Graduate Thesis Or Dissertation
 

Effects of x-irradiation on DNA synthesis in salivary gland cells of Drosophila hydei

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  • The effect of X-irradiation on DNA synthesis in larval salivary glands of Drosophila hydei was studied using the technique of liquid scintillation spectrometry and autoradiography. DNA synthesis in salivary glands of synchronized larvae was measured at successive stages of third instar development by in vitro uptake of ³H-thymidine for a period of 30 minutes in ten pooled glands. A "Major Period of DNA Synthesis, " hereafter referred to as the "MPS," which lasts for about 5% of larval life, was found in glands just before the mid-point of third instar development. DNA synthesis before and after this period is relatively low. In vivo exposure of salivary glands before the MPS to acute Xray doses of 1250, 2500, 5000 and 10,000 R resulted in stimulation of DNA synthesis shortly after irradiation, as determined by liquid scintillation counting. Autoradiographs prepared from glands exposed to 1250 R of X-rays before the MPS show an increase in the number of cells in very active DNA synthesis at the anterior part of the gland. This stimulation of DNA synthesis was explained by an apparent precocious entry of cells into the MPS. The possibility remains that some of the induced DNA synthesis is due to a small amount of DNA repair that is not detectable. Autoradiographic studies showed that stimulation of DNA synthesis also occurred in glands irradiated at the end of larval development immediately after irradiation. However, this stimulation occurred only after exposure of glands to the higher doses (5000 and 10,000 R), and is taken as a possible sign of DNA repair although one cannot rule out the possibility that the increased synthesis is of a semiconservative nature. The short term effect of X-irradiation on DNA synthesis when glands were irradiated during the MPS is one of depression. This supports the interpretation that increased DNA synthesis in glands irradiated before the MPS is due to precocious DNA synthesis since cells in the MPS have already entered the synthetic phase. The long term effect of X-irradiation on DNA synthesis when glands were irradiated before or during the MPS is clearly one of depression. Depression of DNA synthesis was explained on the basis of observations in mammalian cells that ionizing radiation alters various cellular processes associated with DNA synthesis. In addition, the long term depressive effect was accounted for by an X-ray induced prolongation of the larval period which implies a slowing down in the rate of DNA synthesis in all tissues including the salivary glands. The mechanisms for precocious DNA synthesis, repair of DNA damage and depression of DNA synthesis are discussed.
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