182 



PAUL C. MANGELSDORF 



(Jones and Mangelsdorf, 1925; Neal, 1935) which means that inbreds, selfed 

 once and having lost half of their heterozygosity, should yield 75 per cent as 

 much as the open-pollinated varieties from which they were derived. Assum- 

 ing that the single-cross combinations involved are at least equal to the top- 

 cross combinations — 132 per cent — we compute the theoretical F2 yield of 

 the topcrosses at 117 per cent, which is considerably less than the 126 per 

 cent actually obtained in the experiments. From the results it can be con- 

 cluded that hybrid combinations including open-pollinated varieties of maize 

 retain a considerable proportion of their vigor in advanced generations. 



There is also some evidence to indicate that the amount of heterosis which 

 occurs when open-pollinated varieties are used in hybrid combinations may be 



TABLE 11.2 



SUMMARY OF EXPERIMENTS DEMONSTRATING EQUILIBRIUM 

 REACHED IN Fo AND NO ADDITIONAL YIELD REDUC- 

 TION IN F3 OF MAIZE CROSSES 



considerably higher with Latin-American varieties than with varieties com- 

 monly grown in the United States. Wellhausen and Roberts report single 

 topcrosses yielding up to 173 per cent of the open-pollinated variety and 

 double topcrosses up to 150 per cent. A recent report from the Ministry of 

 Agriculture of El Salvador (1949) shows four different hybrids between open- 

 pollinated varieties yielding about 50 per cent more than the average of the 

 parents. Such increases are not surprising, since the varieties used in the 

 experiments are quite diverse, much more so than Corn Belt varieties. 



All of the data which are available on the yields of advanced generations 

 of maize crosses, whether the parents be inbred strains or open-pollinated 

 varieties, tend to show that a substantial part of the hybrid vigor charac- 

 teristic of the Fi is retained in subsequent generations. Thus maize under 

 domestication is potentially and no doubt actually a self-improving plant. 

 Distinct more-or-less stable varieties or races evolve in the isolation of 

 separated regions. Man brings these varieties or races together under condi- 

 tions where cross-fertilization is inevitable, and a new hybrid race is born. 

 Repeated cycles of this series of events inevitably lead to the development, 



