The Tetrahymena Conjugation Junction
Eric S. Cole
Life History: Sexual and Asexual Reproduction
Those who study ciliates have struggled over the years to establish a place in the pantheon of model organisms.1 Eukaryotic cell biologists have turned profitably to yeast models for the powerful genetic tools at their disposal, while developmental biologists have cultivated a gallery of metazoan embryos with contributions from the plant and fungal worlds. Yet ciliates continue to contribute to fundamental aspects of both cell biology and development, often by extreme example, and among the ciliates, Tetrahymena has emerged as one of the stars.
Tetrahymena is a freshwater, unicellular organism, approximately 40 to 60 microns in length. They feed on bacteria which they capture by means of four ciliated “membranelles” that form a set of three combs that brush particles into a curved buccal cavity rimmed by a fourth “undulating membrane” (Fig. 1). Ciliates are characterized by three conspicuous features, ciliature that can be specialized for locomotion or food capture, alveolar membranes that lie just beneath the plasma membrane forming a set of flattened sacks, and “nuclear duality”.2 Ciliates posses both a somatic, transcriptionally active macronucleus, and a germinal, transcriptionally silent micronucleus. It has been attractive to borrow the language of germline and soma from the world of metazoan embryos and recent findings suggest that this may reflect more than creative license. Indeed, ciliates may represent the simplest form of life to generate distinct somatic and germinal nuclear lineages.
During vegetative growth, ciliates reproduce by binary fission. In Tetrahymena, this involves a remarkable reorganization of the cortical cytoskeleton. The equatorial fission zone separates the anterior division product (proter) containing the functional oral apparatus from the posterior fission product (opisthe) containing the functional water-expulsion organelle, (the contractile vacuole system) along with the cell “anus” or cytoproct (Fig. 1). Each division product would perish without the coordinate synthesis of a new oral structure (Oral Primordium, see Fig. 2) in the posterior opisthe, and a new Contractile Vacuole Pore system in the proter. Events that attend the cortical patterning associated with vegetative cell division have been studied extensively.3
When Tetrahymena cells are removed from nutrient medium and starved, they have a repertoire of morphogenetic responses that seem adaptive. Starved cells frequently undergo a process of “oral replacement”.4 This involves disassembling the existing oral apparatus and reforming a new one through de novo basal body synthesis. Cells that are maintained under nutritional stress may undergo a second type of transformation to a “dispersal” form, or fast swimmer.5,6 Finally, nutritional challenge can predispose Tetrahymena for sexual reproduction including meiosis, chromosomal recombination and the exchange of meiotic products with another cell. Sex as a strategy for grooming ones’ genome has also been richly discussed in ciliates.7