Cytological observations on the interaction between two inversions responsible for position-effect variegation in Drosophila melanogaster.

The strain of Drosophila melanogaster In (1)mK; In (2LR) RevB shows more miniature variegation than the strain In(1)mK and less Revolute variegation than the strain In(2LR) RevB. Observations on heterochromatisation in the larval salivary gland chromosomes of the three strains revealed that the mK chromosome is heterochromatised in a higher proportion of nuclei and the RevB chromosome is heterochromatised in a lower proportion of nuclei in the double-inversion strain than in the corresponding single-inversion strain. Single- and double-inversion strains did not however differ in the mean number of bands heterochromatised per affected chromosome. The difference between incidence and extent of heterochromatisation was further exposed by comparisons between and within strains: the incidence of heterochromatisation in different chromosome regions within a nucleus was positively correlated, but a significant positive correlation was found in only one of the eight possible comparisons between extents of heterochromatisation in different chromosome regions in a given nucleus, two of the comparisons showing significantly negative correlations. The results in general are compatible with the view that the initiation and progression of heterochromatisation are distinct phenomena, under separate control.