184 PAUL C. MANGELSDORF 



curred abundantly as a weed permission was obtained from the owner to tag 

 and harvest 500 consecutive plants. Of the 500 plants tagged, 288 proved to 

 be maize, 219 were teosinte plants, and 3 were Fi hybrids. Of the 288 ears 

 classified as maize, 4 showed definite evidence of contamination with teosinte 

 in earlier generations. In addition, one ear was found in an adjacent row (not 

 part of the sample of 500 plants) which was identical in its characteristics 

 with a first backcross to teosinte. 



The plants in this field therefore furnished unmistakable evidence of hy- 

 bridization, both present and during the recent past, between maize and 

 teosinte. One plant out of every 167 plants in the field was a vigorous Fi hy- 

 brid shedding abundant pollen which became part of the general pollen mix- 

 ture in the field. The Fi hybrids themselves, in spite of their vigor, have a low 

 survival value. The Mexican farmer makes no distinction between teosinte 

 and the Fi hybrids. Both are left standing in the field when the corn is har- 

 vested. The pure teosinte disperses its seeds which are enclosed in hard bony 

 shells, and a new crop of teosinte plants appears the following spring. But 

 the Fi hybrids have no effective means of seed dispersal, and their seeds, only 

 partially covered, are quite vulnerable to the ravages of insects and rodents. 



Both maize and teosinte are quite successful in occupying distinct niches 

 in Mexican corn fields. The one, a cultivated plant, depends for its survival 

 upon its usefulness to man. The other, a weed, depends for survival upon its 

 well-protected kernels and its efficient method of dispersal. There is no such 

 niche for the Fi hybrid. It is discarded by man as a cultivated plant, and it 

 cannot compete with teosinte as a weed. "Finding no friend in either nature 

 or man" (to use Weatherwax's apt description) the Fi hybrids would be of 

 no evolutionary significance were it not for the fact that they hybridize with 

 both parents. Thus there is a constant introgression of teosinte into maize and 

 of maize into teosinte. In the vicinity of Chalco, in Mexico, this process has 

 gone on so long and the teosinte has become so maize-like in all of its charac- 

 ters, that maize and teosinte plants can no longer be distinguished until after 

 the pistillate inflorescences have developed. The teosinte of Chalco has "ab- 

 sorbed" the genes for hairy leaf sheaths and red color characteristic of the 

 maize of the region. Individual plants of teosinte have been found which have 

 the yellow endosperm color of corn, although teosinte is normally white- 

 seeded. 



The introgression of teosinte into maize in Mexico today is an established 

 fact. The question is how long this process has been going on and whether it 

 is strictly a local phenomenon or whether it has affected the maize varieties 

 of America. 



Practically all students of maize and its relatives recognize that teosinte 

 varieties differ in the degree to which they have become maize-like. Longley 

 (1941), for example, considers the teosinte of Southern Guatemala to be the 

 least maize-like and that of Mexico the most maize-like. 



