The relation of radiosensitivity to nuclear and chromosomal volumes and DNA contents has been studied in whole-body-irradiated adult amphibians. Data are presented on liver cell nuclear parameters-volume and DNA contents, per nucleus and per chromosome (NV, ICV; DNA/N, DNA/ch)-on correlation of LD50 with nuclear parameters, and on mean survival times for a number of species. In a few species, some individuals have detectable polyploid subpopulations in their liver cells. Nuclear and chromosomal volumes (NV and ICV) are linearly related to DNA contents. On the average, for 25 amphibian species an increase of <tex-math>$1.0\ \mu {\rm m}^{3}$</tex-math> in nuclear DNA content causes an increase of <tex-math>$17\ \mu {\rm m}^{3}$</tex-math> in NV, and residual NV is <tex-math>$72\ \mu {\rm m}^{3}$</tex-math> when DNA content goes to zero. It is clear that a large fraction (about 80%) of amphibian NV and ICV is nonchromosomal, i.e., is nuclear "sap." This difference between actual genome size and its estimate as measured by NV may account for some of the difference in radiosensitivity, at the same measured NV or ICV, between amphibians and other higher organisms. Whole-body-irradiated amphibians are very slow to die, requiring 70 to >150 days to complete radiation-induced deaths, with mean survival times (MST) at the LD50 of 31-190 days. MST increases with increasing nuclear parameter size, but variability is high. Three modes of death (a three-component MST vs dose curve) are apparent. At the minimal LD50, animals die of a hematopoietic death; the mechanism for death at greater doses is not apparent, but is not intestinal damage. LD50 decreases with increasing nuclear parameter size, but less rapidly than linearly, with a slope of -0.58 for the eight species analyzed (13-fold range of NV). Sensitivity-volume data are available for only two other groups of whole-body-irradiated organisms. At the same ICV, amphibians are about seven times as radiosensitive as herbaceous plants, about three times as sensitive as woody plants. Compared to the four radiosensitivity groups (radiotaxa) found previously among single-cell systems from higher organisms, the correlation of D0 against ICV for these whole-body-irradiated amphibians falls very close to the <tex-math>$D_{0}-{\rm ICV}$</tex-math> correlation line for the single-cell group which contained amphibians.

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