Fig 1. Vegetative and sexual cycles of Tetrahymena thermophila.

During vegetative growth (left), the 2 nuclei divide asynchronously. First, the germline nucleus (the MIC) divides mitotically, then the somatic nucleus (MAC) stretches and splits amitotically, and, finally, the cleavage furrow closes to divide the cell. Meiosis is induced when starved cells of 2 different mating types meet (right). The cells mate (“conjugate”), and their MICs undergo synchronous meioses. Three of the 4 meiotic products are degraded, while the remainder is selected and recruited to the conjugation junction, where it undergoes a postmeiotic mitosis. This division produces 2 gametic nuclei, one migratory and the other stationary. Migratory nuclei are exchanged between the partner cells and fertilize their stationary nuclei. The resulting genetically identical zygote nuclei divide twice, and the daughter nuclei differentiate into the new MICs and MACs of 4 sexual progeny cells, while the old MAC is eliminated via autophagy. As soon as new MACs form, they start to polyploidize. The 5 MIC chromosomes are fragmented into about 200 MAC chromosomes, and about one-third of the MIC genome is eliminated by the removal of thousands of internal eliminated sequences (see [76]). MAC, macronucleus; MIC, micronucleus.