MANNER OF PRODUCTION OF MUTATIONS 573 



9100 per second applied simultaneously with and for 5 minutes subse- 

 quent to X rays was found by Conger (1948a, b) to increase the fre- 

 quency of chromosome aberrations in Tradescantia microspores, although 

 without X rays they failed to affect the frequency. It is highly probable 

 that the effect in this case was not on breakage but on the probability of 

 restitutional union of the broken ends, as in the work by Faberge (1947) 

 in which mere changes in temperature, presumably by causing convec- 

 tion currents, exerted such an influence. In the same general category 

 belongs the earlier finding by Sax (1943), following still earlier work by 

 Kostoff (see p. 589) on chromosome changes induced by centrifuging 

 alone, that centrifugation after X irradiation increases the frequency of 

 the induced chromosome aberrations in Tradescantia microspores. These 

 data did not show, however, the extent of the efi'ect that centrifuging 

 alone would have had; therefore it is not possible to judge the extent 

 to which the effects of centrifuging and of X rays were synergistic. 



Pressure is another physical condition which has been thought to influ- 

 ence the frequency of mutations, at least when in combination with a 

 mutagenic agent. McElroy and de la Haba (1949) have presented data 

 which show that a lower frequency of morphologically expressed mutants 

 is produced by nitrogen mustard when a high pressure is applied to 

 Neurospora spores during or after irradiation, and McElroy and Swan- 

 son (1951) state that they have found that the same effect is produced 

 by pressure when mutations are produced by either X rays or ultraviolet. 

 They interpret these results as meaning that the process of formation of 

 the mutant gene (supposedly as a second step, following the production 

 of a " semiactivated " state in the original gene by the mutagen) entails 

 an increase in volume. They state that the differences in frequency in 

 question cannot be due to selective elimination of mutant spores by the 

 adverse influence of pressure since the rate of survival was not reduced 

 by the pressure. However, their data show that the pressurized spores 

 did have a far lower survival rate than the corresponding nonpressurized 

 spores. It is, of course, to be expected that under such circumstances 

 the mutants, being w^eaker, would be killed off at a higher rate than the 

 normals and that this factor alone would result in a lower frequency of 

 mutants being recovered in that series which was subjected to the more 

 adverse conditions, just as was shown to be the case by Hollaender and 

 Emmons in their experiments with ultraviolet. 



A further indication that selection is at the basis of the results with 

 pressure is to be seen in the finding by McElroy and de la Haba (1949) 

 that mutants which are expressed in the form of special nutritional 

 requirements, rather than through morphological abnormalities, failed to 

 show a decrease in frequency wdth pressure ; in fact these mutants showed 

 some increase. This would correspond with the fact that these "bio- 

 chemical mutants" are at no disadvantage, compared with normals. 



