442 RADIATION BIOLOGY 



virulent, would usually be very difficult, except where various other 

 recognizable traits were known to be regularly associated with different 

 types of non virulence ; such traits then could be successively superposed 

 on one another. 



Fortunately, such 100 per cent stability in regard to a given charac- 

 teristic is not reciuired in the case of other genetically desirable types of 

 organisms. Thus the question arises, to what extent is radiation useful 

 in the furtherance of the artificially directed evolution of organisms in 

 the service of man? Certainly the main objection raised in the discussion 

 of its proposed application for speeding human evolution does not apply 

 with other organisms, since in their case one need have no moral com- 

 punction in producing innumerable inferior individuals and discarding 

 them, if at the same time a few desired types arise. The chief questions 

 then remaining are those of economy and of practicability in general. 



Commercially valuable mutations in mushrooms affecting color, growth 

 rate, and fruiting time have been produced by radiation (U.S. Atomic 

 Energy Commission, 1952, p. 98). The experiences of Gustafsson with 

 barley and other crop plants, mentioned on p. 436, showed that about 

 1 in 10 offspring of his irradiated seeds carried some definitely recognizable 

 recessive mutation, and that of these mutations something of the order of 

 1 in 800 were useful to man in some way, as by increasing the yield under 

 the conditions of cultivation or by causing the product to be better 

 adapted for being gathered, processed, or consumed. Thus the plants 

 had to be bred (and inbred) on a considerable scale, and a multiplication 

 of about 8000-fold was necessary before the original numbers were reestab- 

 lished, with the mutation now incorporated in the strain. With plants of 

 the rapidly and comparatively inexpensively multiplying kinds here used, 

 however, neither the time nor the expense of this operation was unduly 

 great. It would probably have taken longer, and cost more, to find and 

 to multiply, to an equivalent extent, the comparable mutants which must 

 have been present, in far more scattered condition, in untreated popu- 

 lations of the same varieties. Moreover, in some such cases the chosen 

 varieties may not yet be widely grown, and other varieties would prob- 

 ably have to be resorted to for finding the desired spontaneous mutants, 

 if they could be found at all. The transfer of the mutant gene from these 

 other varieties to the chosen one might then require a very lengthy proc- 

 ess of backcrossing with the latter, in order to maintain all the desired 

 features of its genetic complex. This roundabout procedure would in 

 some cases be less economical than that involving irradiation of the chosen 

 variety. The reader interested in further instances of the successful use 

 of irradiation for the obtaining of improved varieties of plants of eco- 

 nomic importance may be referred to the recent review by Gustafsson 

 (1952), which cites many cases both in his own \vork and in that of others, 

 carried out largely in Sweden. 



