Longevity in the protozoa.

In ciliates there are examples of cells which have different proliferation potential in the macronucleus. Those species with limited macronuclear proliferation potential require sex to activate the reserve nucleus. In terms of the capital investment theory, some ciliates invested in their spare nucleus without loss of their original potential, while others accumulated debts and needed the reserve account to maintain life. Other cells neglected maintenance of their reserve account and failed unless their venture capital account was not a self-sustaining venture. Sex provided access to the reserve account and had to occur before deterioration of the reserve account. The question is not when cellular immortality was lost, but rather when immortality was partitioned from a mortal segment. The separation provided the option both for senescence and evolution in multicellular organisms. In colonial flagellates, separation of cells with infinite and finite cell lifespan potential occurred in some species, while in others the separation did not involve loss of immortality. In colonial flagellates, sex did not become an obligate stage. The immortal cells are haploid and could not accumulate damage and live (in contrast with the diploids in the ciliated protozoans). The present theory predicts that differences between species or cells with infinite versus finite lifespan potential may reveal differences in the critical determinants of longevity. Senescence could arise as an accident, as well as a design of nuclear differentiation. Cells therefore may have a much greater reserve for totipotency than would be predicted if they were assumed to lose immortality simply by the act of differentiation.