Chromosomal organization of Drosophila tumours

Abstract

In otu mutants of Drosophila melanogaster ovar- ian tumours develop because of the high mitotic activity of the mutant cystocytes; the latter are normally endopo- lyploid. In certain alleles of otu, however, a varying propor- tion of the mutant ovarian cystocytes undergo polyteny. Mutant cystocytes with polytene chromosomes are termed pseudonurse cells (PNC). Polytene chromosome morpholo- gy and banding patterns in PNC of otul/otu ~ flies were cytologically analysed. Extensive variability was noted in the quality of the banding pattern of the PNC chromosomes which ranged from highly condensed (condensed PNC chromosomes) to those with a banding pattern (banded PNC chromosomes) similar to that in larval salivary gland cells (SGC). Both the condensed and banded PNC chromo- somes frequently enter into a diffuse state characterised by weakened synapsis of the polytene chromatids and alter- ations in their banding pattern (diffuse PNC chromosomes). Analysis of DNA synthesis patterns in the various morpho- logical forms of PNC polytene chromosomes by 3H-thymi- dine autoradiography revealed a basic similarity to the pat- tern seen in polytene nuclei of larval SGC. Independently replicating sites, however, could be unambiguously identi- fied only in banded PNC chromosomes. Comparison of late replicating sites in such PNC chromosomes with those of larval SGC showed a remarkable similarity in the two cell types. These results suggest a close correlation between the polytene chromosome banding pattern and its replica- tive organization.