Tetrahymena thermophila

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Classification

Higher order taxa

Eukaryota; Chromalveolata; Ciliophora; Oligohymenophorea; Hymenostomatida; Tetrahymenidae

Species

Tetrahymena thermophila

Description and Significance

Tetrahymena thermophila is a large, motile, phagocytic, unicellular eukaryote. The organism is about 30 x 50 µm. T. thermophila live in temperate freshwater environments.

Describe the appearance, habitat, etc. of the organism, and why you think it is important.

Genome Structure

T. thermophila contain two nuclei, a macronucleus and a micronucleus. Each has been sequenced[1]. The macronucleus is ultimately derived from the micronucleus. The micronucleus is diploid and contains 5 pairs of chromosomes. The macronucleus contains 45 copies of 275 chromosomes formed by fragmentation, as well as 9000 copies of the rRNA gene. The macronucleus contains 15% less genetic information than the micronucleus, which represents noncoding DNA and transposable elements. The macronucleus is kept intact through asexual reproduction by telomerase activity. Additionally the mitochondrial genome has been sequenced [2].

Vegetative cell with macronucleus and micronucleus present

Cell Structure

Tetrahymena is a large and complex cell yet a rather small and simple ciliate. The impression of its being animal-like is reinforced by its having all the basic structures of animal cells, with the exception of intermediate filaments, and its lacking the structures characteristic of plant cells (chloroplasts, vacuole, cell wall). What is distinctive about ciliates, including Tetrahymena, is nuclear dualism and the arrangement of structures in the surface region (cortex) of the cell. Tetrahymena possesses a diploid micronucleus possessing two complete haploid genome sets and capable of both mitosis and meiosis, plus a macronucleus that is made up of multiple copies of a rearranged subset of the complete (micronuclear) genome. The micronucleus is mostly inert transcriptionally, whereas the macronucleus is transcriptionally very active. These two nuclei represent an intracellular separation of germ (micronucleus) and soma (macronucleus).

Metabolism

How it gains energy; what important molecules it produces.

Life Cycle

The presence of the two nuclei reflects T. thermophila’s ability to reproduce both sexually and asexually. During vegetative growth the macronucleus is transcribed, translated and transmitted to the next asexual generation by mitosis. Food stress induces T. thermophila to reproduce sexually, producing pronuclei by meiosis of the micronucleus and exchange with a cell of a different mating type. The macronucleus is ultimately derived from the micronucleus.

T. thermophila life cycle

Ecology and Pathogenesis

Habitat; symbiosis; biogeochemical significance; contributions to environment.
If relevant, how does this organism cause disease? Human, animal, plant hosts? Virulence factors, as well as patient symptoms.