INDUCED MUTATIONS IN PLANTS 1275 



in general as gene mutations. This assumption is obviously invalid in 

 the polyploid species, and is of doubtful validity in other species. 



Since chromosomal alterations (particularly small deficiencies) are 

 known to simulate gene mutation in some instances, and since irradiation 

 regularly induces chromosomal alterations (including deficiencies), there 

 is obvious ground for the suspicion that the apparent gene mutations 

 induced by irradiation are in fact due to extragenic alterations. Are the 

 apparent mutations merely typical chromosomal aberrations involving 

 small or relatively unimportant portions of the chromosomes, which 

 may be lost or reduplicated without lethal effect? Or are they changes of 

 a different type (whether mechanical or chemical) which are necessary 

 consequences or antecedents of the accompanying chromosomal aberra- 

 tions? Or are the mutations wholly unrelated to the grosser chromosomal 

 modifications except in the circumstance that both result from the chain of 

 reactions initiated by irradiation? Whatever their relation to the 

 induced chromosomal aberrations, are the induced mutations representa- 

 tive of mutation in general, or only of some limited class of mutation? 

 If induced variations can be shown to include qualitative as well as 

 quantitative changes in chromosome constitution, may the mendelizing 

 variations as a group be assigned to the class of qualitative gene altera- 

 tions? Or do the mendelizing variations include both intragenic and 

 extragenic alterations? 



Questions of this sort are the chief concern of current genetic investi- 

 gations with radiation. None of the questions stated above can be given 

 an unequivocal answer from the evidence now available. The experi- 

 ments on mutation in Drosophila (see Paper XXXIX) have yielded more 

 information of value in this connection than have the comparable experi- 

 ments with plants, and the reader is referred to discussions of this 

 evidence by Patterson and Muller (35) and by Muller (32). The evi- 

 dence from plants, bearing on the nature of induced mutation, has been 

 discussed by Stadler (46). 



The evidence from plants may be summarized under three main 

 heads: (A) evidence indicating types of mutation not affected by irradia- 

 tion; (B) evidence relating the induced mutations to the induced chro- 

 mosomal aberrations; and (C) genetic and cytological evidence bearing 

 on the nature of specific induced mutations. 



A. A striking characteristic of induced mutation in nonpolyploid 

 plants is the extreme rarity, or perhaps total absence, of dominant muta- 

 tion. This is in contrast to the experimental results in Drosophila, 

 in which induced dominant mutations are fairly frequent. However, 

 most of these dominant mutations, like most of the spontaneously occur- 

 ring dominant mutations in Drosophila, are lethal when homozygous. 

 Thus they correspond in genetic behavior to gametophytic lethals in 

 plants. Numerous induced deficiencies with gametophytic lethal effect 



