NOR activity in Chironomus riparius Mg. (Diptera, Chironomidae) is affected by chronic exposure to trace-metal pollution

We compared the activity of the Nucleolar Organizer Region (NOR) of fourth-instar larvae of Chironomus riparius from a heavy-metal-polluted station with that of larvae from a control by means of FISH with a 28S-rDNA probe. This probe should hybridize with the 28S-rDNA genes and their transcripts, also pointing out the transcriptional activity. In addition, at the cytogenetic level we expected to find that changes of gene expression would appear as variation of Nucleolar size, as indicated by FISH signals. The mean frequency of the active NORs (0.834 ± 0.10) in the larvae from the heavy-metal-polluted station was significantly lower than that in the larvae of the control (0.976 ± 0.055). These results confirm the hypothesis that trace metals reduce the level of transcription activity of the rDNA of the NORs.