GENETIC AND CYTOLOGICAL EFFECTS 2C3 



But, in the ring chromosome, reproduction must take place in a single 

 plane; otherwise the daughters either will be interlocked or will form a 

 double ring with two centromeres. "Crosswise" restitution in the ring 

 chromosome is therefore likely to result in loss of the ring. 



Schultz (1951) found that ultraviolet treatment very greatly increased 

 the frequency of loss of the ring, both in the endosperm and in the embryo. 

 The frequency of deficiencies for A'' among the Fi plants was about as 

 high as the frequency of entire (i.e., nonfractional) endosperm deficiencies. 

 This indicates that the low frequency of broken chromosomes found in 

 plants from irradiated pollen, as compared with the endosperm, is largely 

 due to a higher rate of restitution in the plant. 



Another interesting result of the study was the absence of any evidence 

 of the production of a rod chromosome from the ring. The type of break 

 which would ordinarily result in a stable terminal deficiency should, if it 

 occurred in a ring chromosome, convert the ring to a rod. This would be 

 detectable as a nondeficient nonvariegated plant. Among more than a 

 thousand seedlings in progenies from ultraviolet-treated pollen, no such 

 plant was found, although the number of A* deficiencies presumably 

 induced by the treatments was almost 200. This result is not in conflict 

 with the observed occurrence of terminal deficiencies in other studies. 

 The frequency of deficiencies for a given locus as observed in the Fi 

 plants is extremely low, and the primary breaks or potential breaks which 

 occur in a rod chromosome proximal to the locus concerned may be much 

 more numerous than those which would occur within the small ring. 

 Presumably the proportion of potential breaks resulting in deficiencies 

 realized in the embryo may be well below one in 200. 



The results with the ring chromosome show that the frequency of poten- 

 tial breaks induced by ultraviolet radiation is extraordinarily high, and 

 that in the embryo all but an extremely small proportion of these are fol- 

 lowed by restitution. The types of chromosomal alterations found in the 

 deficient plants that make up this small proportion may be quite mislead- 

 ing as to the primary chromosomal effects of the radiation. 



The frequency of deficiencies detectable in the endosperm following 

 ultraviolet treatment is comparable with that found following X-ray 

 treatment. The two agents differ widely in the relative frequency of 

 deficiencies affecting the endosperm as a whole and deficiencies affecting 

 only a part of the endosperm ("fractionals"). With ultraviolet, about 

 75 per cent of the endosperm deficiencies observed are fractionals, and the 

 fraction showing the deficiency is most frequently about one-half of the 

 endosperm, as estimated by the surface area. The deficient sectors vary 

 widely in form and in relative size, sometimes covering only a small frac- 

 tion of the surface and sometimes covering the entire surface except for a 

 small fraction. The frequency distribution of these fractions of varying 

 size is approximately normal about the modal class of )^. Presumably 



