Fig. 1.

Life cycles of Tetrahymena thermophila. Tetrahymena cells have a transcriptionally active polyploid somatic nucleus and a transcriptionally silent diploid germline nucleus. Tetrahymena can reproduce by either vegetative or sexual reproduction. During vegetative growth, the germline nucleus undergoes mitosis, whereas the somatic nucleus undergoes an amitotic division where the chromosomes are divided roughly equally between the daughter cells. Under starvation conditions, two cells of different mating types can mate, and the germline nuclei of both cells divide by meiosis, whereas the somatic nuclei become degraded. During meiotic prophase, the germline nucleus elongates to form a crescent structure, in which homologous chromosomes pair and meiotic DSBs are formed and repaired by homologous recombination. Condensed bivalents are aligned at metaphase, which is followed by the first and second meiotic divisions. One meiotic product from each cell is selected for pronuclear exchange and fertilization, and the resulting zygotic nucleus divides twice to produce the new germline and somatic nuclei of the four sexual progeny