IRWIN I. OSTER 



absence of either one or both indicates the presence of pre-existing lethals) 

 and divided into two groups according to the marker, forked (f), thereby 

 distinguishing the lethal tests of the two maternally-derived chromosomes. 

 They can then be placed individually into vials since they have already been 

 fertilized. The two classes of F^ flies arising from each treated or control 

 female should each be kept separately (by placing a rubber band around the 

 groups of vials) in order to enable one to detect lethals of common origin 

 arising spontaneously during early cleavage and/or in young germ cells. In 

 the Fj, the frequency of non-yellow females, sc^.Y(y+)/y sc* f InS w^/y w 

 In49, and yellow males, X'^^, y f, will indicate the rate of non-disjunction 

 occurring spontaneously and induced by the treatment. Examination of the 

 F2 generation for the absence of sc®. Y(y+)/y sc^ f InS w^ or sc^.Y(y+)/y w 

 In49 f males, depending on which chromosome is being scored, and at least 

 10 sc^. Y(y+)/X"=-, y f males will then indicate the presence of a sex-linked 

 lethal mutation of maternal origin. This scoring is relatively simple since it 

 involves the recognition of light-eyed males amongst red-eyed males. Please 

 note that the light-eyed flies must be non-yellow in body colour and any 

 yellow flies which might appear merely represent cases of non-disjunction 

 and should not be considered in deciding whether or not a lethal is present. 

 Even though this scheme does not allow for the retesting of individuals where 

 there is some doubt as to the presence of a lethal due to an insufficient yield 

 of flies of all classes, in practice, it has been found that such cases are very rare 

 because the stocks for this scheme had been selected for good fertility and 

 viability. 



Large numbers of homogeneous samples of reproductive cells for treatment 

 can be obtained by irradiating accurately determined stages of the life-cycle 

 which contain a preponderance of one type of germ cell and the utilization 

 of only this one stage at a time for genetic analyses by the use of short 

 mating periods following the maturation of the cells. Thus spermatogonia 

 or oogonia can be treated in third instar larvae, immature oocytes in freshly 

 eclosed females, mature oocytes in 96-hour-old females, spermatids in 48-hour- 

 old pupae, mature sperm in adult males and/or in inseminated females. When 

 testing immature cells for the presence of mutations it is advisable to take 

 only a few offspring for analysis from each treated parent in order to reduce 

 the error resulting from the sampling of mutations which have multiplied 

 by cell division after their origination in one cell. All the stages were exposed 

 to X-rays delivered at a rate of 160 r/minute (135 kV, 20 m A, 1 mm alumin- 

 ium filtration) or 400 r/minute ( 1 75 kV, 20 mA, 1 mm aluminium filtration) 

 in leucite chambers in nitrogen, air, or oxygen, depending on the experiment. 

 These diflferences in intensity had previously been found by us to have no 

 detectable effect on the yield of mutations obtained while Clark ^~ has reported 

 a higher frequency of induced changes following an exposure oi Drosophila to 

 X-rays delivered at the rate of 2000 r/minute than following an equivalent 

 exposure delivered at the rate of 100 r/minute. The flies were treated and 

 bred at a temperature of 25 ± 1°C. 



RESULTS AND DISCUSSION 



Radio-sensitivity of the male germ line 



Previously it had been reported that the spermatid stages of Drosophila 



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