22 MUTATION AND PLANT BREEDING 



Mention should be made of a phenomenon at the R locus in 

 maize which is a rather close parallel of that involving A b . It is now 

 apparent (3, 4) that loss of the plant color effect of the (P) element in 

 the (P) (S) Cornell complex, whether or not it is associated with 

 recombination of marker genes, is attributable to physical loss rather 

 than to gene mutation of the (P) element in the complex. Spontaneous 

 deficiency of the (P) element or, alternatively, loss of this element in 

 response to some type of activator, are two of the mechanisms pro- 

 posed (4) to account for the event in question. However, neither of 

 these explanations satisfactorily accounts for the noncrossover alpha 

 derivatives from the betaralpha complex since it has been shown 

 that the isolated beta element, removed from its association with 

 alpha, is surprisingly stable. 



Thus, an acceptable model on which to account for the loss of 

 beta from the complex must satisfy the evidence that the event does 

 not require the participation of the homologue and that it is uniquely 

 dependent on the cis association with alpha. Moreover, the event must 

 be highly restricted as to time of its occurrence since the vast majority 

 of the derivatives occur as single kernels on ears of tested individuals, 

 thus giving strong indication that meiosis is involved. Several years 

 ago we suggested (12) a mechanism which accommodated the evidence 

 available at that time and which appears not to be in conflict with 

 that available now. It is based on the finding (9, 11) that the A b com- 

 plexes are tandem, serial duplications whose members, at meiosis, 

 engage in oblique synapsis with their complements in the homologue; 

 and it assumes only that the adjacent members of the duplication 

 have the alternative of pairing with each other, intrachromosomally, 

 in a double-loop configuration at meiosis which will be referred to 

 as A. A. (auto-association). As illustrated in Figure 4, rare exchanges 

 between the beta and alpha segments comprising the double loop 

 would result in the loss and/or gain of one complete member of the 

 duplication, depending on the strands involved in the event. Occa- 

 sional losses of the beta-carrying segment by this mechanism would 

 yield apparent noncrossover alpha derivatives which, because of their 

 origin through "deficiency" of beta, would be indistinguishable in 

 phenotype from the crossover alpha derivatives. Moreover, since the 

 event is dependent on a pairing phenomenon ordinarily restricted to 

 meiosis, the observed individual occurrences of the alpha derivatives 



