Reproduction and life cycles
Reproduction in the Protozoa may be either asexual or sexual. The commonest type of asexual reproduction is binary fission, i.e., each individual divides into two. The plane of fission is longitudinal in the flagellates and transverse in the ciliates. Cytoplasmic division follows nuclear fission and separation of the daughter nuclei. Vesicular nuclei and micronuclei divide mitotically; macronuclei divide amitotically.
Multiple fission or schizogony is found mostly in the Telosporasida. In this type of fission, the nucleus divides several times before the cytoplasm divides. The dividing cell is known as a schizont, agamont or segmenter, and the daughter cells are merozoites or schizozoites. Nuclear division, again, is mitotic.
A third type of asexual division is budding. In this process, a small daughter individual is separated off from the side of the mother and then grows to full size.
Internal budding or endodyogeny has been described in Toxoplasma and Besnoitia. Two daughter cells are formed within the mother cell and then break out, destroying it (Goldman, Carver and Sulzer, 1958).
Several types of sexual reproduction have been described, but only two occur in parasitic protozoa. In conjugation, which is found among the ciliates, two individuals come together temporarily and fuse along part of their length. Their macronuclei degenerate, their micronuclei divide a number of times, and one of the resultant haploid pronuclei passes from each conjugant into the other. The conjugants then separate, and nuclear reorganization takes place.
In syngamy, two gametes fuse to form a zygote. If the gametes are similar in appearance, the process is called isogamy; if they are different, it is anisogamy, the smaller gamete being the microgamete and the larger one the macrogaynete. The gametes may be produced by special cells, the microgametocytes and macrogametocytes, respectively. These are also sometimes called gamonts. The zygote may or may not then divide by multiple fission to form a number of sporozoites. The process of gamete formation is known as gametogony. It may differ in different groups, and will be described in the appropriate places below.
Some protozoa form resistant cysts or spores. A cyst results from the formation of a heavy wall around the whole organism. Spores are produced within the organism by the formation of heavy walls around a number of individuals which have been produced by multiple fission or otherwise. This process, known as sporogony, ordinarily follows syngamy. Each spore may contain one or more individual organisms or sporozoites.
The vegetative, motile stage of a protozoon is known as a trophozoite.