Relationship between DNA content of polytene chromosome bands and heterochromatisation in Drosophila

Abstract

THE variegated position effect is associated with chromosomal rearrangements involving heterochromatin (reviewed in ref. 1). In Drosophila melanogaster, cytological studies of the salivary gland chromosomes of strains carrying such rearrangements show that in some cells, the chromosome regions adjacent to broken heterochromatin lose their characteristic arteristic banded appearance and become heterochromatised. The extent of this morphological change varies from cell to cell, but it has been noted that heterochromatisation seems to terminate preferentially just short of relatively large deeply staining bands^2,3. This observation might be due to the fact that such bands are more readily visible, and hence more likely to be scored as euchromatin than smaller bands. The involvement of other factors is however suggested by the observation³ that heterochromatisation associated with a given inversion in two genetic backgrounds terminates with different frequencies adjacent to certain bands which were morphologically similar in the two strains. A factor which, it has been suggested⁴, may affect the progress of heterochromatisation is a prior change in the nucleic acid content of affected regions. Early measurements of nucleic acid content using ultraviolet photographic microdensitometry of whole bands⁴ or spot estimations of optical density⁵ yielded equivocal results. We decided to use scanning microdensitometry to investigate the possibility that the differences in extent of heterochromatisation observed in our strains might be related to the DNA content of the bands concerned.