THE RELATIVELY HIGH FREQUENCY OF WHOLE-BODY MUTATIONS COMPARED WITH FRACTIONALS INDUCED BY X-RAYS IN DROSOPHILA SPERM

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Abstract
Drosophila females were examined for the relative proportion of whole-body vs. fractional mutations induced by X-rays (3000 r) in postmeiotic stages, in the X chromosome of their male parent, at 14 visible loci. Parallel studies were made in a chemically treated and an untreated series. In the X- ray series, 93% of the mutations appeared to be whole-body, the remaining 7% being fractional. By contrast, in the chemically treated series and in the untreated (spontaneous) series, the per cent of fractionals was relatively high (30-67%). The high proportion of whole-body mutations induced by X-rays poses a problem in connection with the double strand model of the DNA molecule, but can be accounted for on the recent theory of Muller, Carlson and Schalet, that radiation may break the bonds holding both bases of a nucleotide pair to the DNA backbone, thereby allowing their "rotational substitution" with resultant whole-body mutation. In all three series the mutant tissue of the fractionals as a rule seemed to involve half of the body (not a quarter or less), so far as could be determined upon external examination of the fractionals. This half-body amount of mutant tissue indicates that these fractionals arise not as delayed effects followed by errors in replication at the first or later divisions of the fertilized egg (events which would lead to one quarter or less mutant tissue), but rather that the fractionals in question arise as permanent changes in an already existing DNA molecule, these observations thus supporting the recent conclusions of Muller and his co-workers as to the usual nature of the mutational event. However, some of the mutants may possibly have been one fourth mosaics. If they were, they would indicate that mutations might sometimes be due to an error in the replication of the gene.

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