Asexual Reproduction: Binary and Multiple Fission

Asexual Reproduction: Binary and Multiple Fission

Protozoans reproduce asexually chiefly by fission — the division of a single parent cell into two or more daughter cells, each essentially a genetic copy of the parent.

Binary fission is the division of one parent cell into exactly two daughter cells, and the plane along which this division occurs is characteristic of the organism:

• In Paramecium, binary fission is transverse — the plane of division runs roughly perpendicular to the cell's long axis, splitting the cell into an anterior daughter and a posterior daughter, each of which must then regenerate the structures (such as the oral groove) that it lacks after the split.
• In Euglena, binary fission is longitudinal — the plane of division runs roughly parallel to the cell's long axis (essentially splitting the cell lengthwise), so that each daughter cell inherits a share of the parent's organelles (such as the chloroplasts) running the length of the body, and each must regenerate a full complement of structures (such as a new flagellum, if only one daughter retains the original).

Multiple fission is a distinct mode of asexual reproduction in which the parent cell's nucleus divides repeatedly first, producing many nuclei within a single, still-undivided cell body; only afterward does the cytoplasm divide around each nucleus, producing many daughter cells simultaneously from a single parent — in contrast to binary fission, which always produces just two daughters from one division event. Multiple fission is often associated with the formation of a protective cyst around the parent cell beforehand, allowing the resulting daughter cells to be released together, sometimes in large numbers, once conditions are favourable. …