DNA-synthesis in larval salivary glands during culture in starved female flies of Drosophila melanogaster.

If imaginal disks are transplanted into host flies that are kept on a protein-free sugar diet, their developmental processes come to a more or less reversibel standstill. This block is generally attributed to absence of cell divisions. Since cell division and DNA-synthesis are intimately coupled, we have used the polytene system of the salivary glands in order to study the question whether DNA-synthesis is possible in starved hosts.Nuclear DNA was determined with a modified Feulgen technique using the fluorescent dye BAO. Salivary glands of 72 hrs old male larvae were cultured in vivo and in vitro under various conditions (Fig. 2, Table 2, 3). In young starved hosts the nuclei can complete an already initiated S-phase, but further synthesis is blocked (Figs. 4, 5, 6). Older starved hosts are more effective in blocking DNA-synthesis. This block is largely reversible: in hosts that are transferred onto complete yeast food, the nuclei resume DNA-synthesis at a normal rate (Table 2, Kg. 5). Cytoplasmic differentiation as indicated by vacuolization of cultured gland cells has also been shown to be reversibly blocked in starved hosts (Fig. 7). Contrary to these findings starvation seemed to cause some irreversible alterations at the chromosomal level (Fig. 8).We suggest that in starved hosts protein synthesis is blocked and that this in turn will prevent initiation of new S-phases.