flow chart of multiple fission
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Multiple fission also occurs among protists and is common in some parasitic species. The nucleus divides repeatedly to produce a number of daughter nuclei, which eventually become the nuclei of the progeny after repeated cellular divisions. There are several kinds of multiple fission, often correlated with phases or stages in the full life cycle of a given species. The number of offspring or filial products resulting from a multiple division (or very rapid succession of binary fissions) may vary from four to dozens or even hundreds, generally in a short period of time. Modes of such multiple fission range from budding, in which a daughter nucleus is produced and split from the parent together with some of the surrounding cytoplasm, to sporogony (production of sporozoites by repeated divisions of a zygote) and schizogony (formation of multiple merozoites, as in malarial parasites). The latter two phenomena are characteristic of many protists that are obligate parasites of more advanced eukaryotes. Some multicellular algal protists reproduce via asexual spores, structures that are themselves often produced by a series of rapid fissions.
Even under a light microscope, differences can be seen in the modes of division among diverse groups of protists. The flagellates, for example, exhibit a longitudinal, or mirror-image, type of fission (symmetrogenic fission). The ciliates, on the other hand, basically divide in a point-by-point correspondence of parts (homothetogenic fission), often seen as essentially transverse or perkinetal (across the kineties, or ciliary rows). Many amoebas exhibit, in effect, no clear-cut body symmetry or polarity, and thus their fission is basically simpler and falls into neither of the categories described above.
Sexual phenomena are known among the protists. The erroneous view that practically all protists reproduce asexually is explained by the fact that certain well-known organisms, such as species belonging to the genus Euglena, do not demonstrate sexuality. Even many of the unicellular species can, under appropriate conditions, form gametes (sex cells), which fuse and give rise to a new, genetically unique generation. In fact, sexual reproduction—the union of two gametes (syngamy)—is the most common sexual phenomenon and occurs quite widely among the protists—for example, among various flagellated organisms and pseudopods and among many parasitic phyla (e.g., in Plasmodium, a malaria-causing organism).
Conjugation, the second major kind of sexual phenomenon and one occurring in the ciliated protists, has genetic and evolutionary results identical to those of syngamy. The process involves the fusion of gametic nuclei rather than independent gamete cells. A zygotic, or fusion, nucleus, not a true zygote, is produced and undergoes a series of meiotic divisions to produce a number of haploid pronuclei; all but one of these pronuclei in each organism will disintegrate. The remaining pronuclei divide mitotically; one pronucleus from each organism is exchanged, and the new micronuclei and macronuclei of the next generation are formed. Following the exchange of the pronuclei and the subsequent formation of new micronuclei and macronuclei in each organism, a series of asexual fissions, accompanied by mitotic divisions of the new diploid micronuclei, occurs in each exconjugant line. The new polyploid macronuclei are distributed passively in the first of these divisions; in subsequent fission, the macronuclei duplicate themselves through a form of mitosis. This last stage constitutes the only reproduction involved in the process.
Answer:
Multiple fission also occurs among protists and is common in some parasitic species. The nucleus divides repeatedly to produce a number of daughter nuclei, which eventually become the nuclei of the progeny after repeated cellular divisions. There are several kinds of multiple fission, often correlated with phases or stages in the full life cycle of a given species. The number of offspring or filial products resulting from a multiple division (or very rapid succession of binary fissions) may vary from four to dozens or even hundreds, generally in a short period of time. Modes of such multiple fission range from budding, in which a daughter nucleus is produced and split from the parent together with some of the surrounding cytoplasm, to sporogony (production of sporozoites by repeated divisions of a zygote) and schizogony (formation of multiple merozoites, as in malarial parasites). The latter two phenomena are characteristic of many protists that are obligate parasites of more advanced eukaryotes. Some multicellular algal protists reproduce via asexual spores, structures that are themselves often produced by a series of rapid fissions.