Characterization of the toromeric structure in Drosophila lummei

The toromere, a structure previously reported only in several strains of Drosophila melanogaster, is found in salivary gland nuclei of three populations of Drosophila lummei, a member of the virilis group. The toromere is characterized by being quinacrine-bright and Feulgen-positive. Further staining and enzyme digestion procedures reveal that the toromere is composed of double-stranded DNA with little or no protein complexed with it. The toromere appears as a small quinacrine-bright dot in diploid cells and apparently undergoes polytenization, as it is observed as a large quinacrine-bright doughnut-shaped structure in salivary gland preparations. We describe the effect of larval culture temperature on the morphology and number of toromeres per cell, and lend support to the proposal that the toromere could be a highly replicated episome.