A. J. MANGELSDORF 



Experiment Sfation, Hawaiian Sugar Planters Assodaiion , Honolulu, T.H. 



Chapter 20 



Gene Interaction in Heterosis 



Sugar cane behaves very much like corn in its reaction toward inbreeding 

 and outcrossing. Although the sugar cane flower is normally provided with 

 both male and female organs, male sterility is not uncommon. Among the 

 varieties that produce an abundance of pollen, many are partially or highly 

 self-sterile. As a consequence, cross-fertilization by wind-borne pollen is the 

 rule in sugar cane, as in corn. When sugar cane is subjected to self-pollina- 

 tion, the usual result is a reduction in seed setting and a marked reduction in 

 the vigor of the offspring. 



The sugar cane breeder enjoys one great advantage over the corn breeder: 

 sugar cane can be propagated asexually. Each node on the stalk is provided 

 with a bud and with a number of root primordia. In field practice, stalks of 

 the selected variety of sugar cane are sectioned into cuttings of two or more 

 internodes each. These cuttings are then placed horizontally in furrows and 

 covered lightly with soil. In due course the cutting sends out its roots, the 

 buds develop into shoots, and a new plant is established. 



Were it possible to apply this procedure to corn, and thus to perpetuate 

 outstanding individuals from whatever source, it is unlikely that the corn 

 breeder would have felt obliged to resort to the laborious procedures now 

 employed. 



When sugar cane varieties are propagated by cuttings, the traits by which 

 we are able to distinguish one variety from another maintain their integrity 

 through many cycles of clonal propagation. This is true not only of morpho- 

 logical traits, but also of physiological traits. 



Sugar cane has a number of relatives growing in the wild, some of which 

 may be ancestral to the original cultivated forms. Wild Saccharums are wide- 

 ly distributed in the tropical and sub-tropical regions of the Old World, from 

 central Africa through Asia and Malaya, to and including the Indonesian 

 and many of the more westerly Pacific islands. This heterogeneous array of 



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