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A natural population of Chironomus riparius from the heavy metal-polluted Piedmont station (Italy) was analysed. Chromosome G, on which the Balbiani rings BRa, BRb and BRc are located, was very sensitive tothe pollution. The BR system was considered to be an interesting model for studying the response of the genome to the heavy metal pollution. Together, with a standard activity of BRs (BRc>BRb), we observed a clear reversed level of activity of BRb and BRc: BRc was repressed while BRb became activated. Also, there were a number of cells in which BRb and BRc were collapsed. A change in the activity of the nucleolar organizing region (N) was found, from a very high activity till completely collapsed. For the first time it was established that therewas an activation at the telomere region of chromosome G. Additionalactive sites in sections Dc and E2de were observed. The conjugation of both homologues was often disturbed. A high frequency of ectopic pairings was also observed. In addition, structural rearrangements as inversions and deletions were found. Some deletions led to the formation of pompon-like chromosome G. Chromosome G of C. riparius is suggested to be used to test for the presence of genotoxic concentrations of polluting agents in an aquatic ecosystem. [ABSTRACT FROM AUTHOR]