HETEROSIS IN A NEW POPULATION 419 



cesses and failures of standard techniques in this new i)opulation, together 

 with certain modifications that are being tried, will be discussed. 



HETEROSIS IN NATIVE OPEN-POLLINATED VARIETIES 



The first obvious step in any breeding program in a new area is adequate 

 testing of the varieties at hand. In the early years of the program, therefore, 

 considerable time was devoted to a study and classification of the ])resent-day 

 varieties in Mexico (Wellhausen el al., in collaboration with Mangelsdorf, 

 1951). Evidence presented in this report strongly indicates that many factors 

 have been involved in the evolution of corn in Mexico, the most important 

 of which are repeated here as follows: 



1. Varieties in the ancient pod-pop corn type were probably at first chiefly 

 brought about through mutation and by a partial release from the pressure 

 of natural selection by man. There are four ancient races in Mexico which 

 definitely trace back to a common parent. Where this common parent origi- 

 nated is still unknown. All have a sufficient number of different characters 

 to warrant their classification as separate races, yet they all have a number of 

 characters in common; namely, all are pop corns, two of the four are pod 

 corns, all are early maturing, all have a low chromosome knob number, and 

 all are relatively low in yield capacity compared to modern varietal stand- 

 ards. Since no record of the common ancestor is available, no direct compari- 

 sons can be made of the yield capacities of the ancient indigenous races as 

 they exist today in Mexico and of their common ancestor. Judging from the 

 Bat Cave material (Mangelsdorf and Smith, 1949) it is not at all unlikely 

 that considerable increase in yield capacity was brought about through gene 

 mutation alone. 



2. It is distinctly evident from a study of the various collections that some- 

 time during the history of the Mexican corns there was an influx of e.xotic 

 types from countries to the south. As a result of the introgression of the an- 

 cient indigenous types into the exotic types, and vice versa, many new varie- 

 ties and races came into existence. 



3. Superimposed upon the above two evolutionary mechanisms was the 

 introgression of teosinte germplasm. If Mangelsdorf and Reeves (1939) are 

 right in their theory that teosinte originated as a cross between corn and 

 Tripsacum, then this teosinte germplasm is largely Tripsacum germplasm. 

 Practically all the modern more-productive types of corn contain some teo- 

 sinte germplasm. 



4. The fourth important factor in the evolution of corn in Mexico has been 

 the geography of Mexico itself. Mexico is a mountainous country with many 

 different climates and geographically isolated valleys. Corn is grown from 

 sea-level up to 10,000 feet elevation under a wide range of rainfall conditions. 

 In some areas rainfall is limited to five to ten inches for a i)eriod of four 

 months. Other areas receive up to 100 or more inches in a period of six to ten 



