SENSiriNin' (IF i)K()S()i'iiii,.\ (;i-,RM cKi.i, s'iAf;r.s m x irradiation 



melatwiiaster are much more susceptible to tin- mutagenic efiects of X-rays 

 than mature spermatozoa ejaculated by the male on the first clay after 

 irradiation, with tlie latter being somewhat more sensitive than sperm 

 released on the second day following treatment^- ^•^•'*'^' *• ^^, while the work 

 of Bonnier and Liming^'*, and Abrahamson^^ has indicated that sperma- 

 tozoa in the seminal receptacles and spermathecae of females are also more 

 susceptible than mature spermatozoa treated in the male. Following our 

 demonstration in 1956 that the spermatid stages are even more radio- 

 sensitive than sperm in the female', we have carried out additional tests on 

 the different stages and Table 1 summarizes our results. 



Table 1. Results of X irradiation of different cell stages in the male Drosnphila melanngaster 



Cell stage Dose Per cent lethal Percent 



mutations translocations 



Untreated male germ cells 

 Spermatogonia (larvae) 

 Spermatids (48-hour-old pupae) 

 Spermatozoa (adult malc-s) 



(released 1 day after treatment) 

 Spermatozoa (adult males) 



(released 2 days after treatment) 

 Spermatozoa (inseminated females) 



Thus it can readily be seen that although the spermatid stages received 

 less than one-half as much radiation as the spermatozoa in the female they 

 yielded one-half a^am as many translocations. According to our results there 

 does not seem to be too large a difference between the radio-resistance of 

 spermatozoa released one day after treatment and those released two days 

 following treatment. This finding agrees with the results of Nordbach^^, 

 who has failed to obtain consistent differences amongst the rates of induced 

 changes in sperm released during the first and second days following treat- 

 ment with X-rays. The cause for this variation has not yet been determined. 

 Arranging the male germ cells in order of decreasing sensitivity to the 

 mutagenic effects of X-rays we have, spermatids, spermatozoa in the female, 

 sperm released one day after treatment, sperm released two da\s after treat- 

 ment, and spermatogonia. Although we have not irradiated spermatocytes 

 Khishin has found them to be somewhat more sensitive than spermatogonia 

 and less sensitive than mature spermatozoa^. 



It is interesting to note that although it has been thought that there is 

 something in the physico-chemical nature of condensed chromosomes which 

 makes them most sensitive to X-rays, our results demonstrating the greater 

 sensitivity of spermatids as compared to spermatozoa, the latter's chromo- 

 somes being even more spiralized than those of the former, seem to indicate 

 that some other factor(s) is responsible, in part at least, for the heightened 

 sensitivity of spermatids. A previous attempt to find the cause for this 

 heightened sensitivity of the spermatid stage to the mutagenic action of 

 radiation by the utilization of stocks containing either rod-shaped or ring- 

 shaped sex chromosomes which yielded substantially similar frecjuencies 

 of recessive lethal mutations, had indicated that this is not due merely to 



258 



