Smith-Sonneborn J
Department of Zoology and Physiology, University of Wyoming, Laramie 82071.
Basic Life Sci. 1987;42:101-9. doi: 10.1007/978-1-4613-1939-9_7.
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.
在纤毛虫中，有一些细胞在大核中具有不同的增殖潜力。那些大核增殖潜力有限的物种需要有性生殖来激活储备核。根据资本投资理论，一些纤毛虫在不损失其原始潜力的情况下对其备用核进行了投资，而另一些则积累了债务，需要储备账户来维持生命。其他细胞忽视了对其储备账户的维护，除非其风险投资账户是一个能够自我维持的企业，否则就会失败。有性生殖提供了进入储备账户的途径，并且必须在储备账户恶化之前发生。问题不在于细胞永生何时丧失，而在于永生何时从有死的部分中分离出来。这种分离为多细胞生物中的衰老和进化都提供了选择。在群体鞭毛虫中，一些物种出现了具有无限和有限细胞寿命潜力的细胞分离，而在其他物种中，这种分离并不涉及永生的丧失。在群体鞭毛虫中，有性生殖并没有成为一个必经阶段。永生细胞是单倍体，不会积累损伤并存活（这与纤毛原生动物中的二倍体细胞形成对比）。目前的理论预测，具有无限与有限寿命潜力的物种或细胞之间的差异可能揭示了寿命关键决定因素的差异。衰老可能是一种意外，也可能是核分化的一种设计。因此，细胞的全能性储备可能比假设它们仅仅通过分化行为就丧失永生的情况下所预测的要大得多。
