Developmental changes in the responsiveness to ecdysterone of chromosome region I-18C of Chironomus tentans.

The presence of nascent RNA in chromosome region I-18C was revealed by the method of induced hybrid formation (IEH) and by use of anti-RNA:DNA antibodies. IEH signals were quantitated, and used as a measure of transcriptional activity in the region. The expansion of the I-18C region was also determined and used as an indication of local chromatin decondensation. Chironomus tentans larvae may undergo oligopause or they may develop subitaneously. It was found that the course of basal activity in I-18C differs substantially in these two modes of development. Most interestingly they also differ in activation of I-18C in response to in vitro ecdysterone treatment. Changes in hormonal responsiveness of I-18C parallel changes in the preexisting state of I-18C decondensation. Oligopause stage 5 animals, being most refractory in this respect, exhibit the most condensed I-18C chromosome region.