In the rhabditid nematode Caenorhabditis briggsae the incorporation of thymidine-H3 has been studied by autoradiography after Feulgen staining, with animals maintained under axenic conditions in a medium of only partly defined composition. Labeling has been followed in adults left in the presence of thymidine-H3 for periods of from ½ to 24 hours, as well as in adults reared from larvae in the presence of the tritiated nucleoside. A massive incorporation is found in the nuclei of the gonads and intestine; also a less intense particulate cytoplasmic incorporation is clear in certain cells, especially those of the intestine. In general, all labeling has proved to be sensitive to DNase, but resistant to RNase. The label's stability has been tested by the transfer of adults into a medium containing "cold" thymidine. They remain there for up to 48 hours. A transfer for 24 hours results in a considerable decrease in the intensity of nuclear and cytoplasmic labeling; a stay of 48 hours leads to its complete disappearance from non-dividing (intestinal) as well as dividing (gonadal) nuclei. A phenomenon of DNA turnover is envisaged and discussed as a possible physiological attribute of C. briggsae.

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