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Eukaryotes (such as protists and unicellular fungi) may reproduce in a functionally similar manner by mitosis; most of these are also capable of sexual reproduction. Multiple fission at the cellular level occurs in many protists , e.g. sporozoans and algae .
Even protists that no longer reproduce sexually still retain a core set of meiosis-related genes, reflecting their descent from sexual ancestors. [155] [156] For example, although amoebae are traditionally considered asexual organisms, most asexual amoebae likely arose recently and independently from sexually reproducing amoeboid ancestors. [157]
Obligate parthenogenesis is the process in which organisms exclusively reproduce through asexual means. [41] Many species have transitioned to obligate parthenogenesis over evolutionary time. Well documented transitions to obligate parthenogenesis have been found in numerous metazoan taxa, albeit through highly diverse mechanisms.
Reproduction in Protozoa can be sexual or asexual. [38] Most Protozoa reproduce asexually through binary fission. [39] Many parasitic Protozoa reproduce both asexually and sexually. [38] However, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically. [40]
Parthenogenesis is a mode of asexual reproduction in which offspring are produced by females without the genetic contribution of a male. Among all the sexual vertebrates, the only examples of true parthenogenesis, in which all-female populations reproduce without the involvement of males, are found in squamate reptiles (snakes and lizards). [1]
Many protist species can switch between asexual reproduction and sexual reproduction involving meiosis and fertilization. [6] In contrast to the cells of prokaryotes, the cells of eukaryotes are highly organised. Plants, animals and fungi are usually multi-celled and are typically macroscopic. Most protists are single-celled and microscopic.
Many protists reproduce sexually, as do many multicellular plants, animals, and fungi. In the eukaryotic fossil record, sexual reproduction first appeared about 2.0 billion years ago in the Proterozoic Eon, [64] [65] although a later date, 1.2 billion years ago, has also been presented.
Paramecium reproduction is asexual, by binary fission, which has been characterized as "the sole mode of reproduction in ciliates" (conjugation being a sexual phenomenon, not directly resulting in increase of numbers). [3] [32] During fission, the macronucleus splits by a type of amitosis, and the micronuclei undergo mitosis. The cell then ...