Tetrahymena as a Model Organism Tetrahymena
Tetrahymena thermophila is a useful model organism for molecular research at all levels. A ciliated single-celled protozoan, Tetrahymena is a freshwater organism that inhabits streams, lakes, and ponds and can be found almost everywhere, in a range of climates. The cells are large (40-50 um) and their complexity rivals human cells, making them a good alternative to human tissues (1). The cells are inexpensive to grow, requiring little more than a shaker, and they grow rapidly to high density in a variety of media and conditions. Tetrahymena possesses many core processes conserved across a wide diversity of eukaryotes (including humans) that are not found in other single-celled model systems (such as yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe) (3). Conventional tools for genetic analysis, and molecular genetic tools for experimental analysis of gene function have been developed (3) for use in Tetrahymena. Using these techniques homologues of human genes can be characterized, knocked out, and overexpressed among many other things.
Tetrahymena is a good model organism for molecular research in part due to its unique nuclear dimorphism. The organism has two nuclei with different functions: a germline micronucleus (MIC) and a somatic macronucleus (MAC). The MIC is the store of genetic information for sexual prodigy, is diploid and contains five pairs of chromosomes, andis transcriptionally silent (1). The MAC is expressed during vegetative replication. It contains ~300 chromosomes derived from the MIC chromosomes. The entire macronuclear genome has been sequenced. There are many implications for molecular research due to this nuclear dimorphism. For example, tagging a gene with a reporter such as GFP (green fluorescent protein) gives information about where the gene of interest is localizing in the cell. Because the MAC and the MIC have different functions, the localization of a gene gives valuable information as to its function.
Tetrahymena Life Cycle
The life cycle of Tetrahymena provides the opportunity to study factors important for different cellular processes. The life cycle consists of alternating haploid and diploid stages. Conjugation is the sexual stage of the life cycle. It involves meiosis, fusion of haploid MIC gametes to produce a new zygotic MIC, and differentiation of new MACs (called anlagen; An) from copies of the MIC. In addition, the old MAC (OM) degrades. After the anlagen is formed, cells can reproduce asexually. During asexual ("vegetative") growth, all gene expression occurs from the MAC while the MIC remains silent (3).
As shown for conjugation, two cells form a junction, exchange their gamete nuclei, and generate the nuclei of their progeny (1). To conjugate, cells must be starved of nutrients, be of different mating types, and be sufficiently mature. There are seven mating types of Tetrahymena. It is possible to learn about the proteins that are important to these different processes because all cells in a culture can be easily synchronized to undergo these events simultaneously. For example, a profile for the expression of any gene can be obtained throughout the entire conjugation pathway. From this, one can determine the stages of the cell cycle in which the gene of interest is being expressed, which provides insight into gene function.
Studying Gene Function
Tetrahymena cells facilitate the functional characterization of genes. It is possible to perform DNA-mediated transformation, gene 'knock-out' through homologous recombination, epitope-tagging, and inducible/repressible gene expression. These are complicated procedures with mammalian cells that can be more easily performed with Tetrahymena cells. They provide an ideal way to study gene function in vivo. In addition, nuclear and chromosomal activities can be monitored with DAPI staining and fluorescence microscopy, so phenotypic studies can be easily performed to examine gene function in the cell. The ciliate provides a simple way of looking at many genes, and has proved useful as a molecular research tool.