A Ciliate with Complicated Mating
In freshwater ponds and streams around the world lives a microscopic, pear-shaped organism named Tetrahymena thermophila. This single-celled protist navigates its aquatic environment using hundreds of tiny, hair-like structures called cilia. While common, its reproductive life is anything but. Instead of the typical two sexes, T. thermophila possesses seven distinct "mating types," designated I through VII. A cell of any given type can successfully mate with a cell from any of the other six types, but not with one of its own.
This organism also features a peculiar nuclear arrangement. Each cell contains two separate nuclei: a large somatic macronucleus and a small germline micronucleus. The macronucleus is the "working" nucleus, actively transcribing genes that control the cell's daily functions and appearance. The micronucleus is a silent, diploid archive of the genome, analogous to the germline cells in animals, and is used exclusively for sexual reproduction. The coordinates for this point of interest mark the University of California, Berkeley, an important center for research into the cell biology of this organism.
The Genetic Lottery
The determination of a Tetrahymena's mating type is a stochastic process that occurs after two cells conjugate. The mating types of the parent cells have no influence on the mating type of the progeny. Instead, the outcome is decided by a process of programmed DNA rearrangement.
The germline micronucleus contains a tandem array of incomplete gene pairs, with each pair corresponding to a potential mating type. During the formation of a new macronucleus in the offspring, a complex "cut-and-paste" mechanism is activated. One of the incomplete gene pairs is randomly chosen, completed by joining DNA segments from the ends of the array, and installed in the new macronucleus. All other potential mating-type gene pairs are excised and permanently deleted from the somatic genome. This leaves the new cell with a single, randomly determined mating type for life. This system dramatically increases the odds of finding a compatible partner compared to a two-sex system, boosting genetic diversity.
This organism's unique biology has made it an model for scientific discovery. Research on T. thermophila led to the Nobel Prize-winning discovery of telomeres, the protective caps on the ends of chromosomes, and the enzyme telomerase that maintains them.