Localization and characterization of rDNA in Drosophila tumiditarsus

Using in situ nucleic acid hybridizations, the genes that code for 28, 18 and 5S rRNA have been localized in the polytene chromosomes of Drosophila tumiditarsus. The 5S genes are found at a single site near the centromere of the second chromosome, whereas the 28 and 18S genes are found at the nucleolar organizer region of the dot chromosome. The dot chromosome has been previously described as alpha-heterochromatic. However, our cytochemical and autoradiographic results do not support such a conclusion. The autoradiographic results reveal that the dot chromosome is transcriptionally active and is not late-replicating, as is expected of alpha-heterochromatin. Further, the dot chromosomes possess none of the usual staining characteristics of heterochromatin except for its lack of polytene bands. Using rRNA-DNA filter hybridizations, we find that the rDNA of D. tumiditarsus salivary glands is under-replicated. This is the first species of Drosophila where the rDNA in not found on the sex chromosomes, and is the first report of an under-replicated autosomal locus which is not located in heterochromatic blocks.