Generalized ciliate life cycle. (0) Vegetative cells. (Small and large circles) MIC and MAC, respectively. (1) Two paired cells, homozygous for alternative alleles at one locus. (2) MICs undergo meiosis, and four haploid nuclei are produced. Only the anterior meiotic product remains functional; the other three disintegrate. This is the stage at which meiotic crossing-over, used for genetically mapping the MIC genome, occurs. (3) Mitotic division of functional meiotic product yields genetically identical migratory (anterior) and stationary (posterior) gamete pronuclei. (4) Migratory pronuclei are reciprocally exchanged and fuse with stationary pronuclei of the recipient cell, forming the zygote nucleus, which is diploid and, in this instance, heterozygous. (5) The zygote nucleus undergoes two mitotic divisions, giving rise to four genetically identical diploid nuclei. (6) Two of those nuclei (checkerboard-filled) have differentiated into macronuclei; the other two (solid and white halves) remain diploid micronuclei. The old MACs (at the bottom of each conjugant) are being resorbed and will be lost. This is the stage at which chromosome fragmentation and other site-specific DNA rearrangements occur in the differentiating MAC. The two exconjugants have separated and undergo their first binary fission, restoring the normal nuclear composition (back to stage 0). (7) Vegetative cell dividing by binary fission. The diploid MIC has divided mitotically; the polyploid MAC is undergoing “amitotic division,” pinching off into roughly equal halves. This life cycle scheme is highly conserved among ciliates, although differences of detail occur in particular groups and species.
