CHROMOSOME ABERRATIONS IN ANIMALS 683 



melanogaster cultures. Parallel genetical and cytological studies of 

 fertile progeny in the virilis cultures indicated that radioactive P 32 pro- 

 duces both mutations and chromosomal aberrations. The melanogaster 

 flies were not tested for mutations. 



There are some reports that the rate of occurrence of aberrations may 

 differ in different stocks of the same species. For example, the frequency 

 of dominant lethals was found to be higher in the Oregon-R strain of 

 D. melanogaster than in the Swedish-b strain when both were exposed 

 simultaneously to X rays (Demerec, Kaufmann, and Hoover, 1938; 

 Dempster, 1941b). In a more recent study, however, Demerec and Fano 

 (1941) questioned the statistical significance of the observed differences 

 because of the considerable range of variability from experiment to 

 experiment in studies of frequencies of dominant lethals. 



4-2. EFFECT OF PLOIDY 



It has been shown in several experiments that the radiation sensitivity 

 of a cell is related to the number of sets of chromosomes it contains. 

 Haploid microspores of Tradescantia are about twice as sensitive as 

 diploid microspores with respect to the production of chromosomal aber- 

 rations by a given dose of X rays (Sax and Swanson, 1941). Tetraploid 

 seeds of barley are more resistant than diploid seeds to the effects of 

 irradiation, as determined by growth rates and vigor (Miintzing, 1941). 

 Haploid yeast plants are more sensitive than diploids, as determined by 

 rates of survival (Latarjet and Ephrussi, 1949). In contrast with these 

 results, it was reported by Lamy and Muller (1939) that the mortality 

 rates are not significantly different in triploid and diploid embryos of 

 Drosophila exposed to X rays. From this it was concluded that the 

 lethal effects of X rays on embryos must be "physiological" rather than 

 genetic in nature. However, it has been shown that in another insect, 

 the wasp Habrobracon, haploid male larvae are more sensitive than 

 diploid female larvae (Whiting and Bostian, 1931); and these observa- 

 tions have recently been extended by Clark and Kelly (1950). Lethal 

 effects were determined by Clark and Kelly by measuring rates of eclosion 

 after treatment of prepupae and pupae. Haploid males were found to 

 be more sensitive than either diploid males or diploid females. 



4-3. RELATIVE SENSITIVITY OF CHROMOSOMES IN DIFFERENT TYPES 



OF CELLS OF THE SAME SPECIES 



The frequencies of aberrations induced by X-ray treatment in various 

 types of cells of the plant Tradescantia were determined by Sax and 

 Swanson (1941). The order of sensitivity in the cells examined, begin- 

 ning with the most sensitive, was as follows: microsporocytes, micro- 

 spores, root-tip cells, and the generative cell. Microspores and root-tip 

 cells of Allium were less sensitive than those of Tradescantia. Differ- 



