Whaley • The Gifts of Hybridity 



more desirable than their parents. The 

 occurrence of this hybrid advantage, 

 generally referred to as hybrid vigor, 

 provides one of the most intriguing of 

 biological puzzles. 



Our discussion of it must be pref- 

 aced with a note about the use of the 

 word "hybrid." In a narrow sense, a 

 hybrid is an offspring of two different 

 species. In the broader sense in which 

 we shall use it here, hybrids are the 

 offspring of genetically different types. 

 These may be species, varieties, or lines, 

 types which differ too litle to be called 

 separate varieties. 



The discovery that among both 

 plants and animals certain cross-fertili- 

 zations result in the production of prog- 

 eny more vigorous than either parent 

 was made long ago. There are records 

 of plant hybridizers taking practical ad- 

 vantage of hybrid vigor as early as the 

 last quarter of the eighteenth century. 

 The value of the mule, which is often, 

 though perhaps not correctly, cited as 

 a classic example of hybrid vigor, was 

 recognized by the contemporaries of 

 Moses. Through the years it has been 

 learned that hybrid vigor is the result 

 of interbreeding within plant genera 

 as diverse as com and oaks and within 

 animal genera as different as fruit 

 flies, fish, cattle and man. 



THE BASIC PROBLEM 



There is a vast literature on the sub- 

 ject of hybrid vigor. Part is devoted to 

 the usefulness of hybrid vigor as a 

 manageable biological occurrence, and 

 part is devoted to the opportunities 

 that hybrid vigor presents for approach- 

 ing certain complex questions of inheri- 

 tance and development. When one 

 examines the results of research, it 

 becomes apparent that there is no 

 single definition of what constitutes 

 hybrid vigor. Different plant types ex- 

 hibit hybrid vigor in different ways. 



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For example: In com, in which great- 

 est practical use has been made of the 

 characteristic, the hybrid advantages 

 are generally recorded in terms of total 

 yield, weight, length of the ears, num- 

 ber of grains per row on the ear, and 

 number of nodes per plant. There ap- 

 pear to be "quality" factors involved 

 also, but these are much more difficult 

 to evaluate. Among them are likely to 

 be differences in chemical composition, 

 making for differences in nutritive 

 value. In the tomato, on the other 

 hand, the effects are mostly in increas- 

 ing the number of parts rather than the 

 size of the individual parts. 



The differences in manifestation 

 of hybrid vigor in com and tomato are 

 related to differences in growth habit. 

 Com is a determinate plant in which, 

 after a certain number of intemodes 

 have been formed, the growing shoot 

 apex matures as a flower structure. To- 

 mato is an inderterminate plant which 

 forms an indefinite number of lateral 

 branches and bears lateral rather than 

 terminal flowers. Apparently, in de- 

 terminate plants like com, added vigor 

 increases the magnitude of the parts, 

 the number of which is more or less 

 fixed by the genetic pattern. In inde- 

 terminate plants like tomato, added 

 vigor results in faster or longer-con- 

 tinued production of the new parts. 

 There may be an important clue as to 

 the nature of hybrid vigor in these 

 differences in expression, for they seem 

 to suggest that we are not dealing so 

 much with a change in the basic ge- 

 netic pattem as with an increase in 

 the efficiency of growth and develop- 

 ment. 



Hybrid vigor in com presents the 

 best example for consideration. Com 

 has been grown in the Westem Hemi- 

 sphere for hundreds of years. It has been 

 subject to so much artificial selection 

 that it was long ago modified to the 

 point where it can now exist only in 



