Location of rDNA in the heteromeric macronucleus of Chilodonella steini

As it was shown previously the chromatin of the heteromeric macronucleus of Chilodonella steini consists of two specific zones, the outer one (the orthomere) made up from late replicating electron dense material, and the inner zone (the paramere) of medium electron density and early replicating, with a concentration of electron dense material called the endosome in its central part. New details regarding the reorganization of the chromatin after macronuclear division are presented here. After the division no endosome is visible. It reappears within the first two hours after the separation of sister macronuclei. With the use of in situ rDNA — DNA hybridization it was shown that the rDNA in the macronucleus of Chilodonella steini is located in the outer zone (orthomere) and the endosome. Since it has been found earlier that the DNA content of the macronucleus depends on culture conditions the authors suggest that the quantitative DNA changes are caused preferentially by differential replication of rDNA dependent on the food conditions of the cells.