Protozoan Asexuality

Abstract

Asexuality is a mode of reproduction that, unlike sexual reproduction, occurs without meiosis and the fusion of gametes. Its consequence is multiplication and conservation of the genotype in the offspring. In protozoa, asexual reproduction occurs by cell division.

Keywords: asexuality; reproduction; mitotic division; cell cycle; protozoa

Figure 1.

Binary fission in the ciliate Euplotes octocarinatus (cells stained with acetocarmine acid). (a) In S phase, two replication bands (RB) migrate from the tips of the C‐shaped macronucleus (MA) to its centre. The area concerned is involved indeoxyribonucleic acid (DNA) replication and histone synthesis. (b) The macronucleus is constricted in its centre and divides amitotically into two equal parts. The micronucleus (MI) has already divided in a mitotic process. Finally, the parent cell gives rise to two daughter cells (cytokinesis) by a fission plane (FP) that is transverse to the cell's long axis.

Figure 2.

Life cycle of Plasmodium sp. Two asexual (schizogony, sporogony) and one sexual (gamogony) reproduction phases are distinguished. Sporozoites enter liver cells where they begin a phase of asexual multiplication called ‘exoerythrocytic schizogony’, during which thousands of uninucleate merozoites are formed. These enter erythrocytes, in which they undergo the erythrocytic schizogony, during which fewer than 24 merozoites are formed. Some of the merozoites develop into sexual stages, which are taken up by a mosquito. The resulting zygote or ookinete bores through the gut wall and forms an oocyst. Within the oocyst asexual multiplication occurs, resulting in the formation of sporozoites that enter the salivary glands of the mosquito.

Figure 3.

Types of fission in protozoa (not to scale; for details, see text). (a) Elementary form of binary fission in Amoeba proteus. (b) Division of a cryptic rhizopod, Euglypha alveolata. (c) Symmetrogenic division in trypanosomes. (d) Homothetogenic division in Paramecium caudatum. (e) Multiple fission in Ichthyophthirius multifiliis. (f) Budding in Eufolliculina uhligi. (a)–(c), after Grell ; (e) after Mehlhorn and Piekarski ; (f), after Mulisch .

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References

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Further Reading

Dönges J (ed.) (1988) Parasitologie. Stuttgart: Thieme‐Verlag.

Margulis L, Corliss JO, Melkonian M and Chapman DJ (eds) (1990) Handbook of Protoctista. Boston: Jones and Bartlett.

Mehlhorn H (ed.) (1988) Parasitology in Focus. Berlin: Springer‐Verlag.

Mehlhorn H and Ruthmann A (eds) (1992) Allgemeine Protozoologie. Jena: Fischer‐Verlag.

Pickett‐Heaps JD (ed.) (1975) Green Algae. Structure, Reproduction and Evolution in Selected Genera. Sunderland, MA: Sinauer.

Trager W (ed.) (1986) Living Together. The Biology of Animal Parasitism. New York: Plenum Press.

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How to Cite close
Kuhlmann, Hans‐Werner(May 2001) Protozoan Asexuality. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001937]