TILLINA MAGNA: MICRONUCLEAR NUMBER, ENCYSTMENT AND VITALITY IN DIVERSE CLONES; CAPABILITIES OF AMICRONUCLEATE RACES

¹ Department of Zoology, University of North Carolina, Chapel Hill

The number of micronuclei was examined in 50 trophic specimens and 50 resting cysts of each of 20 clones of Tillina magna, three of which were amicronucleate.

In any particular clone trophic specimens and resting cysts contained approximately equivalent mean numbers of micronuclei. In different micronucleate clones the mean number varied from 4.21 to 12.61. The mean for 1,700 specimens of 17 clones was 7.07.

The number in the individuals of any particular micronucleate clone was variable; some clones showed relatively little variation, e.g., 3 to 5 micronuclei; others, considerable variation, e.g., 2 to 11 micronuclei. The smallest number observed in any micronucleate individual was 2; the largest, 16.

All the clones produced normal resting cysts upon depletion of the food supply (Pseudomonas fluorescens). The cysts of different clones were equally viable and capable of excystment. Their size was unaffected by the number of micronuclei. Amicronucleate cysts showed the usual macronuclear reorganization. Hence, neither the number of micronuclei nor the absence of micronuclei affected encystment, viability and size of cysts, excystment or macronuclear reorganization.

An attempt was made to maintain each clone in pure-line culture for 60 days and thereby to examine the division rate and vitality. Four clones were refractory and encysted before 60 days expired. The remaining 16 clones, including the three amicronucleate ones, survived with undiminished vigor and were discontinued. The 13 micronucleate clones produced from 149 to 176 generations during the 60-day period; the three amicronucleate clones produced 154, 164, and 172 generations, respectively. Hence, the 16 surviving clones showed slight differences in their average daily division rates, but neither the divison rate nor the vitality of these clones was correlated with variations in micronuclear number or with the absence of micronuclei. Division cysts of amicronucleate clones showed the usual macronuclear reorganization after each division of the macronucleus. The four refractory clones had high micronuclear numbers.

Since conjugation is rare and endomixis and aütogamy are unknown in Tillina, it is probable that amicronucleate races arise at division by an unequal distribution of the daughter micronuclei.

Some of the literature on amicronucleate ciliates and on the regeneration of various types of nucleate merozoa is reviewed. The evidence shows that the macronucleus is the indispensable nuclear element in the so-called vegetative life of the organism, whereas the micronucleus during this period appears to be a relatively passive organelle. Its chief function concerns the periodic replacement of the macronucleus and the production of new hereditary combinations. Special attention is directed to the fact that inheritance in an amicronucleate race is no less precise than in a typical micronucleate race, although the division of the macronucleus is amitotic and usually reveals no suggestion of true chromosomes. It is evident that the hereditary mechanism of amicronucleate races, and perhaps of ciliates generally, differs radically from the conventional chromosomal mechanism of metazoa.