424 E. J. WELLHAUSEN 



gions to which Conico is best adapted, neither Chalqueno nor Tuxpeno will 

 mature, and in addition Tuxpeno is very heavily attacked by rust. On the 

 other hand, in the lowland areas where Tuxpeno is best adapted, Chalqueno 

 produces very little and Conico produces practically nothing. A similar rela- 

 tionship has held with an artificial cross between Conico and Tuxpeno in 

 which half the germplasm was from one parent and half from the other. The 

 artificial hybrid, as one might expect, was not as well adapted to the Chal- 

 queno area as Chalqueno itself. Chalqueno over a period of many years of 

 gene sifting no doubt concentrated those favorable growth genes best suited 

 to its present environment and practically eliminated the frequency of other 

 genes which were unnecessary or deleterious to its maximum development. 

 The hybrid, nevertheless, was much superior to the two parents in the envi- 

 ronment best suited to Chalqueno. 



Here then is an example of the process which often happens in nature or 

 in planned breeding programs. A hybrid between two parents adapted to 

 widely different environments shows no hybrid vigor, as measured in yield, 

 over either one of the two parents when grown in the native habitat of either 

 parent. But in a new environment different from the one under which either 

 parent developed, the hybrid may show extreme vigor. It is highly probable 

 that many new varieties came into existence when new areas of land were 

 brought under cultivation or when the native inhabitants migrated to new 

 areas. Very often several different corns were brought together in these new 

 environments which were not as well adapted as the hybrids that resulted 

 between them. It is also highly probable that the first varietal hybrids were 

 not as good on the whole as the varieties that developed from them through 

 successive generations of backcrossing and gene sifting. 



From a study of the origin and development of the various types of corn 

 in Mexico, it seems that the most important factor in the evolution of the dif- 

 ferent productive races has been the gradual accumulation of favorable 

 growth or yield genes in combination or balance with the proper "governing 

 or regulating" genes in each specific environment. Maximum yield in a spe- 

 cific environment is not only dependent on favorable genes for such quantita- 

 tive characters as ear and kernel size, number of ears, leaf area or photosyn- 

 thetic efficiency, but also on genes which govern such functions as maturity, 

 disease and drought resistance, or general adaptation. 



The latter group might well be genes which some investigators have 

 termed botlleneck genes. Such genes may inhibit the full expression of certain 

 quantitative yield genes and thus prohibit the organism from reaching its 

 maximum production allowed by a specific environment. An increase in yield 

 capacity, therefore, often may not involve the accumulation of more favor- 

 able yield genes, but rather the removal of certain bottleneck genes. In new 

 environments these bottleneck genes might be relic genes carried over from 

 their old native habitat where they existed because they had survival value. 



