Replication in Drosophila chromosomes. XII. Reconfirmation of underreplication of heterochromatin in polytene nuclei by cytofluorometry

Abstract

It is widely known that the bulk of the pericentromeric heterochromatin (α-heterochromatin) does not replicate during polytenization in Drosophila. However, a recent DNA-Feulgen cytophotometric study (Dennhöfer 1982a) has claimed equal polytenization of all heterochromatin regions. To re-examine this issue, the amount of Hoechst 33258-bright heterochromatin in non-polytene and polytene nuclei in salivary glands and Malpighian tubules of late third instar larvae of D. nasuta has been compared by cytofluorometry. Since the amount of Hoechst 33258-bright heterochromatin is similar in non-polytene and polytene nuclei in spite of the latter having an enormously high euchromatin DNA content, it is concluded that the α-heterochromatin does not replicate during polytenization. The present results further indicate that in the polytene nuclei of Malpighian tubules the α-heterochromatin remains at the 2C level whereas in salivary gland polytene nuclei it varies between the 2C and 4C levels.