EXPERIMENTAL PRODUCTION OF CHAINS AND ITS GENETIC CONSEQUENCES IN THE CILIATE PROTOZOAN, COLPIDIUM CAMPYLUM (STOKES)

Abstract

In a clone of Colpidium campylum (Stokes), a small proportion (less than 1.2 per cent) of the individuals formed chains when cultivated in a rye infusion inoculated with the bacterium Micrococcus sp. (probably sensibilis), but not when the infusion was inoculated with Achromobacter sp. (probably candicans). Other factors, one of which possibly was the concentration of colpidia in the culture fluid, affected the proportion of chains formed when the colpidia were fed Micrococcus. Chains thus produced went through a series of developments including the formation of heteropolar doubles and multiple monsters, and culminating often in the formation of homopolar doubles of a self-perpetuating, relatively stable sort. Similar biotypes of doubles were also formed once as a result of a "pseudo-conjugation." In clones of homopolar doubles, singles sometimes arose by ordinary transverse fission across a double with a deep median anterior cleft. Consequently, mass cultures begun with doubles eventually contained singles also. As the two types multiplied side by side, the relative proportion of singles gradually increased. When the cultures were regularly renewed by taking a sample of the old culture to start a new one, eventually, after many such renewals, doubles entirely disappeared from the cultures, leaving only singles. The change occurred in both Achromobacter and Micrococcus fluid, but more rapidly in the former; this was probably due to the more rapid reproduction in that fluid. The change in the mass cultures was not due to dying out of doubles or to the transformation of all of them into singles. In isolation culture, lines of doubles were maintained as long as the period required for doubles to disappear entirely from mass cultures. Furthermore, when doubles were deliberately salvaged at each renewal of culture, they were maintained in cultivation, partly in isolation, partly in mass, for 194 days, during which about 582 generations passed. The disappearance of doubles from mass cultures in 41 days or less must therefore have been due to other factors than the inability of doubles to live and reproduce their kind. One of these factors was a differential rate of fission : the singles produced by the doubles of one clone multiplied 0.373 fissions per line per day more than the doubles of this clone. On the other hand, differential mortality counteracted this to some extent, for the mortality rate of singles was higher than the mortality rate of the doubles that produced them. The gradual disappearance of doubles in series of mass cultures was therefore due partly to their repeated production of singles and partly to their lower fission rate. The persistence of doubles during nearly 600 generations, by the end of which time no evidence of inability to maintain themselves had yet appeared, indicates that the type could have maintained itself indefinitely, even when the bacterium that led to its formation was absent or present in but very small quantities. Further evidence of stability of organization was the passage of a line of doubles through encystnient without loss of the double organization. The question of whether diverse biotypes existed among the experimentally produced doubles and their descendants was extensively investigated. (1) Different clones of doubles differed (a) in the rate at which doubles disappeared from series of mass cultures, (b) in rate of multiplication, (c) in rate of mortality. (2) Singles derived from diverse clones of doubles differed in rate of mortality to about the same extent as the clones of doubles from which they had been derived. (3) There were two very different kinds of singles derived from the same clone of doubles: the usual kind and a rarer kind formed by transverse fission across an unequally cleft double. The singles formed from the narrower anterior part of these were narrower, paler, and shorter than ordinary singles and had a much higher rate of mortality.(4) Singles not descended from doubles had a lower rate of mortality than singles descended from doubles. (5) Within a clone of doubles there were genetic differences in the frequency with which singles were produced, for long-continued selection with in lines of doubles brought to light very great changes in this frequency. Attempts to isolate by selecdon biotypic diversities in rate of fission within clones of doubles and of singles failed, although the coefficient of variation of fission rate was higher for doubles than for singles. The general picture of the genetic consequences of environmental action in the ciliate protozoan, Colpidium campylum, is strikingly similar to the picture in the rhabdocoel turbellarian, Stenostomum incaudatum. The similarity in these and the difference of both from the genetic effects of environmental action in higher organisms were ascribed to the method of reproduction. In asexual reproduction hereditary changes may arise without altering the nature of the chromatin; they may be due simply to changes in the number of units or arrangement of units in the self-perpetuating parts. The changes induced in Colpidium and Stenostomum were of this sort and, in this respect, resemble translocations, inversions, and reduplications in Drosophila.