PREFERENTIAL SEGREGATION IN MAIZE 11 



Fig. 4.3). No such regularity is found in those infrequently occurring dyads 

 which are longitudinally displaced from the spindle {)late at metaphase II. 

 Their true centric regions divide prematurely. Consequently, the two 

 chromatids of these displaced dyads no longer remain conjoined, but fall 

 apart to become independent monads which lie side-by-side, parallel with 

 the longitudinal axis of the spindle. 



The neo-centric activity which these monads now manifest is similar to 

 that found at anaphase II for those monads derived from normally oriented 

 dyads lacking precocious neo-centromeres at metaphase II, in that neo- 

 centromeres may arise from the ends of both arms. When this occurs, the 

 orientation of the two neo-centromeres of each monad is usually to opposite 

 poles, but sometimes both ends of a monad are directed toward the same 

 pole. Although the monads from displaced dyads have neo-centromeres at 

 the end of each arm, one end being attracted to the nearest pole and the other 

 to the more distant pole, normal disjunction usually occurs. This requires 

 one monad to move away from the nearest pole toward which one of its ends 

 is attracted, and to pass to the more distant pole, while the other monad goes 

 to the nearest pole. It is difficult to interpret this phenomenon in terms of 

 strength of attraction as a function of distance from centromere to pole. 



The formation of neo-centric regions requires the presence of the abnormal 

 chromosome 10. In its absence, no such regions are found. It appears highly 

 probable that heterochromatic knobs located on other chromosomes also are 

 concerned in the formation of precocious centric regions at both meiotic 

 metaphases, since the cytological observations show a correlation between 

 number of knobs and number of precocious centric regions. Knobless arms 

 later form neo-centric regions, but not until anaphase movement has already 

 been initiated by the true centric region. 



It is possible that maize chromosomes possess latent centric regions which 

 are activated by the abnormal 10. It has been demonstrated, however, that 

 the true centric region is involved in the formation of neo-centromeres. 

 Plants homozygous for abnormal 10 and heterozygous for the long para- 

 centric inversion in chromosome 4, studied by McClintock (1938) and Mor- 

 gan (1950), were obtained. Both the normal and inverted chromosome 4 

 carried a large knob in the long arm which is included in the inverted seg- 

 ment. Single crossovers within the inversion give rise to two non-crossover 

 monocentric chromatids, one dicentric chromatid which forms a bridge at 

 anaphase I, and an acentric fragment. The knobbed acentric fragment lies 

 passively on the spindle with no indication of spindle fiber activity. Neo-cen- 

 tromeres arise from the same chromatin segments comprising the acentric 

 fragment when they constitute a portion of a whole chromosome 4. It fol- 

 lows that the true or primary centromere plays an essential role in the pro- 

 duction of neo-centromeres. 



The localized centromeres of maize chromosomes are concerned with the 



