Protozoan Sexuality


Sexuality in the mostly unicellular Protozoa (lower eukaryotes), as in the higher eukaryotes (animals, fungi, plants), basically involves an activity, obligate or facultative in the life cycle, in which differentiated haploid gametes (or gametic nuclei) fuse to form a diploid zygote (or zygotic nucleus) which, in turn, develops – via a series of asexual cell divisions, the first of which sometimes includes secondary reduction to nuclear haploidy – into a new population or generation of the organism.

Keywords: hologamy; isogamy; pedogamy; anisogamy; oogamety; syngamy; zygote; monoecy; dioecy; hermaphroditism; parthenogenesis; gamontogamy; gamonty; conjugation; autogamy; cytogamy; pronucleus; synkaryon; senescence; rejuvenescence

Figure 1.

Asexual reproduction (stages 1–4) only in the rather complex life cycle of the haploid schizopyrenid sarcodinid amoeba Naegleria gruberi. Cycling flagellated (stage 4′) and encysted (stage 4′′) forms may alternate in the life cycle of this amoeboflagellate, following the induction of different environmental factors.

Figure 2.

Sexual reproduction (better, process) (stages 1–6) with isogametes (from + and − strains) in the life cycle of the haploid phytoflagellate Chlamydomonas eugametos. F! indicates fusion of gametes. Notice that meiotic reduction (M!) is zygotic. Successive empty circles indicate repeated cycles of purely asexual reproduction of haploid organisms.

Figure 3.

Pedogamy in the diploid heliozoan sarcodinid Actinophrys sol (stages 1–6). Within the original cystic membrane surrounding the previously vegetative organism (stage 1), sister diploid gamonts (stage 2) produce many isogametes (stages 3–4), following meiosis (M!), which behave differently in defusing with each other (stage 5 and F!) to produce the diploid zygote in the life cycle (stage 6).

Figure 4.

Oogamety (marked gametic dimorphism) in the complex anisogamous life cycle of an eimerian coccidian sporozoon with sporic reduction (M!). A single host is envisioned in this diagram; but when two hosts are involved (such as in the haemosporidian malarial organisms), stages 2–7 would be found in the intermediate host (such as a human) and 8–11 (including fusion of anisogametes, F!) plus 1 would occur in the definitive or final host (such as the mosquito).

Figure 5.

Sexual reproduction (better, process) with isogametes in the unique life cycle of the sarcodinid single‐chambered foraminiferan Myxotheca arenilega, showing its obligatory heterophasic alternation of generations: haploid sexual (stages 2–5) and diploid asexual (6–9). F! indicates time and place of occurrence of fusion of the isogametes; M! the location of meiosis in the cycle.

Figure 6.

Conjugation in the diploid heterokaryotic hypotrichous ciliate Euplotes crassus, showing (counterclockwise) principal stages (1–11) in the process for this species. Heterozygous individuals are depicted with white and black micronuclei and macronuclei; these nuclei are entirely white in homozygous individuals. Note exchange of the migratory pronuclei in stage 7. Maupasian phases in the life cycle are indicated in the labels near the open circles (extended stage 11): im, clonal sexual immaturity; ma, maturity; se, senescence; cd, clonal death, occurring if eventually no rejuvenating sexual activity, such as conjugation or autogamy, occurs or is allowed to occur. M! indicates time/place of meiosis in the cycle; F! of fusion of the pronuclei.

Figure 7.

Autogamy (self‐fertilization) in encysted cells of the diploid heterokaryotic holotrichous ciliate Tetrahymena rostrata, showing some of the successive stages in the process and the locations of meiosis (M!) and fusion of pronuclei (F!) in the cycle. Mic, the diploid micronucleus (germinal nucleus); Mac, the polyploid macronucleus (somatic nucleus). The excysted diploid trophont (stage 8) of this intensive inbreeder may go through many stages of repeated asexual divisions (indicated by the repetitive empty circles) before returning to the presumably rejuvenating act of autogamy (stages 2–6). No conjugation has ever been discovered in this unusual species of Tetrahymena.



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Further Reading

Anderson OR (1988) Comparative Protozoology: Ecology, Physiology, Life History. Berlin: Springer.

Grell KG (1967) Sexual reproduction in Protozoa. In: Chen T‐T Research in Protozoology, 2: pp. 149–213.

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Hausmann K and Hülsmann N (1996) Protozoology, 2nd edn. Stuttgart: Thieme.

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Raikov IB (1982) The Protozoan Nucleus: Morphology and Evolution. Vienna: Springer.

Sleigh MA (1989) Protozoa and Other Protists, 2nd edn. London: Arnold.

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Dini, Fernando, and Corliss, John O(Apr 2001) Protozoan Sexuality. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0001938]