Repressive function of the micronucleus during asexual reproduction in Paramecium tetraurelia: Experimental analysis with defective-micronucleates generated by laser microbeam irradiation

The present paper reports on the experimental analysis of a novel regulatory function of the micronucleus of Paramecium tetraurelia. Previous studies have made clear that amicronucleate cell lines shortly after their generation generally suffer depression, in exhibiting low viability, slow growth and abnormal oral development in binary fission, but they eventually recover to near-normal. A compensatory mechanism is thus activated in the absence of the micronucleus, to allow recovery of the amicronucleate cell line. Implicit in this conclusion is the role of the micronucleus in repression of the compensatory mechanism. The present study tested this notion by perturbing the micronucleus with laser microbeam irradiation. This operation generated cell lines possessing defective micronuclei; during their asexual propagation, some cells lost the micronucleus and gave rise to amicronucleates. The viability of amicronucleate cell lines derived in this manner was found to be higher, compared to others generated by a different operation involving instantaneous removal of normal micronuclei from the cell with a microinjection needle. Some evidence also suggested that their oral development was less abnormal during the initial depression period. Hence, damage of the micronucleus has apparently facilitated the activation of the compensatory function, and the latter might have occurred even before the loss of the defective micronucleus. The present findings provide support for a regulatory role of the micronucleus during asexual propagation. Previous studies have indicated that the physical basis of the compensatory mechanism resides with the macronucleus. The micronuclear repressive function may be directed against this compensatory mechanism of the macronucleus.