G. ROY LANE 



the chromosome breaks have their origin in interchange of the type he 

 describes. This may well account for the greater than linear relation 

 between chromosome breaks with sister reunion (so-called isochromatid 

 breaks) and the dose, reported by some workers in irradiated material. I 

 have observed an apparent chromosome break with sister reunion of Ijoth 

 centric and acentric fragments with the two fragments interlocked by the 

 reunion. This clearly cannot be the result of failure of reproduction of 

 broken chromosome ends. 



However, other evidence suggests that only a fraction of the sister reunions 

 are the result of minute interchange at loops. Firstly, if all B" are the result 

 of interchange at loops, one would expect the frequency of the three alter- 

 native types of detectable interchange to be approximately equal. In fact, 

 the B" with sister reunion far outnumber the other types {i.e. minutes (m) 

 with or without B' and unequal or looped chromatids). Secondly, after 

 treatment with diepoxide, breaks cutting straight across both chromatids 

 without sister reunion in the centric or acentric fragment occur fairly 

 frequently. In addition to these clear breaks, constrictions or lesions occur 

 either on both chromatids at apparently the same locus or on one chromatid 

 opposite a chromatid break. These constrictions have been observed by 

 McLeish^ in Vicia after treatment with maleic hydrazide and by many 

 workers in irradiated material. They are usually interpreted as restitution 

 and may be considered as evidence supporting the view that diepoxide and 

 other radiomimetic substances produce direct chromosome breakage by 

 local inhibition of chromosome synthesis, and that the aberration is not 

 always an exchange as RevelP"^ has suggested. 



Finally, RevelFs suggestion that the diepoxide effects are to be considered 

 as a kind of artificially induced chiasma formation is a tempting one to 

 follow, at least in regard to the clear cases of interchange. The similarities 

 between certain types of interchange and the chiasma are obvdous, and 

 they both occur at a restricted period in the cell cycle, probably at or near 

 the time of chromosome reduplication. However, it should not be forgotten 

 that the differences are many, a most significant one being that the chiasma 

 is never an asymmetric interchange. This suggests a different mechanism 

 for the two types of chromatid exchange, the chiasma and the artificially 

 induced interchange. On the other hand, the complete absence of chromo- 

 some reunion (R"), even with treatment during early resting phase, suggests 

 that breakage occurs near to the time of reduplication at whatever part of 

 the resting phase the treatment is given. 



SUMMARY 



(/) Di(2 : 3-epoxypropyl) ether causes chromosome structural changes in 

 the meristematic cells of the roots of Tradescantia paludosa. These changes 

 are similar to those reported in Vicia faba root after similar treatment. This 

 substance also has a delaying effect on mitosis, the effect increasing with dose. 

 {2) The chromosome breaks and reunions do not appear to be localized 

 as in Vicia nor is there the same tendency towards apparently homologous 

 chromatid interchange betw^een homologous chromosomes. From the 

 limited data so far obtained, interchange appears to be favoured by higher 

 dose. 



273 T 



