Macronuclear chromatin changes during encystment in the ciliate Colpoda inflata: Formation of crystal-like structures in the resting cyst chromatin and nucleolar condensation

We analysed the macronuclear chromatin structure changes during encystment of the ciliate Colpoda inflata, using both standard transmission electron m icroscopy and a spreading chromatin method, from isolated vegetative and re sting cyst macronuclei. The vegetative macronucleus of C, inflata, like the majority of ciliates, is formed by discrete condensed chromatin clumps, wh ich became fused during encystment. In mature resting cysts the macronuclea r chromatin was similar to vegetative or resting cyst micronuclear chromati n probably indicating the transcriptional inactivity of the macronucleus in this stage. Likewise, the numerous nucleolar masses of the vegetative stag e fused in only one nucleolar body during resting cyst formation, and in th is nucleolar mass neither a granular zone nor nucleolar organizers were obs erved. Hybridization with a ribosomal DNA probe, after pulsed field gel ele ctrophoresis of vegetative and mature resting cysts, showed that ribosomal DNA molecules could be compacted with some nucleolar elements, corroboratin g the nucleolar inactivation inferred from the ultrastructural data. The ch romatin spreading method has revealed that vegetative macronuclear chromati n is formed by a net of long fibres organized in polynemic cords probably r epresenting amplified regions of the macronuclear genome. It represents, as in the colpodid ciliate Bursaria, a type of non-classical polyteny. In thi s study we report for the first time in the genus Colpoda, a very special s tructural organization of the macronuclear chromatin in mature resting cyst s; the existence of crystal-like hexagonal bodies. We also discuss widely a ll these macronuclear chromatin modifications and the factors that could be involved in the formation of polygonal structures.