50 MUTATION AND PLANT BREEDING 



ever, there is no compelling reason to ascribe cases of gene conversion 

 to intragenic changes since extragenic mechanisms are capable of 

 producing the observed resnlts. Whatever may be the cause of the 

 intriguing cases of paramutation in maize described by Brink and his 

 associates and by Coe, there is no evidence in these studies of 

 intragenic modifications. 



It should be emphasized that in none of the many carefully con- 

 ducted and extensive mutational investigations in maize is there 

 unassailable proof of true gene mutation. In all well-analyzed cases an 

 extragenic mechanism has either been demonstrated to be responsible 

 for the mutant change or else one appears to be highly probable. 

 Insofar as the mutation spectrum is concerned, it would be surprising 

 if a comparable situation does not exist in other higher plants. Muta- 

 tional studies are of concern not only to the geneticist but also to the 

 plant breeder who utilizes mutations as a source of variation and who 

 needs to be aware of the diverse mechanisms which simulate true 



oene mutation. 



Comments 



Kramer: Since the waxy gene in corn produces pollen grains whose 

 starch stains brown rather than blue with iodine, Dr. O. E. Nelson, 

 by looking at stained pollen from ¥ 1 plants of crosses between ivx stocks 

 derived from independent reoccurrences of wx mutations, has been 

 able to identify recombinants between different wx mutants by the rare 

 occurrences of blue staining grains. A number of mutants have been 

 placed in linear order within the waxy locus by this method. The large 

 populations of pollen grains which can be screened, make this locus an 

 ideal one and comparable to phage for genetic fine structure studies. 



Caspar: The latest chromosome theory, based on the information provided 

 from electron microscope pictures of chromosomes and theoretical molecu- 

 lar biology, states that there are 32 to 64 strands of DNA in the chromo- 

 some of higher plants and animals. The theory holds that "gene" 

 mutation is due to rearrangements in the base pair configurations of the 

 DNA. A mutation is expressed when the chromosomes of a cell all carry 

 the same mutated base pair configuration. It would seem that a chromo- 

 some would have to go through at least 16 reduplications before this could 

 happen. 



How, then, does the investigator, testing for induced mutations in 

 the higher plants and animals, score, let alone find, mutations which are 

 due to induced changes in the base pair configurations? 



