Main Content

The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 7530

Chandra K. Pyne; Electron Microscopic Autoradiographic Studies on DNA Synthesis during Macronuclear Development in the Ciliate Chilodonella uncinata. Protistologica XV(1):67-77, 1979

Reprint

In File

Notes

The DNA synthesis during macronuclear development in the ciliate Chilodonella uncinata has been studied using tritiated thymidine and electron microscopic autoradiography. During the first stage of macronucler development, the DNA content of the macronuclear anlage increase from 2 C to 32 C, and the chromosomes appear as distinct osmiophilic structures in the electron microscope. During this stage an active DNA synthesis is observed in the macronuclear anlage, corresponding to the 4 duplication cycles; these rounds of replication may be separated by short periods when there is no DNA synthesis. At the end of the initial polyploidisation phase, the chromosomes start despiralizing; during this transition period there is no incorporation of tritiated thymidine. When the chromatin has reached its extreme state of despiralization in stage II macronuclear anlage, it appears as thin filamentous structures which show a singular banding pattern. At this stage, the macronuclear anlage again shows a heavy generalized labeling which appears to be associated with the filamentous structures. Our experimental procedure did not make it possible to ascertain whether this labeling represents a duplication of the whole genome or an over-replication of certain parts of the genome. During the third stage of development, the macronuclear anlage contains chromatin strands and small electron dense granules; the latter become progressively more and more numerous, and probably fuse amongst themselves to form the chromatin granules characteristic of the vegetative macronucleus. During this stage the incorporation of tritiated thymidine by the macronuclear anlage decreases progressively; moreover the labeling is observed to be associated with the chromatin strands rather than with the dense granules. The old macronucleus in ex-conjugants may go on synthesising DNA, though at a lower rate compared bo the macronucleus of ciliates in vegetative growth. This DNA synthesis decreases progressively along with the development of the macronuclear anlage until it is finally suppressed.