384 Inbreeding , Selection, and Heterosis 



a ready sale on the local markets during a period in which 

 normal-sized tomatoes had to be imported. 



Heterosis 



The sorting out of genotypically different homozygous lines 

 as the result of inbreeding opens up an interesting question. 

 What will be the result if these homozygous lines are crossed 

 together? We have had the answer to that since G. H. Shull's 

 report in 1908 showing that what may appear to be a uniform 

 variety of maize is really a series of very complex hybrids in- 

 volving a number of distinct biotypes. He pointed out, as we 

 have discussed under inbreeding, that these biotypes can be iso- 

 lated by continued self-fertilization and that if they are crossed 

 together after isolation the hybrids have much greater vigor 

 and strength than the inbred lines. He showed also that one or 

 two of them even exceeded the original hybrid combinations that 

 comprised the maize field, thus indicating that the way to maxi- 

 mum yields with respect to any desired characteristic was to find 

 the right pair of inbreds and repeat the cross. The vigor of 

 certain hybrids had been recognized for at least two hundred 

 years before that, but it was the work of Shull that clarified the 

 phenomenon. To this vigor of the hybrids the terms hybrid 

 vigor and heterosis have been applied. The vigor manifests it- 

 self differently in the numerous plants and animals in which it 

 has been recorded, but it frequently is revealed in some form as 

 greater plant height, greater size and weight of the fruit, greater 

 yield per acre, greater length of ear, number of rows, and num- 

 ber of kernels per row in maize, more internodes per plant, and 

 greater gross weight. The difference between inbred lines and 

 the hybrids can be readily appreciated from Fig. 100. This 

 vigor, however, is not maintained at the same high level unless 

 provision is made to repeat the same effective heterozygous geno- 

 type, and this repetition can be achieved only by maintaining 

 the purebreds and repeating the cross between them each year 

 or by some vegetative method of reproduction. 



The cause of the greater vigor in the hybrids has puzzled 

 geneticists for a number of years. Several theories have been 

 suggested and various modifications and extensions of some of 

 these theories have been offered, but no explanation yet pro- 

 posed is entirely satisfactory. It is, of course, possible that sev- 



