smith: directed mutation 417 



Animals 



The most extensive series of experiments on differential muta- 

 genicity reported with metazoan material are those of O. G. and M. j. 

 Fahmy on Drosophila melanogaster. As in the experiments on higher 

 plants, these involve selectivity of mutagenic agents among loci, and 

 are concerned with "forward" mutations which are frequently asso- 

 ciated with deficiencies. In earlier publications the Fahmy's reported 

 the following differences in mutants recovered after treatment with 

 certain alkylating agents compared to X-rays: a higher proportion of 

 visible to lethal mutations, new visible mutations not previously pro- 

 duced spontaneously or after irradiation, and a different distribution 

 of breakpoints and lethals in the X chromosome. These claims have 

 been critically reviewed recently by Auerbach (4) and Altenburg and 

 Altenburg (3) and will not be discussed further here. 



Subsequently, additional papers by Fahmy and Fahmy (19, 20, 

 21) have been published giving further experimental evidence of 

 differentia] induction of mutations in Drosophila. In one of these (20) 

 was reported differential response of different regions of the X 

 chromosome in Drosophila to the same mutagenic compound (phenyl- 

 alanine mustard). In the region proximal to / they found an excess 

 of visibles in the chromosome segment f-car and an excess of lethals 

 in the segment from car to the centromere. The Fahmy's contend that 

 their data indicate a qualitative difference in the physical nature of 

 visibles and lethals. They suggest that visibles may result from 

 molecular re-orientation which alters the transmission of the genetic 

 code, whereas lethals can be attributed to elimination or inactivation 

 of the gene. Obviously, it is not presently possible to test this 

 hypothesis. The Fahmy's have recently shown (21) that S-2-chloro- 

 ethylcysteine is virtually specific to a particular cell stage of the male 

 germ line in Drosophila melanogaster. This compound, they reported, 

 is ineffective on mature sperm, but is very effective on the early 

 spermatogonia. 



In the silkworm Nakao, Tazima, and Sakurai (51) found differ- 

 ences in the relative frequencies of two egg color mutants (pe and re) 

 following treatment of the males with two structurally different muta- 

 genic mustards, nitromine and alanine "mustard". With the former 

 the ratio of pe to re was over 1 (1.06-1.58), with the latter it was 0.6. 

 Most of the mutations were due to deficiencies so that tentatively 



