196 



PAUL C. MANGELSDORF 



found, however, that when teosinte germplasm was introduced into another 

 inbred strain, 127C, the results obtained in the hybrids were somewhat dif- 

 ferent. In 1946, 6 hybrids out of 25 were better than the check, 3 of them 

 significantly so. In 1947, 15 hybrids out of 49 were better than the check, 

 6 of them significantly so. Reeves suggested that the difference between 4R-3 

 and 127C in their response to teosinte introgression lies in the fact that 

 4R-3 already contained considerable amounts of teosinte germplasm while 

 127C does not. The suggestion is supported by differences in the morphologi- 

 cal characteristics of the two lines. 



There was also an indication in Reeves' experiments that the entries with 



TABLE 11.5 



AVERAGE NUMBERS OF SEEDS PER PLANT IN HYBRIDS OF 



CORN BELT INBREDS WITH TEXAS 4R-3 AND ITS 



TEOSINTE-MODIFIED DERIVATIVES 



* Difference probably significant. 



teosinte genes made their best showing in 1947, a season of severe drought. 



Considering all of the results together it may be concluded that: (c) 

 blocks of teosinte genes in the heterozygous condition do in some instances 

 improve the total yield of the plants which contain them ; (b) even more fre- 

 quently do such blocks of genes increase the total number of seeds produced ; 

 (c) there is some evidence that the teosinte derivatives impart resistance to 

 heat and drought to their hybrids. 



In those crosses in which the heterozygous combination is superior to 

 either of the homozygous combinations, a block of maize genes or a block of 

 teosinte genes, natural selection would undoubtedly favor, at least initially, 

 the heterozygous combination. If the block of genes were one involving the 

 region of the centromere where crossing-over is reduced, it is quite possible 

 that the block of genes would be retained more or less intact for a consider- 

 able number of generations. The maintenance of heterozygosity through 

 natural selection also would be promoted if, as in the case of Drosophila 

 studied by Dobzhansky, one set of genes is superior in adapting the organism 

 to one kind of environment while the other set contributes to adaptation 



