PREFERENTIAL SEGREGATION IN MAIZE 75 



the fiber-producing activity of the neo-centric regions is a ])r()<luct of the true 

 centric region. 



The appearance of neo-centric fibers in prophase II is not the rule. Usually 

 the dyads come to lie with the true centric region on the spindle plate at 

 metaphase II before any pronounced activity of neo-centric regions is ap- 

 parent. Before the primary centric region divides, thus permitting a normal 

 anaphase, chromosomal fibers again arise near the distal ends of the long 

 arms of some dyads. These newly formed fibers move the long arms poleward 

 while the dyad is still held on the metaphase plate by the undivided true 

 centric region. This poleward movement is so rapid that the ends of the 

 chromosomes may reach the spindle poles before the true anaphase occurs. 

 Eventually the true centric region becomes functionally split, and the two 

 monads fall apart and pass poleward. It is evident from Figures 4.4 and 4.5 

 and Figure 7 of Plate II that the configurations of the disjoining monads 

 (chromatids) at anaphase II are greatly different from normal. 



Neo-centric activity, as shown by formation of additional chromosomal 

 fibers, occurs in plants both homozygous and heterozygous for the abnormal 

 10, but it is much more striking in homozygous plants. Plants trisomic for 

 abnormal 10 were not greatly different from homozygous disomic sibs. 



Precocious chromosomal fiber formation by neo-centromeres at metaphase 

 II appears in general to be confined to the long arms of the dyads, although 

 it is often difficult to differentiate between two unequal arms when one is 

 stretched poleward. Some chromosomes have arm ratios so extreme that 

 the distinction between long and short arms is clear, and in these chromo- 

 somes the precocious fibers at metaphase II arise from the long arms. It is 

 perhaps significant that, with the exception of the terminal knob on the short 

 arm of chromosome 9, all remaining knobs in our material were situated in 

 the long arms. (Chromosome 6 had two small knobs in its long arm but a 

 maximum of one knob was present in the other chromosomes.) Corre- 

 spondingly, only one of the two arms of any chromatid had neo-centric 

 activity at metaphase 11.^ The number of dyads with precocious spindle 

 fibers, as judged by the number of arms pulled poleward at metaphase II, 

 varied in different strains. The maximum number in some plants was seven, 

 in others five, etc. Plants with seven knobbed chromosomes had a maximum 

 of seven dyads with arms stretched poleward at metaphase II. Those with 

 four knobs had four such dyads. That is, a strong correlation exists between 

 knob number and the number of dyads with neo-centric activity at meta- 

 phase II. 



A further observation of some interest was that in plants homozygous for 

 all knobs both homologous arms of a dyad usually were pulled poleward at 

 metaphase II; while in plants heterozygous for some knobs many of the 

 dyads had only one arm with neo-centric activity (see Figure 4.5 and Figures 



1. With the possible exception of chromosome 6. See Figure 5 of Plate II. 



