| PubMed Abstract |
Neural ganglia of wild type third-instar larvae of Drosophila melanogaster were incubated for 13 hours at various concentrations
of BUdR (1, 3, 9, 27 micrograms/ml). Metaphases were collected with colchicine, stained with Hoechst 33258, and scored under
a fluorescence microscope. Metaphases in which the sister chromatids were clearly differentiated were scored for the presence
of sister-chromatid exchanges (SCEs). At the lowest concentration of BUdR (1 microgram/ml), no SCEs were observed in either
male or female neuroblasts. The SCEs were found at the higher concentrations of BUdR (3, 9, And 27 micrograms/ml) and with
a greater frequency in females than in males. Therefore SCEs are not a spontaneous phenomenon in D. melanogaster, but are
induced by BUdR incorporated in the DNA. A striking nonrandomness was found in the distribution of SCEs along the chromosomes.
More than a third of the SCEs were clustered in the junctions between euchromatin and heterochromatin. The remaining SCEs
were preferentially localized within the heterochromatic regions of the X chromosome and the autosomes and primarily on the
entirely heterochromatic Y chromosome.--In order to find an alternative way of measuring the frequency of SCEs in the Drosophila
neuroblasts, the occurrence of double dicentric rings was studied in two stocks carrying monocentric ring-X chromosomes. One
ring chromosome, C(1)TR94--2, shows a rate of dicentric ring formation corresponding to the frequency of SCEs observed in
the BUdR-labelled rod chromosomes. The other ring studied, R(1)2, exhibits a frequency of SCEs higher than that observed with
both C(1) TR94--2 and rod chromosomes.
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