1300 BIOLOGICAL EFFECTS OF RADIATION 



Some data on the frequency of induced deficiency of known endosperm 

 genes have been presented by Stadler (50, 53). Endosperm characters 

 were selected for these quantitative studies because large numbers are 

 more readily obtained, compensating for the disadvantage that they 

 cannot be checked cytologically. The induced deficiencies are roughly 

 in proportion to the X-ray dosage given. But for a given dosage each 

 gene has a different characteristic frequency. Of the genes studied, A 

 became lost most frequently and Y least frequently. The most instruc- 

 tive are the data involving C Sh and Wx. These are all in the short 

 arm of chromosome 9 with Wx known to be closest to the spindle-fiber 

 insertion. The linkage order is C Sh Wx, with about 4 per cent crossing 

 over between C and Sh and about 22 per cent between Sh and Wx. 

 Of the seeds deficient for C, the majority were deficient also for Sh and 

 Wx. About 25 or 30 per cent were deficient for Sh but not for Wx. A 

 few were deficient for C only. Most of the deficiencies were considered 

 terminal ones. Some of those deficient for all the genes were probably 

 losses of the entire chromosome. 



An interesting feature of endosperm deficiencies is the frequent 

 appearance of one or more small islands of cells carrying the dominant 

 genes (Stadler 49, 50). These Stadler interprets as due to recovery of a 

 chromosome affected by X-ray. On this interpretation the deficiency 

 is due to loss of the power of reproduction of the affected chromosome 

 or section of chromosome. At each succeeding mitosis this chromosome 

 goes to one daughter cell or the other. If later it recovers its power of 

 reproduction, it then may divide regularly and form an island of normal 

 (nondeficient) tissue. Since later cytological studies on deficiencies 

 have shown them to be actual losses rather than inactivations, a modifica- 

 tion of Stadler 's hypothesis seems required. We may assume for whole 

 chromosome losses, that the missing chromosome remains free in the 

 common cytoplasm of the early endosperm, while the nuclei divide for 

 several successive generations, and then becomes incorporated in one of 

 the free nuclei and divides regularly thereafter. Several separate spots, 

 usually clustered as they are, might be formed, for instance, by one or 

 more sporadic divisions of the free chromosome, before being associated 

 with a nucleus. Recovery of a fragment would require that the fragment 

 become attached to one of the chromosomes or, less probably, develop 

 its own spindle fiber. Such things might happen in the endosperm where 

 they would not in ordinary tissue, since there are a number of divisions 

 of the endosperm nucleus in a common cytoplasm followed by migration 

 to the periphery before cell-wall formation takes place. The one case 

 cited by Stadler (49) as recovery in the sporophyte is open to interpreta- 

 tion as an ordinary chimaera in which only a small portion was non- 

 deficient. Similar islands of green tissue are occasionally found in 

 maternally inherited plastid chimaeras. 



