INDUCED CHROMOSOMAL ALTERATIONS IN MAIZE 1303 



such that the two chromosomes 1 and 1^ go to opposite poles. Likewise 

 2 and 2' go to opposite poles. But the distribution of 2 and 2' seems 

 to be independent of the distribution of 1 and 1^ so that four types of 

 spores are formed in equal numbers. These have the following chromo- 

 some constitution : 



1, 2, normal; 



1*, 2', interchange; 



1, 2', and P, 2, inviable. 



The normal and interchange spores are viable and fully functional. 

 The other two types have a portion of one chromosome duplicated, and 

 a portion of the other deficient. They are not functional. Consequently 

 half of the ovules fail to develop and half of the pollen is abortive. The 

 chromosomes may occasionally be distributed irregularly, three going 

 to one pole, and one to the other. 



In linkage studies, the interchange may be followed by means of 

 pollen semisterility which behaves in outcrosses like a dominant gene 

 at the locus of the interchange in both chromosome maps. 



Of the interchanges described semisterile 1 (Brink, 6), semisterile 3 

 (Burnham, 13), semisterile 4 (Rhoades, 41, 44) and semisterile 5 (Cooper 

 and Brink, 20) may be taken as fairly typical interchanges. Semisterile 2 

 (Burnham, 13, 18) shows about 59 per cent of bad pollen. The chromo- 

 some distribution is somewhat more irregular. About 7 per cent of the 

 diakinesis figures show a chain instead of a ring of chromosomes. 



A low sterile described by Burnham (14) forms chains consistently 

 but never rings. The interchange had taken place in the satellite 

 of chromosome 6. The point of interchange is within one or two chro- 

 momeres of the end of the chromosome. The interchanged end of the 

 satellite does not show pairing with the normal satellite. This gives 

 a T-shaped pachytene figure, opening out to form a chain at diakinesis. 

 One of the duplicate-deficient classes of spores lacks only a few chro- 

 momeres of having all parts of the chromosomes represented, and forms 

 pollen which is not visibly abnormal. 



A more extreme case has been described by Clarke and Anderson (19) 

 in which chains are found in only about one-third of the diakinesis figures, 

 the balance showing two slightly unequal "pairs." Here too the inter- 

 change is in the satellite of chromosome 6 and also near the end of 

 chromosome 3. 



One interchange from irradiation, involving chromosome 6, divided 

 the reticulate region, which is the attachment to the nucleolus (Anderson, 

 1). This interchange has been used by McClintock (33) for an intensive 

 study of the function and development of the nucleolus. The nucleolus 

 is shown to be organized by this reticulate region or nucleolar-organizing 

 body. When this body is severed by the interchange each part organizes 

 its own nucleolus during the mitotic telophases. 



