Differences in the number of nucleolus organizers in Chironomus tepperi shown by in situ hybridization.

Two stocks of Chironomus tepperi could be isolated. One stock, N(IV)+, contains nucleolus organizers in chromosome I and IV, whereas the other one, N(IV)-, shows only one nucleolus in chromosome I. It is demonstrated by in situ hybridization with radioactive rRNA that the absence of the nucleolus in chromosome IV of stock N(IV)- is not related to an inactivation of the nucleolar DNA, as might have been suggested, but is due to the lack of ribosomal cistrons.