates.
Fusion of protoplasm occurs at the place of contact. Macronuclei break up and disappear. Micronuclei undergo meiotic division now all but one micronuclei degenerate. This remaining micronucleus again divides forming two gametic micronuclei. Out of these two, one is considered male pronucleus and other female pronucleus. Male pronucleus of one conjugant moves through fused protoplasm into the other conjugant. In each conjugant, these male and female pronuclei fuse together forming a zygote nucleus. Now two individuals separate and are called exconjugants. Each exconjugant undergoes further nuclear and cytoplasmic divisions forming four daughter individuals.
Association in conjugation is not at random but indicates a high level of specialization. Sonneborn has recognized different syngens in a species of Paramecium and each syngen includes two mating types. Conjugation can only take place between individuals of the same syngen but belonging to opposite mating types. The unique feature of conjugation is an exchange of hereditary material so that each conjugant benefits from a renewed hereditary constitution.
In Actinophrys, the gametes which fail at cross-fertilization, develop parthenogenetically. It also occurs in Chlamydomonas and others when syngamy has been missed. Individuals of Polytoma, which are potential gametes, can grow and divide parthenogenetically.
Most Protozoa can regenerate their lost parts, as normally displayed at fission or encystment. Parasitic Protozoa usually have slight regenerative capacity. Nucleus plays an important role in the process. Relative quantities of nuclear and cytoplasmic materials and the size of the broken piece affect the rate and result of the process of regeneration.
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