198 PAUL C. MANGELSDORF 



and teosinte have hybridized, and after there has been an introgression of 

 teosinte into maize: (1) the teosinte genes must be eliminated or, (2) their 

 effects must be changed through the accumulation of a new modifier com- 

 plex, or (3) they must be kept in a heterozygous state. There is evidence, but 

 not final proof, that both of the two last-named factors have operated during 

 the evolution of maize. Interracial and interspecific hybridization accom- 

 panied by sustained heterosis are therefore regarded as two important fac- 

 tors in the evolution of maize. 



SUMMARY 



1. Evidence is presented to show that both interracial and interspecific 

 hybridization, accompanied by heterosis, have been factors in the evolution 

 of maize. 



2. The races of maize of Mexico are cited as an example of interracial hy- 

 bridization. Of the 25 Mexican races described by Wellhausen et al., 14 are 

 considered to be the products of interracial hybridization. 



3. The hybrid vigor, which occurs when races of maize are crossed, is 

 capable of persisting in part in subsequent generations. Maize under domesti- 

 cation is therefore potentially a self-improving plant. 



4. Interspecific hybridization of maize and teosinte is occurring in Gua- 

 temala and Mexico today, and there is evidence — archaeological, morphologi- 

 cal, and cytological — that it has occurred in the past. 



5. Introgression of teosinte into maize in experimental cultures is some- 

 times beneficial when the teosinte genes are heterozygous, but is always 

 deleterious when they are homozygous. 



6. It, therefore, seems probable that the persistence of teosinte germ- 

 plasm in races of maize has been accompanied either by development of 

 modifier complexes which have made the teosinte genes recessive in their 

 action, or by the maintenance of a continued state of heterozygosity. 



7. The possibility that heterozygosity in maize has been preserved by 

 natural selection as it has been in Drosophila is discussed. 



