28 MUTATION AND PLANT BREEDING 



Grobman: Examination of a large number of archeological ears of corn 

 in Peru has not disclosed the presence of phenotypes with lower pigmen- 

 tation intensity than those corresponding to the A h or a? levels (brown 

 and brown-red pericarp, respectively), during at least the first 2,000 

 years of a 2,900-year period represented by those ears. It is believed 

 that A h , aP, or both represent the wild condition alleles at the A 1 locus 

 in maize. The stability of A b phenotype seems to have been relatively 

 high during a long period in the evolution of corn, suggesting stability 

 of A h (a/3) allele complex, until such time as tripsacoid races of corn 

 and lower level expression of A b down to a became prevalent. 



This apparent evolutionary stepping up of the changes at A, which 

 we find coincides with the appearance of other conspicuous mutational 

 events in other genes, suggests a direct or indirect effect of Tripsacum 

 genetic material incorporated into corn. 



In the light of the information he has accumulated, would Doctor 

 Laughnan care to comment on the possibility that the mutational events 

 at A h were accelerated and are now being influenced by extragenic ele- 

 ments originating from some kind of Zea-Tripsacum interaction? 



Laughnan: Experiments conducted by R. A. Emerson and E. G. Ander- 

 son reported in 1932 established that A b and «p, in the presence of the 

 P factor, govern the synthesis of brown pigment in the pericarp of 

 maize. However, since a number of other loci are known to affect pig- 

 mentation of the pericarp, even in the absence of P, and since a variety 

 of intergrading phenotypes is possible with respect to both quality and 

 pattern of pigmentation, there is no way, other than by crossing and 

 progeny test, to establish with certainty whether a particular pericarp 

 phenotype is governed by the P locus. Under these circumstances, I 

 consider it unsafe to make inferences about the /J-locus constitution 

 on the basis of pigmentation in archeological ears of corn, and even 

 more hazardous to argue a shift in frequencies of alleles on this cri- 

 terion. 



So far as I can see, there is nothing about the A'' events which can 

 be taken to support or contradict the mentioned Zea-Tripsacum inter- 

 action. I suppose some better understanding of this would come from 

 genetic analysis of the presumed A elements of Andean Tripsacum. 



Auerbach: I wonder whether you think it is safe to generalize from 

 maize to other organisms, in particular to animals. You said in your 

 opening remarks that in maize intergenic changes are so much more fre- 

 quent than gene mutations that, in over-all estimates, the bona fide 



