The Relation of Autoǵamy to Senescence and Rejuvenescence in Paramecium aurelia

Two problems were posed for experimental analysis: (1) What is the effect of omission of both conjugation and autogamy on fission rate and other characteristics of P. aurelia? (2) What effects does autogamy have when it occurs after intervals of varying length since the last previous fertilization? These questions were explored most thoroughly in stock R of variety 1 and stock W of variety 2; but some data are also presented for stocks S and 60 of variety 1, stock E of variety 2, and stocks 29, 32, 47, and 51 of variety 4 and their hybrids. In essential features the results on all stocks and varieties are in agreement, as far as they go; but the details differ. To the first question, the answer is simple and clear: under the conditions of culture employed, fission rate declines and death eventually occurs in daily isolation lines that omit both conjugation and autogamy. The maximum number of fissions until death was about 350 in stock R (variety 1), about 300 in stock W (variety 2) and perhaps as low as 200 in variety 4. Early in life the fission rate is high and shows little or no decrease for a considerable time; it then decreases progressively and slowly for a long time; and may finally decline at a faster rate. Decline and death are preventable by permitting autogamy to occur repeatedly without unduly long intervals between them. Viable lines maintaining high fission rate may be continued in this way without apparent limit. The answer to the second question is more complex, for autogamy may have any one of several consequences. Even in controls undergoing recurrent autogamy at relatively short intervals, autogamy sometimes yields nonviable clones or clones with low fission rate; but it usually yields viable clones manifesting normal high fission rate. Likewise, the most vigorous clones arising at autogamy from parents that have not begun to decline in fission rate show no increase in fission rate, but undergo a whole life cycle consisting of as many fissions as the total life cycle of the parent from its origin at fertilization until its death. As the parents progressively decline in fission rate, the most vigorous clones to which they give rise at autogamy again initiate new life cycles characterized at the start by normal high fission rate. Since the latter is above the fission rate level of the parent, rejuvenescence is at once apparent. However, not all autogamous offspring of aging parents initiate new life cycles at the normal maximum fission rate level. Some exhibit an immediate marked decrease in fission rate below the level of the controls and the parents and may maintain about the same fission rate for long periods. Others drop at once to low fission rate and appear to start to decline further very soon. Still others show very little or no change from the level of the parental fission rate, even when the latter has declined greatly at the time of their origin. Variation in the fission rate of offspring arising at autogamy is one of the capital facts brought out by this study. Rejuvenescence at autogamy occurs, but only in some autogamous offspring. At the other extreme, autogamy in aging clones—as in controls—sometimes leads to death at once or within several fissions. The frequency with which this happens increases markedly with the age of the parent and eventually it becomes impossible to obtain viable autogamous offspring, as also reported by other investigators. At this stage in the life history, the doom of the strain is sealed: it will die eventually without autogamy and it will die quickly if it undergoes autogamy. Finally, in the last stage of the life cycle, autogamy can be induced only with difficulty and in a small proportion of the animals, if at all. Yet, in spite of all the risks involved, the animals must undergo autogamy (or conjugation) or perish. Of those that take the risk, if they have not waited too long to do so, some will survive and be rejuvenated. The end result of our study is thus to place on a firm experimental basis the conclusion to which Woodruff and Erdmann (47) were forced forty years ago by the logic of the situation, even in the absence of experimental analysis: conjugation is not essential for P. aurelia; autogamy (endomixis) alone can maintain the organisms in life and vigor. The main hypotheses that have been advanced to account for senescence and rejuvenescence in ciliates are discussed in the light of the foregoing results and other available knowledge. It is concluded that present information does not yet permit a decision as to the correct interpretation. Certain values and difficulties of the various hypotheses are pointed out, as are some opportunities to bring recent advances in knowledge to bear decisively on a choice among them. It appears possible that the ultimate solution of these problems will involve elements of more than one of the current hypotheses.