Academic journal article Genetics

Tetrahymena as a Unicellular Model Eukaryote: Genetic and Genomic Tools

Academic journal article Genetics

Tetrahymena as a Unicellular Model Eukaryote: Genetic and Genomic Tools

Article excerpt

GENETIC model systems have a long-standing history as important tools to discover novel genes and processes in cell and developmental biology. The ciliate Tetrahymena thermophila is a model system that combines the power of forward and reverse genetics with a suite of useful biochemical and cell biological attributes. Moreover, Tetrahymena are evolutionarily divergent from the commonly studied organisms in the opisthokont lineage, permitting examination of both unique and universally conserved biological processes. Here we highlight the advantages of Tetrahymena as a model system, such as its unique and easily manipulated life cycle, that have contributed to important discoveries. The growing suite of molecular-genetic and genomic tools described here provides a system to couple gene discovery to mechanistic dissections of gene function in the cell.

T. thermophila (Figure 1) is a ciliate model organism whose study has led to fundamental biological insights covering the central dogma and beyond. Indeed, this singlecelled, motile eukaryote provides the tools and techniques not only for novel gene discovery, but also for unveiling the important molecular mechanisms behind those genes' functions. As such, Tetrahymena's utility as a genetic model organism is revealed by its short life cycle, easy and cost-effective laboratory handling, and its accessibility to both forward and reverse genetics. Despite its affectionate reference as "pond scum" (Blackburn 2010), the beauty of Tetrahymena as a genetic model organism is displayed in many lights.

Tetrahymena has a long and distinguished history in the discovery of broad biological paradigms (Figure 2), beginning with the discovery of the first microtubule motor, dynein (Gibbons and Rowe 1965). Others include the Nobel Prize winning discoveries of catalytic RNA (Kruger et al. 1982) and telomere structure and telomerase (Greider and Blackburn 1985). The first histone-modifying enzyme (histone acetyl transferase) and its role as a transcription factor were discovered in Tetrahymena (Brownell et al. 1996), which gave birth to the "histone code" and the field of epigenetic control of gene expression by chromatin modification. The role of small interfering RNAs in heterochromatin formation and the massive, programmed excision of transposon-related DNA from the somatic genome (Mochizuki et al. 2002; Taverna et al. 2002) is another major Tetrahymena contribution. These fundamental discoveries in Tetrahymena have helped usher in the modern era of molecular and cellular biology. Many of the reasons why Tetrahymena was an advantageous system for these groundbreaking discoveries are the same that make Tetrahymena useful today and in the future, augmented by the ever-expanding toolkit available to Tetrahymena researchers.

In this review, we discuss major biological questions to which T. thermophila is amenable and the genetic tools available to answer them. It begins with the general biology of Tetrahymena and its unique advantages as an experimental model system. We then describe both forward and reverse genetic strategies in Tetrahymena to facilitate gene discovery and to interrogate the mechanistic underpinnings of those genes. Ultimately, we seek to engage future researchers by describing the wealth of experimental advantages (both historical and modern) that Tetrahymena can provide and preview its promising future.

Tetrahymena Biology

Tetrahymena are unicellular, ciliated eukaryotes that live in fresh water over a wide range of conditions. In the wild, Tetrahymena feed on bacteria, but laboratory strains typically live as axenic cultures in nutrient-rich media (derived from tissue extracts) or chemically defined media. Populations grow quickly, with cells dividing every 2-3 hr under optimal conditions. Each cell is large, 30-50 mminlength,making them ideal for light and electron microscopy-based investigations.

Tetrahymena, like all ciliates, are nuclear dimorphic, which means that the germline genome and the somatic genome exist within two separate nuclei in one cell: the micronucleus (MIC) and the macronucleus (MAC), respectively (Table 1, Table 2, and Figure 1). …

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