Influence of bromodeoxyuridine on the stability and function of polytene chromosomes

Polytene chromosomes of Drosophila hydei have been employed to study the effect of bromodeoxyuridine (BrdU) on chromosome function and structure. Two media have been tested for their capacity to permit substitution of thymidine by BrdU in larval salivary glands. In medium L up to 80% of the polytene DNA is replicated with maximum substitution of 80% of the thymidine in one strand by BrdU. The degree of substitution achieved with this medium is uniform within individual nuclei but varies between nuclei. An alternate medium (S) results in more complete substitution but less total DNA synthesis. Autoradiographic analyses reveal that extensive BrdU substitution is correlated with an inhibition of chromosomal RNA synthesis. Nucleolar RNA synthesis is almost totally suppressed by exposure to BrdU. Chromosomes which exhibit extensive BrdU incorporation produce puffs in response to a temperature shock or in vitro incubation. Preliminary studies suggest that puffing at some loci is anomalous. The data, therefore, indicate that moderate BrdU incorporation completely suppresses nucleolar RNA synthesis and reduces chromosomal RNA synthesis but does not generally prevent the induction of new puffs. — Exposure of larvae to BrdU also stabilizes the morphology of their polytene chromosomes in the presence of high pH and high ionic strength. Chromosome morphology that is lost at 0.6 M NaCl is regained at 0.15 M NaCl in isolated nuclei derived from BrdU-fed larvae. No recovery is observed in chromosomes of unsubstituted control larvae.