DEVELOPMENT OF THE HETEROSIS CONCEPT 51 



ly superior, on the average, to those of 1910. Jones's discovery about 1917 

 of the double cross plan of producing hybrid seed in maize, and the subse- 

 quent proof by many workers that double crosses can be obtained that closely 

 approach the vigor of Fi crosses between selfed lines, furnished the basis for 

 the utilization of hybrid vigor in field corn. With sweet corn, however, F] 

 crosses between selfed lines are used very widely today for the commercial 

 crop. 



East and Hayes emphasized that Fi crosses probably would be of com- 

 mercial value in some truck crops where crossing was easy. Eggplants, to- 

 matoes, pumpkins, and squashes were considered to offer promise for a prac- 

 tical use of such vigor. The writers also mentioned the fact that heterozygosis 

 had been used in vegetatively propagated plants, though not purposely, and 

 that it seemed feasible to make a practical application in the field of forestry. 



The use of heterosis in practical plant and animal improvement has borne 

 out and surpassed these early predictions as shown in Table 3.1. 



TABLE 3.1 



USE OF HETEROSIS IN CROP PLANTS AND LIVESTOCK 



Farm crops: Maize, sugar beets, sorghums, forage crops, and grasses 



Horticultural crops: Tomatoes, squashes, cucumbers, eggplants, onions, 



annual ornamentals 

 Silkworms 



Livestock: Swine, poultry, beef and milk cattle 



Vegetatively propa- 

 gated plants 



In the corn belt of the United States nearly 100 per cent of all maize is 

 hybrid. Hybrid corn is rapidly being developed in other countries of the 

 world, and is one of the best illustrations of the practical utilization of mod- 

 ern genetics. Considerable evidence leads to the conclusion that heterosis can 

 be used extensively in farm crops, including such widely different plants as 

 sugar beets, sorghums, tobacco, forage crops, and grasses. 



With horticultural plants, where the individual plant is of rather great 

 value, planned heterosis has proven worth while. First generation crosses 

 of tomatoes, onions, egg plants, cucumbers, and squashes have proven their 

 value and are being grown extensively by home and truck gardeners. Similar 

 use is being made of heterosis in some annual ornamentals. 



Heterosis has become an important tool of the animal breeder. Its use in 

 silkworm breeding is well known. Practical utilization of hybrid vigor has 

 been made in swine and poultry, and applications are being studied with beef 

 cattle, dairy cattle, and sheep. A somewhat better understanding of the 

 effects of inbreeding and crossing by the breeder has aided in applications 

 with livestock. As in plants, inbreeding makes controlled selection possible, 

 while controlled crosses may be grown to utilize favorable gene combinations. 



