Abstract

From Immortality to Mortality

Horizontal to Vertical Genetic Axis

From Small to Large Cells

Evolution of cellular lifespan

Paramecium as a model

Discussion
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I present a hypothesis on the origin of the cellular lifespan, or the limitation of division potential in somatic cells, and try to apply the implications to experimental studies on the clonal lifespan of Paramecium, the ciliate protozoan. The core of the hypothesis is summarized as a proposal that the safety devices for enlarged eucaryotes, i.e., 'differ- entiation into germ and soma' and 'diploidization' have coupled to the brake systems to control replication in soma and to initiate meiosis in germ. The contents are: 1. From immortality to mortality 2. From horizontal to vertical genetic axis 3. From small to large cells 4. A possible scenario of the evolution of cellular lifespan 4.1. Differentiation into germ and soma 4.2. Alternation between haploid and diploid 4.3. When do diploid cells become haploid? 5. Paramecium as a model for the study of cellular lifespan 5.1. Sonneborn limits in ciliates 5.2. Association of the clonal lifespan with other life cycle features 5.2.1. Clonal lifespan versus the length of sexual immaturity 5.2.2. Clock rewinding during sexual reproduction 5.2.3. Clonal lifespan versus cultural lifespan 5.3. Coupling between the lifespan and sexual reproduction

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Presentation Number SAtakagi0233
Keywords: Paramecium, lifespan, evolution, germ-soma, meiosis