Protozoa reproduce by various sexual and asexual processes. Asexual reproduction occurs by simple division, which can be equal or unequal, originating equal or not daughter cells, respectively. If there are more than two daughter cells, the division will be multiple. Budding is a variant of uneven cell division.
To remember, budding is the formation of one or more smaller cells from an original cell. In its strict use in protozoology, this designation should be reserved for those cases where the mother cell remains sessile and releases one or more swimming daughter cells. These differ from the mother cell, not only in terms of their lower degree of differentiation, but also because they have special locomotor organelles.
Two related processes should be mentioned: plasmotomy and schizogony. In the first of these, the multinucleated body of a protozoan divides into two or more small multinucleated individuals, whose maternal nuclei are randomly distributed among them. In schizogony, the multinucleated organism gives rise, within a short space of time, to many mononucleated gemmules being abandoned, usually, a mass of anucleated protoplasm.
Various types of sexual reproduction have been observed among protozoa. One is the sexual fusion of two gametes between protozoa. The sexual fusion of two gametes (singamy or gametogamy) occurs in several groups of protozoa. Conjugation, usually a temporary union of two individuals to exchange nuclear material, is observed exclusively in Ciliophora. After exchanging nuclei, the conjugants separate and each gives rise to its respective progeny by fission or budding. Some ciliates, however, demonstrate “total conjugation”, with complete fusion of the two organisms.
Regeneration: the ability to regenerate lost parts is characteristic of all protozoa, from simpler forms to those with highly complex structures. When a protozoan is cut in two, the nucleated portion regenerates, the anucleate does not. In ciliates, the macronucleus alone (or even part of it) is sufficient to ensure this process.
Laybourn-Parry, Johanna. A functional biology of free-living protozoa. Univ of California Press, 1984.
Sibly, Richard, and Peter Calow. “Asexual reproduction in Protozoa and invertebrates.” Journal of Theoretical Biology 96, no. 3 (1982): 401-424.
Tarín, Juan J., and Antonio Cano, eds. Fertilization in protozoa and metazoan animals: Cellular and molecular Aspects. Springer Science & Business Media, 2012.