DNA synthesis in the giant salivary chromosomes of Drosophila virilis prior to pupation

Both two‐wavelength microspectrophotometry of Feulgen‐stained whole nuclei and autoradiography of H3‐thymidine incorporation by giant salivary chromosomes in Drosophila virilis demonstrate a net decrease in the relative rate of salivary DNA synthesis during the late third instar and prepupal stages of development. Amounts of DNA‐Feulgen per nucleus were distributed into several classes, the means of which closely approximated values projected as geometric multiples of the basic somatic DNA level estimated from hemocyte nuclei of the same larvae. Comparison of DNA polytene class frequencies showed no statistical difference between male larvae of different development stages, although female prepupae showed a greater frequency of nuclei in higher polytene classes when compared to male prepupae of the same age. Comparison of chromosomal H3‐thymidine incorporation with previously described H3‐histidine incorporation suggests that the amino acid labeling, which reaches a maximum during the prepupal period, has a physiological significance distinct from chromosomal endoreplication.