Asexual Reproduction:
| Binary Fission | Division of an individual into approximately 2 equal parts. Karyokinesis followed by Cytokinesis. Division might occur across transverse, longitudinal, oblique or any plane. Some organelles from the parent are distributed, some are regenerated in the daughter cells. Some Protozoa divide only in the encysted stage. e.g. Colpoda. |
| Plasmotomy | Division of multi-nucleate Protozoa into two or more smaller multinucleate daughter individuals. Takes place in Opalina. |
| Budding | A variant of fission which results in a smaller small daughter individual in the form of a bud. When the bud breaks off from the parent, it grows to full size. |
| Multiple Fission | Sporulation. Common in Foraminifera, Radiolaria, Sporozoa Nuclear division is not followed immediately by division of cytoplasm. First nucleus undergoes a series of divisions by repeated binary fissions or by simultaneous multiple divisions. This multinucleate body then divides into as many parts as there are nuclei through cytoplasmic division. Schizogony: Formation of numerous daughter nuclei — formation of cytoplasmic buds each containing a nucleus — Buds are pinched off to grow directly into new organisms |
| Plasmogamy | Two or more individuals may fuse their cytoplasm to form a Plasmodium, in which the nuclei remain distinct, and they separate afterwards. Serves the purpose of digestion of large prey. |
Sexual Reproduction:
| Syngamy | Complete fusion of two sex cells or gametes, resulting in formation of zygote. Classified based on degree of differentiation displayed by the fusing gametes. Hologamy Two protozoan individuals do not form gametes, but themselves behave as gametes and fuse to form zygote. e.g. Sarcodina and Mastigophora. Isogamy Fusing gametes are similar in morphology but different in behaviour. e.g. Foraminifera, Monocystis, Chlamydomonas. Anisogamy Fusing gametes differ in both morphology and behaviour. Conventionally, male gamete is small and motile, female gamete is large and non-motile. e.g. Plasmodium, Volvox. Autogamy Fusion of gametes derived from the same parent cell. e.g. Actinophrys Pseudopodia are withdrawn and a cyst is formed — Meiotic division that leads to two daughter nuclei with half the number of chromosomes (no cell division) — gametic nuclei fuse to form zygote |
| Conjugation | Temporary union of two animals (conjugates) — usually at oral or buccal regions. e.g. Suctoria ciliates Different syngens, and each syngen has 2 mating types. Conjugation can only take place between individuals of the same syngen but opposite mating types. Process Fusion of protoplasm occurs at place of contact. Macronuclei break up and disappear. All but one micronuclei degenerate. Single micronucleus divides forming two gametic micronuclei. Out of these two, one is considered male pro-nucleus and the other female pronucleus. The pro-nuclei move through protoplasm to the other cell and fuse with the opposite sex gamete. Forms a Zygote nucleus. Individuals separate and are called exconjugants. Conjugation thus, involves genetic exchange. Each exconjugant undergoes further nuclear and cytoplasmic divisions forming 4 daughter individuals. |
| Parthenogenesis | Gametes which fail at cross fertilisation, develop parthenogenetically. In Chlamydomonas, it occurs when syngamy has been missed. Individuals of Polytoma which are potential gametes can grow and divide parthenogenetically. |
| Regeneration | Most protozoa can regenerate their lost parts. Ability to regenerate is generally less among parasitic protozoa. Relative quantities of nuclear and cytoplasmic material and size of the broken piece affect the rate and result of process of regeneration. |
Evolution of sex in Protozoa:
Asexual Reproduction
Early Protozoans adopted this method as the primary means of multiplication.
Enabled them to build up large populations in shortest possible time.
Binary fission, multiple fission, budding, plasmotomy etc.
Best suited to these animals which possessed delicate bodies, no apparent means of defence and therefore, needed a speedy way of multiplication.
In asexual method, all individuals produce offspring without the help of other individuals.
Saves energy and time.
Disadvantages: Produces clones and does not produce diversity.
Sexual Reproduction
Sexual reproduction involves two types of individuals producing two types of gametes whose primary aim is to exchange the genetic material and to bring about genetic diversity in the population.
Power of recombination is so great that chances of two individuals being identical are almost nil.
Such a trait is advantageous and is favoured by natural selection.
In sexually reproducing populations, only 50% individuals produce offspring, while the rest half just contribute their genes, thus reducing the reproductive capacity, whereas in asexuals all individuals produce offsprings with no help from others.
Evolution of Sex
First there was asexual reproduction – fast and simple but produced clones and could not get rid of harmful effects of mutation.
Meiosis evolved to reduce the no. of chromosomes to half.
Nucleus and the cell division apparatus were necessary to invent the process of meiosis that produced
gametes carrying half the number of genes.
Isogamy produced equal sized gametes that had equal amount of energy investment.
They were large cells, had low searching capacity and did not have particular attraction towards each other to fuse and form zygote.
Anisogamy: Different mating types originated.
Anisogamy to some extent tried to solve the problem of cell fusion.
Large number of mating types was reduced to two.
Larger gametes (females) specialized in the storage of nutrients and development of embryo. Could now be produced in small numbers. Being in small numbers became limiting resource, triggering competition, and in turn selection.
Small gametes were in large number and competed to fertilise the large gametes.
Evolution of anisogamy from the primitive isogamy happened due to two selection forces:
1) Increased efficiency in finding mating partner in anisogamy.
2) Supply the zygote with sufficient amount of reserve food for development of embryo.