Chironomus polytene chromosomes

Attention has already been briefly drawn to the use of morphological alterations of the polytene chromosomes in the salivary glands of larval Chirono-mus tentans (Bentivegna and Cooper 1993) as an assay for genotoxicity. This could be applied equally to field samples although care would have to be exercised concerning the variability in natural populations which may contain several species of Chironomus. 

In the same way that amphibian oocytes are an accessible scource of giant lampbrush chromosomes (as well as nuclear envelopes), the salivary glands of dipteran larvae are a source of giant polytene chromosomes, which are formed by repeated endomitotic reduplication of the DNA, while retaining lateral association of chromatids. In Chironomus tentans, polytene chromosomes in salivary... 

The majority of hereditary material in polytenic chromosomes is inactive. No more than 10%-15% of the 5000 bands in D. melanogaster and 1900 bands in Chironomus formed puffs. The maximum proportion of bands which are simultaneously active is smaller and is approximately equal to 6% (Felling, 1966). 

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