rnr. r.F.XFTic n.\sis of \-I<\^■ inducfd somatic damagf 



These were employed on the basis of the fact that ring-sliapcd chromosomes 

 whicli are broken lead more often than ordinary rod-shajird chromosomes 

 to the formation of chromosome bridges Ijy the dicentric isochromatids 

 resuhing from the union of centric sister-chromatid fragments and/or hypo- 

 ploid cells lacking a chromosome or part of it, resulting from the failure of 

 the dicentrics to form a permanent bridge followed by the formation of 

 daughter nuclei lacking the sections of the broken chromosomes which had 

 been left outside them. 



RESULTS AND DISCUSSION 



The results sho\\n in Table 1 were obtained : 



Table 1 



Genotype T i Number of Number of Per cent 



of male '^" "'^" larvae imagoes hatching mortality 



The majority of the individuals which survived and reached adulthood 

 after being irradiated as third instar larvae showed extreme wing abnormalities, 

 lack of many bristles, and marked weakness which was presumably due to 

 damage to the musculature. Mortality, in those cases in which it occurred 

 during the pre-imaginal stages, occurred most frequently amongst late pupae 

 and very rarely during the late larval and early pupal instars. It can readily 

 be seen that male larvae bearing a ring-X chromosome were much more 

 susceptible to the deleterious effects of X-rays than male larvae containing 

 a normal rod-X chromosome. In addition, it was noted that the ring-con- 

 taining survivors were also less viable than their rod-bearing counterparts. 

 Such results can be interpreted on the basis of the fact that ring-X chromo- 

 somes are lost more often following irradiation. This would be especially 

 deleterious in the case of males since they contain only one such chromosome 

 per cell. These results demonstrate that this X-ray induced shortening of the 

 life-span has a genetic basis and they complement the data obtained 

 previously by which it was shown that male larvae having one X chromo- 

 some are more susceptible to X-ray induced somatic damage than females 

 having two X chromosomes. 



On the other hand, it was shown that female larvae heterozygous for a ring- 

 X chromosome do not have a higher X-ray induced mortality than females 

 homozygous for two ordinary rod-X chromosomes. This finding as well as 

 the previously o]:)served difference between males and females supports the 

 view that death has been caused by hypoploidy following breakage rather 

 than by bridge formation. If the latter had been true, females (two X chromo- 

 somes) and males (one X, one Y chromosome) would have been found to 

 be equally sensitive to X-rays, since a Y chromosome in the male is expected 



270 



