202 KAUIATION H1()L(KJY 



occurring at the doses here considered with fre(iueiicies as high as 20 per 

 cent. Deficieney of A in the Fi seedlings is rare; a series of progenies 

 grown from seeds which inchuled H)S endosperm deficiencies for A yielded 

 only five -1 deficiencies in the Fi plants. 



Approximately three-fourths of the endosperm deficiencies are frac- 

 lionals, commonly affecting about half of the endosperm. Deficiencies 

 similarly affecting half of the pro-embryo would presumably be present 

 in the resulting Fi plants in only about half of the alTected cases, and this 

 proportion might be reduced by competitive development of the defective 

 and nondefective sectors. But the proportion of fractionals, as shown 

 by the endosperm deficiencies, cannot account for the discrepancy; 

 among the seeds which yielded the five A deficiencies mentioned in the 

 preceding paragraph, there were more than 100 with nonfractional A 

 deficiencies in the endosperm. 



This pronounced disproportion between the frequency of deficiencies 

 in the endosperm and the embryo does not occur wuth X-ray treatment. 

 Ill cultures marked by specific genes affecting both endosperm and plant 

 characters, deficiencies in the Fi plants are somewhat less frequent than 

 in the endosperms, but not more so than might possibly be accounted for 

 by reduced survival. 



Aside from possible differences in the exposure to ultraviolet of the two 

 sperms in the treated pollen grain, a possible cause of the wide disparity is 

 contrasting behavior of chromosome l)reaks in endosperm and embryo 

 development. McClintock's study (1941) of the effects of mechanically 

 broken chromosomes suggests that some alterations induced in the two 

 sperms, though similar in character and in frequency, might have quite 

 disproportionate effects upon the frequency of detectable changes in the 

 embryos and endosperms of the resulting seeds. 



This possibility has l)een investigated by Schultz (1951), using a ring 

 chromosome {Dp 3a) carrying the gene A''. The pollen treated was that 

 of a Dp 3a stock homozygous for a deficiency at the A locus, a-X3, a type 

 in which all functioning pollen carries the duplication (Stadler and Roman, 

 1948). Pollinations on ears of aa constitution show the loss of A'' in 

 endosperm and embryo tissue by the absence of anthocyanin pigment. 

 With untreated pollen a small percentage of deficiencies occurs in both 

 endosperm and embryo, owing to losses of the ring in late microspore or 

 early endosperm or embryo divisions. The sporadic loss of the ring in 

 subse(iuent mitoses results in variegation in the remaining endosperms 

 and plants. 



The result of a break in the ring chromosome differs from that of a 

 break in a rod chromosome chiefly in the effects of restitution. If the 

 break may be followed alternatively by restoration t)f the linear order 

 between the original chromatids or crosswise between the sister chroma- 

 tids, the effect in a rod chromosome would be undetectable in both cases. 



