MYERS: DISCUSSION OF SESSION III 307 



Vettel's results and Doctor Hayes' results is of limited validity, since 

 one has the choice of growing many F 2 populations with few plants in 

 each or few F 2 populations with many plants in each. Perhaps the 

 most important single factor in success is the total number of plants 

 that can be grown. Nevertheless, the plant breeder who selects care- 

 fully the parents he uses, as did Doctor Hayes, will make fewer crosses 

 and have fewer F 2 populations. If, however, he was successful in selec- 

 tion of the parents, the average value of his F 2 populations would exceed 

 that of the F 2 populations of the breeder who did not select the parents 

 so intensively. 



The real point I was trying to make, however, is that the low per- 

 centage of Vettel's crosses from which new varieties were obtained is 

 not, in itself, a valid reason for condemning the pedigree method. II, 

 to take an extreme example, one grew only one F 2 plant of each cross, 

 there would be little likelihood that any superior varieties would result 

 from 5,045 F 2 populations. 



MacKey: The spontaneous mutations occurring in the breeding cul- 

 tures mentioned by Doctor Myers as one cause of variability should 

 definitely not be overlooked. In wheat, using a marker gene on chromo- 

 some IX, I found 0.7 per cent mutations (speltoids) in a homozygous 

 stable variety, close to 20 per cent after optimal X-irradiation, close 

 to 40 per cent after optimal neutron irradiation, but more than 50 

 per cent in heterozygous ¥ 1 material, all calculated on a per plant prog- 

 eny basis. 



Davies: In relation to Doctor Myers' comment on artificially stimulating 

 crossing over, we have already some results in this field. If we expose 

 certain early meiotic stages to low doses of gamma radiation, a definite 

 increase in the number of true chiasmata is observed in the bivalents 

 at metaphase. 



Langham: There seems to be a tendency of new breeders to ignore par- 

 tially the natural genetic variability in a species (such as sesame) and 

 go directly to radiation equipment to look for induced mutations. I 

 believe emphasis should be placed in (1) obtaining a world collection 

 of genetic variability and (2) looking for new methods of screening this 

 material for useful genes. In this connection, the High-Low method 

 of breeding, as used in our sesame program, has given positive results 

 and offers a useful tool in screening available breeding material. The 

 High-Low method is as follows: 



