Evaluation and re-evaluation of genetic radiation hazards in man. I. Interspecific comparison of estimates of mutation rates.

A detailed presentation is made of the experimental data from the various systems used by Abrahamson et al. 2] to conclude that the per locus per rad (low LET) radiation-induced forward mutation rates in organisms, whose DNA content varies by a factor of about 1000, is proportional to genome size. Additional information pertinent in this context is also reviewed. It is emphasized that the mutation rates cited by Abrahamson et al. 2], although considered as pertaining to mutations at specific loci, actually derive from a broad variety of genetic end-points. It is argued that an initial (if not sufficient) condition for sound inter-specific mutation rate comparisions, covering a wide range of organisms and detecting systems of various sensitivities, requires a reasonalbly consistent biological definition of a specific locus mutation, namely, a transmissible intra-locus change. Granting the differences between systems in their resolving power to detect intragenic change, the data cited in this paper do not support the existence of a simple proportionality between radiotion-induced intra-locus mutation rate and genome size for the different species reviewed here. Furthermore, in Drosophila melanogaster, where individual salivary gland chromosome bands (that can differ greatly in DNA content) are usually associated with individual loci or at least distinct complementation groups, radiation-induced intra-locus mutation rates are not correlated with apparent differences in the DNA content of bands. This result is incompatible with the notion that most of the DNA in a band represents a radiation-mutable target capable of eliciting the kind of mutation observed in mutation rate experiments. All these considerations argue against the validity of the hypothesis of Abrahamson et al. 2] and their generalization that, for the evaluation of genetic radiation hazards in man, we can now "extrapolate from mutation rates obtained in lower organisms to man with greater confidence" on the basis of DNA content (italics are ours).