Paramecium is one of the best-known protists, often taught in school biology courses. Paramecium is a ciliate genus. They are a clade of protists which move by the synchronous waves of tiny projections from their cuticle. These projections are called cilia (singular: cilium). The species range from 50 to 350 μm in length. They live in freshwater ponds, and eat bacteria, and other protists such as single-celled algae.
Reproduction in Paramecium has been researched for many years. Paramecium has two nuclei (a large macronucleus and a single compact micronucleus). They cannot survive without the macronucleus and cannot reproduce without the micro-nucleus. Reproduction is either accomplished by binary fission (asexual), conjugation (sexual), or, rarely, by endomixis, a process of self-fertilisation. During binary fission a fully grown organism divides into two daughter cells. Conjugation consists of the temporary union of two organisms and the exchange of micro-nuclear elements. Without the rejuvenating effects of conjugation a paramecium ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation.
This reproductive system is unique to ciliates, and is one of the reasons why we think the Protista is not a natural clade, but just a convenient collection of single-celled organisms of various origin.
Paramecium aurelia [change]
This species consists of 14 "syngens", each genetically isolated from every other, and biochemically unique. Each syngen has two mating types. The syngens are so similar in appearance that they have not been given separate species names.